diff --git a/src/backend/backend.c b/src/backend/backend.c
index 33a3d8bc..5120672b 100644
--- a/src/backend/backend.c
+++ b/src/backend/backend.c
@@ -82,13 +82,17 @@ void handle_device_reset(session_t *ps) {
 }
 
 /// paint all windows
-void paint_all_new(session_t *ps, struct managed_win *t) {
+///
+/// Returns if any render command is issued. IOW if nothing on the screen has changed,
+/// this function will return false.
+bool paint_all_new(session_t *ps, struct managed_win *const t) {
 	struct timespec now = get_time_timespec();
 	auto paint_all_start_us =
 	    (uint64_t)now.tv_sec * 1000000UL + (uint64_t)now.tv_nsec / 1000;
 	if (ps->backend_data->ops->device_status &&
 	    ps->backend_data->ops->device_status(ps->backend_data) != DEVICE_STATUS_NORMAL) {
-		return handle_device_reset(ps);
+		handle_device_reset(ps);
+		return false;
 	}
 	if (ps->o.xrender_sync_fence) {
 		if (ps->xsync_exists && !x_fence_sync(&ps->c, ps->sync_fence)) {
@@ -114,7 +118,7 @@ void paint_all_new(session_t *ps, struct managed_win *t) {
 
 	if (!pixman_region32_not_empty(&reg_damage)) {
 		pixman_region32_fini(&reg_damage);
-		return;
+		return false;
 	}
 
 #ifdef DEBUG_REPAINT
@@ -190,16 +194,15 @@ void paint_all_new(session_t *ps, struct managed_win *t) {
 	auto after_damage_us = (uint64_t)now.tv_sec * 1000000UL + (uint64_t)now.tv_nsec / 1000;
 	log_trace("Getting damage took %" PRIu64 " us", after_damage_us - after_sync_fence_us);
 	if (ps->next_render > 0) {
-		log_trace("Render schedule deviation: %ld us (%s) %" PRIu64 " %ld",
-		          labs((long)after_damage_us - (long)ps->next_render),
-		          after_damage_us < ps->next_render ? "early" : "late",
-		          after_damage_us, ps->next_render);
+		log_verbose("Render schedule deviation: %ld us (%s) %" PRIu64 " %ld",
+		            labs((long)after_damage_us - (long)ps->next_render),
+		            after_damage_us < ps->next_render ? "early" : "late",
+		            after_damage_us, ps->next_render);
 		ps->last_schedule_delay = 0;
 		if (after_damage_us > ps->next_render) {
 			ps->last_schedule_delay = after_damage_us - ps->next_render;
 		}
 	}
-	ps->did_render = true;
 
 	if (ps->backend_data->ops->prepare) {
 		ps->backend_data->ops->prepare(ps->backend_data, &reg_paint);
@@ -219,7 +222,7 @@ void paint_all_new(session_t *ps, struct managed_win *t) {
 	// on top of that window. This is used to reduce the number of pixels painted.
 	//
 	// Whether this is beneficial is to be determined XXX
-	for (auto w = t; w; w = w->prev_trans) {
+	for (struct managed_win *w = t; w; w = w->prev_trans) {
 		pixman_region32_subtract(&reg_visible, &ps->screen_reg, w->reg_ignore);
 		assert(!(w->flags & WIN_FLAGS_IMAGE_ERROR));
 		assert(!(w->flags & WIN_FLAGS_PIXMAP_STALE));
@@ -541,6 +544,7 @@ void paint_all_new(session_t *ps, struct managed_win *t) {
 	for (win *w = t; w; w = w->prev_trans)
 		log_trace(" %#010lx", w->id);
 #endif
+	return true;
 }
 
 // vim: set noet sw=8 ts=8 :
diff --git a/src/backend/backend.h b/src/backend/backend.h
index f572a935..1b4df729 100644
--- a/src/backend/backend.h
+++ b/src/backend/backend.h
@@ -366,4 +366,8 @@ struct backend_operations {
 
 extern struct backend_operations *backend_list[];
 
-void paint_all_new(session_t *ps, struct managed_win *const t) attr_nonnull(1);
+/// paint all windows
+///
+/// Returns if any render command is issued. IOW if nothing on the screen has changed,
+/// this function will return false.
+bool paint_all_new(session_t *ps, struct managed_win *t) attr_nonnull(1);
diff --git a/src/backend/driver.c b/src/backend/driver.c
index b909a622..f17743a4 100644
--- a/src/backend/driver.c
+++ b/src/backend/driver.c
@@ -19,6 +19,13 @@ void apply_driver_workarounds(struct session *ps, enum driver driver) {
 	}
 }
 
+enum vblank_scheduler_type choose_vblank_scheduler(enum driver driver) {
+	if (driver & DRIVER_NVIDIA) {
+		return VBLANK_SCHEDULER_SGI_VIDEO_SYNC;
+	}
+	return VBLANK_SCHEDULER_PRESENT;
+}
+
 enum driver detect_driver(xcb_connection_t *c, backend_t *backend_data, xcb_window_t window) {
 	enum driver ret = 0;
 	// First we try doing backend agnostic detection using RANDR
diff --git a/src/backend/driver.h b/src/backend/driver.h
index e4cc3981..1b0877c1 100644
--- a/src/backend/driver.h
+++ b/src/backend/driver.h
@@ -7,6 +7,7 @@
 #include <stdio.h>
 #include <xcb/xcb.h>
 
+#include "config.h"
 #include "utils.h"
 
 struct session;
@@ -41,6 +42,8 @@ enum driver detect_driver(xcb_connection_t *, struct backend_base *, xcb_window_
 
 /// Apply driver specified global workarounds. It's safe to call this multiple times.
 void apply_driver_workarounds(struct session *ps, enum driver);
+/// Choose a vblank scheduler based on the driver.
+enum vblank_scheduler_type choose_vblank_scheduler(enum driver driver);
 
 // Print driver names to stdout, for diagnostics
 static inline void print_drivers(enum driver drivers) {
diff --git a/src/common.h b/src/common.h
index 707d00e7..52967e60 100644
--- a/src/common.h
+++ b/src/common.h
@@ -139,8 +139,6 @@ typedef struct session {
 	// === Event handlers ===
 	/// ev_io for X connection
 	ev_io xiow;
-	/// Timer for checking DPMS power level
-	ev_timer dpms_check_timer;
 	/// Timeout for delayed unredirection.
 	ev_timer unredir_timer;
 	/// Timer for fading
@@ -214,26 +212,19 @@ typedef struct session {
 	bool first_frame;
 	/// Whether screen has been turned off
 	bool screen_is_off;
-	/// Event context for X Present extension.
-	uint32_t present_event_id;
-	xcb_special_event_t *present_event;
 	/// When last MSC event happened, in useconds.
 	uint64_t last_msc_instant;
 	/// The last MSC number
 	uint64_t last_msc;
-	/// When the currently rendered frame will be displayed.
-	/// 0 means there is no pending frame.
-	uint64_t target_msc;
 	/// The delay between when the last frame was scheduled to be rendered, and when
 	/// the render actually started.
 	uint64_t last_schedule_delay;
 	/// When do we want our next frame to start rendering.
 	uint64_t next_render;
-	/// Did we actually render the last frame. Sometimes redraw will be scheduled only
-	/// to find out nothing has changed. In which case this will be set to false.
-	bool did_render;
 	/// Whether we can perform frame pacing.
 	bool frame_pacing;
+	/// Vblank event scheduler
+	struct vblank_scheduler *vblank_scheduler;
 
 	/// Render statistics
 	struct render_statistics render_stats;
@@ -245,8 +236,18 @@ typedef struct session {
 	options_t o;
 	/// Whether we have hit unredirection timeout.
 	bool tmout_unredir_hit;
-	/// Whether we need to redraw the screen
-	bool redraw_needed;
+	/// If the backend is busy. This means two things:
+	/// Either the backend is currently rendering a frame, or a frame has been
+	/// rendered but has yet to be presented. In either case, we should not start
+	/// another render right now. As if we start issuing rendering commands now, we
+	/// will have to wait for either the the current render to finish, or the current
+	/// back buffer to be become available again. In either case, we will be wasting
+	/// time.
+	bool backend_busy;
+	/// Whether a render is queued. This generally means there are pending updates
+	/// to the screen that's neither included in the current render, nor on the
+	/// screen.
+	bool render_queued;
 
 	/// Cache a xfixes region so we don't need to allocate it every time.
 	/// A workaround for yshui/picom#301
diff --git a/src/config.c b/src/config.c
index f5764c5c..22774f7f 100644
--- a/src/config.c
+++ b/src/config.c
@@ -8,6 +8,8 @@
 #include <limits.h>
 #include <math.h>
 #include <stdbool.h>
+#include <stddef.h>
+#include <stdint.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
@@ -617,6 +619,82 @@ char *locate_auxiliary_file(const char *scope, const char *path, const char *inc
 	return NULL;
 }
 
+struct debug_options_entry {
+	const char *name;
+	const char **choices;
+	size_t offset;
+};
+
+// clang-format off
+const char *vblank_scheduler_str[] = {
+	[VBLANK_SCHEDULER_PRESENT] = "present",
+	[VBLANK_SCHEDULER_SGI_VIDEO_SYNC] = "sgi_video_sync",
+	[LAST_VBLANK_SCHEDULER] = NULL
+};
+static const struct debug_options_entry debug_options_entries[] = {
+    {"smart_frame_pacing", NULL,                 offsetof(struct debug_options, smart_frame_pacing)},
+    {"force_vblank_sched", vblank_scheduler_str, offsetof(struct debug_options, force_vblank_scheduler)},
+};
+// clang-format on
+
+void parse_debug_option_single(char *setting, struct debug_options *debug_options) {
+	char *equal = strchr(setting, '=');
+	size_t name_len = equal ? (size_t)(equal - setting) : strlen(setting);
+	for (size_t i = 0; i < ARR_SIZE(debug_options_entries); i++) {
+		if (strncmp(setting, debug_options_entries[i].name, name_len) != 0) {
+			continue;
+		}
+		if (debug_options_entries[i].name[name_len] != '\0') {
+			continue;
+		}
+		auto value = (int *)((void *)debug_options + debug_options_entries[i].offset);
+		if (equal) {
+			const char *const arg = equal + 1;
+			if (debug_options_entries[i].choices != NULL) {
+				for (size_t j = 0; debug_options_entries[i].choices[j]; j++) {
+					if (strcmp(arg, debug_options_entries[i].choices[j]) ==
+					    0) {
+						*value = (int)j;
+						return;
+					}
+				}
+			}
+			if (!parse_int(arg, value)) {
+				log_error("Invalid value for debug option %s: %s, it "
+				          "will be ignored.",
+				          debug_options_entries[i].name, arg);
+			}
+		} else if (debug_options_entries[i].choices == NULL) {
+			*value = 1;
+		} else {
+			log_error(
+			    "Missing value for debug option %s, it will be ignored.", setting);
+		}
+		return;
+	}
+	log_error("Invalid debug option: %s", setting);
+}
+
+/// Parse debug options from environment variable `PICOM_DEBUG`.
+void parse_debug_options(struct debug_options *debug_options) {
+	const char *debug = getenv("PICOM_DEBUG");
+	const struct debug_options default_debug_options = {
+	    .force_vblank_scheduler = LAST_VBLANK_SCHEDULER,
+	};
+
+	*debug_options = default_debug_options;
+	if (!debug) {
+		return;
+	}
+
+	scoped_charp debug_copy = strdup(debug);
+	char *tmp, *needle = strtok_r(debug_copy, ";", &tmp);
+	while (needle) {
+		parse_debug_option_single(needle, debug_options);
+		needle = strtok_r(NULL, ";", &tmp);
+	}
+}
+
 /**
  * Parse a list of window shader rules.
  */
@@ -819,5 +897,6 @@ char *parse_config(options_t *opt, const char *config_file, bool *shadow_enable,
 	(void)hasneg;
 	(void)winopt_mask;
 #endif
+	parse_debug_options(&opt->debug_options);
 	return ret;
 }
diff --git a/src/config.h b/src/config.h
index 31e6774c..d91dbb45 100644
--- a/src/config.h
+++ b/src/config.h
@@ -73,6 +73,27 @@ enum blur_method {
 
 typedef struct _c2_lptr c2_lptr_t;
 
+enum vblank_scheduler_type {
+	/// X Present extension based vblank events
+	VBLANK_SCHEDULER_PRESENT,
+	/// GLX_SGI_video_sync based vblank events
+	VBLANK_SCHEDULER_SGI_VIDEO_SYNC,
+	/// An invalid scheduler, served as a scheduler count, and
+	/// as a sentinel value.
+	LAST_VBLANK_SCHEDULER,
+};
+
+extern const char *vblank_scheduler_str[];
+
+/// Internal, private options for debugging and development use.
+struct debug_options {
+	/// Try to reduce frame latency by using vblank interval and render time
+	/// estimates. Right now it's not working well across drivers.
+	int smart_frame_pacing;
+	/// Override the vblank scheduler chosen by the compositor.
+	int force_vblank_scheduler;
+};
+
 /// Structure representing all options.
 typedef struct options {
 	// === Debugging ===
@@ -262,6 +283,8 @@ typedef struct options {
 	c2_lptr_t *transparent_clipping_blacklist;
 
 	bool dithered_present;
+
+	struct debug_options debug_options;
 } options_t;
 
 extern const char *const BACKEND_STRS[NUM_BKEND + 1];
diff --git a/src/event.c b/src/event.c
index 807cf4ed..739540dc 100644
--- a/src/event.c
+++ b/src/event.c
@@ -716,7 +716,8 @@ void ev_handle(session_t *ps, xcb_generic_event_t *ev) {
 	// XXX redraw needs to be more fine grained
 	queue_redraw(ps);
 
-	// the events sent from SendEvent will be ignored
+	// We intentionally ignore events sent via SendEvent. Those events has the 8th bit
+	// of response_type set, meaning they will match none of the cases below.
 	switch (ev->response_type) {
 	case FocusIn: ev_focus_in(ps, (xcb_focus_in_event_t *)ev); break;
 	case FocusOut: ev_focus_out(ps, (xcb_focus_out_event_t *)ev); break;
diff --git a/src/meson.build b/src/meson.build
index a608c575..a2f9c36c 100644
--- a/src/meson.build
+++ b/src/meson.build
@@ -9,7 +9,8 @@ base_deps = [
 
 srcs = [ files('picom.c', 'win.c', 'c2.c', 'x.c', 'config.c', 'vsync.c', 'utils.c',
                'diagnostic.c', 'string_utils.c', 'render.c', 'kernel.c', 'log.c',
-               'options.c', 'event.c', 'cache.c', 'atom.c', 'file_watch.c', 'statistics.c') ]
+               'options.c', 'event.c', 'cache.c', 'atom.c', 'file_watch.c', 'statistics.c',
+               'vblank.c') ]
 picom_inc = include_directories('.')
 
 cflags = []
@@ -58,7 +59,7 @@ endif
 
 if get_option('opengl')
 	cflags += ['-DCONFIG_OPENGL', '-DGL_GLEXT_PROTOTYPES']
-	deps += [dependency('gl', required: true), dependency('egl', required: true)]
+	deps += [dependency('gl', required: true), dependency('egl', required: true), dependency('threads', required:true)]
 	srcs += [ 'opengl.c' ]
 endif
 
diff --git a/src/picom.c b/src/picom.c
index 6f3c2908..044a9eb1 100644
--- a/src/picom.c
+++ b/src/picom.c
@@ -16,11 +16,13 @@
 #include <errno.h>
 #include <fcntl.h>
 #include <inttypes.h>
+#include <math.h>
 #include <sched.h>
 #include <stddef.h>
 #include <stdio.h>
 #include <string.h>
 #include <sys/resource.h>
+#include <time.h>
 #include <unistd.h>
 #include <xcb/composite.h>
 #include <xcb/damage.h>
@@ -64,6 +66,7 @@
 #include "options.h"
 #include "statistics.h"
 #include "uthash_extra.h"
+#include "vblank.h"
 
 /// Get session_t pointer from a pointer to a member of session_t
 #define session_ptr(ptr, member)                                                         \
@@ -122,27 +125,6 @@ static inline int64_t get_time_ms(void) {
 	return (int64_t)tp.tv_sec * 1000 + (int64_t)tp.tv_nsec / 1000000;
 }
 
-static inline bool dpms_screen_is_off(xcb_dpms_info_reply_t *info) {
-	// state is a bool indicating whether dpms is enabled
-	return info->state && (info->power_level != XCB_DPMS_DPMS_MODE_ON);
-}
-
-void check_dpms_status(EV_P attr_unused, ev_timer *w, int revents attr_unused) {
-	auto ps = session_ptr(w, dpms_check_timer);
-	auto r = xcb_dpms_info_reply(ps->c.c, xcb_dpms_info(ps->c.c), NULL);
-	if (!r) {
-		log_fatal("Failed to query DPMS status.");
-		abort();
-	}
-	auto now_screen_is_off = dpms_screen_is_off(r);
-	if (ps->screen_is_off != now_screen_is_off) {
-		log_debug("Screen is now %s", now_screen_is_off ? "off" : "on");
-		ps->screen_is_off = now_screen_is_off;
-		queue_redraw(ps);
-	}
-	free(r);
-}
-
 /**
  * Find matched window.
  *
@@ -163,122 +145,248 @@ static inline struct managed_win *find_win_all(session_t *ps, const xcb_window_t
 	return w;
 }
 
+enum vblank_callback_action check_render_finish(struct vblank_event *e attr_unused, void *ud) {
+	auto ps = (session_t *)ud;
+	if (!ps->backend_busy) {
+		return VBLANK_CALLBACK_DONE;
+	}
+
+	struct timespec render_time;
+	bool completed =
+	    ps->backend_data->ops->last_render_time(ps->backend_data, &render_time);
+	if (!completed) {
+		// Render hasn't completed yet, we can't start another render.
+		// Check again at the next vblank.
+		log_debug("Last render did not complete during vblank, msc: "
+		          "%" PRIu64,
+		          ps->last_msc);
+		return VBLANK_CALLBACK_AGAIN;
+	}
+
+	// The frame has been finished and presented, record its render time.
+	if (ps->o.debug_options.smart_frame_pacing) {
+		int render_time_us =
+		    (int)(render_time.tv_sec * 1000000L + render_time.tv_nsec / 1000L);
+		render_statistics_add_render_time_sample(
+		    &ps->render_stats, render_time_us + (int)ps->last_schedule_delay);
+		log_verbose("Last render call took: %d (gpu) + %d (cpu) us, "
+		            "last_msc: %" PRIu64,
+		            render_time_us, (int)ps->last_schedule_delay, ps->last_msc);
+	}
+	ps->backend_busy = false;
+	return VBLANK_CALLBACK_DONE;
+}
+
+enum vblank_callback_action
+collect_vblank_interval_statistics(struct vblank_event *e, void *ud) {
+	auto ps = (session_t *)ud;
+	double vblank_interval = NAN;
+	assert(ps->frame_pacing);
+	assert(ps->vblank_scheduler);
+
+	if (!ps->o.debug_options.smart_frame_pacing) {
+		// We don't need to collect statistics if we are not doing smart frame
+		// pacing.
+		return VBLANK_CALLBACK_DONE;
+	}
+
+	// TODO(yshui): this naive method of estimating vblank interval does not handle
+	//              the variable refresh rate case very well. This includes the case
+	//              of a VRR enabled monitor; or a monitor that's turned off, in which
+	//              case the vblank events might slow down or stop all together.
+	//              I tried using DPMS to detect monitor power state, and stop adding
+	//              samples when the monitor is off, but I had a hard time to get it
+	//              working reliably, there are just too many corner cases.
+
+	// Don't add sample again if we already collected statistics for this vblank
+	if (ps->last_msc < e->msc) {
+		if (ps->last_msc_instant != 0) {
+			auto frame_count = e->msc - ps->last_msc;
+			auto frame_time =
+			    (int)((e->ust - ps->last_msc_instant) / frame_count);
+			if (frame_count == 1) {
+				render_statistics_add_vblank_time_sample(
+				    &ps->render_stats, frame_time);
+				log_trace("Frame count %lu, frame time: %d us, ust: "
+				          "%" PRIu64 "",
+				          frame_count, frame_time, e->ust);
+			} else {
+				log_trace("Frame count %lu, frame time: %d us, msc: "
+				          "%" PRIu64 ", not adding sample.",
+				          frame_count, frame_time, e->ust);
+			}
+		}
+		ps->last_msc_instant = e->ust;
+		ps->last_msc = e->msc;
+	} else if (ps->last_msc > e->msc) {
+		log_warn("PresentCompleteNotify msc is going backwards, last_msc: "
+		         "%" PRIu64 ", current msc: %" PRIu64,
+		         ps->last_msc, e->msc);
+		ps->last_msc_instant = 0;
+		ps->last_msc = 0;
+	}
+
+	vblank_interval = render_statistics_get_vblank_time(&ps->render_stats);
+	log_trace("Vblank interval estimate: %f us", vblank_interval);
+	if (vblank_interval == 0) {
+		// We don't have enough data for vblank interval estimate, schedule
+		// another vblank event.
+		return VBLANK_CALLBACK_AGAIN;
+	}
+	return VBLANK_CALLBACK_DONE;
+}
+
+void schedule_render(session_t *ps, bool triggered_by_vblank);
+
+/// vblank callback scheduled by schedule_render, when a render is ongoing.
+///
+/// Check if previously queued render has finished, and reschedule render if it has.
+enum vblank_callback_action reschedule_render_at_vblank(struct vblank_event *e, void *ud) {
+	auto ps = (session_t *)ud;
+	assert(ps->frame_pacing);
+	assert(ps->render_queued);
+	assert(ps->vblank_scheduler);
+
+	log_verbose("Rescheduling render at vblank, msc: %" PRIu64, e->msc);
+
+	collect_vblank_interval_statistics(e, ud);
+	check_render_finish(e, ud);
+
+	if (ps->backend_busy) {
+		return VBLANK_CALLBACK_AGAIN;
+	}
+
+	schedule_render(ps, false);
+	return VBLANK_CALLBACK_DONE;
+}
+
 /// How many seconds into the future should we start rendering the next frame.
 ///
 /// Renders are scheduled like this:
 ///
-/// 1. queue_redraw() registers the intention to render. redraw_needed is set to true to
-///    indicate what is on screen needs to be updated.
-/// 2. then, we need to figure out the best time to start rendering. first, we need to
-///    know when the next frame will be displayed on screen. we have this information from
-///    the Present extension: we know when was the last frame displayed, and we know the
-///    refresh rate. so we can calculate the next frame's display time. if our render time
-///    estimation shows we could miss that target, we push the target back one frame.
-/// 3. if there is already render completed for that target frame, or there is a render
-///    currently underway, we don't do anything, and wait for the next Present Complete
-///    Notify event to try to schedule again.
-/// 4. otherwise, we schedule a render for that target frame. we use past statistics about
-///    how long our renders took to figure out when to start rendering. we start rendering
-///    at the latest point of time possible to still hit the target frame.
+/// 1. queue_redraw() queues a new render by calling schedule_render, if there
+///    is no render currently scheduled. i.e. render_queued == false.
+/// 2. then, we need to figure out the best time to start rendering. we need to
+///    at least know when the next vblank will start, as we can't start render
+///    before the current rendered frame is diplayed on screen. we have this
+///    information from the vblank scheduler, it will notify us when that happens.
+///    we might also want to delay the rendering even further to reduce latency,
+///    this is discussed below, in FUTURE WORKS.
+/// 3. we schedule a render for that target point in time.
+/// 4. draw_callback() is called at the schedule time (i.e. when scheduled
+///    vblank event is delivered). Backend APIs are called to issue render
+///    commands. render_queued is set to false, and backend_busy is set to true.
 ///
-/// The `triggered_by_timer` parameter is used to indicate whether this function is
-/// triggered by a steady timer, i.e. we are rendering for each vblank. The other case is
-/// when we stop rendering for a while because there is no changes on screen, then
-/// something changed and schedule_render is triggered by a DamageNotify. The idea is that
-/// when the schedule is triggered by a steady timer, schedule_render will be called at a
-/// predictable offset into each vblank.
+/// There are some considerations in step 2:
+///
+/// First of all, a vblank event being delivered
+/// doesn't necessarily mean the frame has been displayed on screen. If a frame
+/// takes too long to render, it might miss the current vblank, and will be
+/// displayed on screen during one of the subsequent vblanks. So in
+/// schedule_render_at_vblank, we ask the backend to see if it has finished
+/// rendering. if not, render_queued is unchanged, and another vblank is
+/// scheduled; otherwise, draw_callback_impl will be scheduled to be call at
+/// an appropriate time. Second, we might not have rendered for the previous vblank,
+/// in which case the last vblank event we received could be many frames in the past,
+/// so we can't make scheduling decisions based on that. So we always schedule
+/// a vblank event when render is queued, and make scheduling decisions when the
+/// event is delivered.
+///
+/// All of the above is what happens when frame_pacing is true. Otherwise
+/// render_in_progress is either QUEUED or IDLE, and queue_redraw will always
+/// schedule a render to be started immediately. PresentCompleteNotify will not
+/// be received, and handle_end_of_vblank will not be called.
+///
+/// The `triggered_by_timer` parameter is used to indicate whether this function
+/// is triggered by a steady timer, i.e. we are rendering for each vblank. The
+/// other case is when we stop rendering for a while because there is no changes
+/// on screen, then something changed and schedule_render is triggered by a
+/// DamageNotify. The idea is that when the schedule is triggered by a steady
+/// timer, schedule_render will be called at a predictable offset into each
+/// vblank.
+///
+/// # FUTURE WORKS
+///
+/// As discussed in step 2 above, we might want to delay the rendering even
+/// further. If we know the time it takes to render a frame, and the interval
+/// between vblanks, we can try to schedule the render to start at a point in
+/// time that's closer to the next vblank. We should be able to get this
+/// information by doing statistics on the render time of previous frames, which
+/// is available from the backends; and the interval between vblank events,
+/// which is available from the vblank scheduler.
+///
+/// The code that does this is already implemented below, but disabled by
+/// default. There are several problems with it, see bug #1072.
+void schedule_render(session_t *ps, bool triggered_by_vblank attr_unused) {
+	// If the backend is busy, we will try again at the next vblank.
+	if (ps->backend_busy) {
+		// We should never have set backend_busy to true unless frame_pacing is
+		// enabled.
+		assert(ps->vblank_scheduler);
+		assert(ps->frame_pacing);
+		log_verbose("Backend busy, will reschedule render at next vblank.");
+		if (!vblank_scheduler_schedule(ps->vblank_scheduler,
+		                               reschedule_render_at_vblank, ps)) {
+			// TODO(yshui): handle error here
+			abort();
+		}
+		return;
+	}
 
-void schedule_render(session_t *ps, bool triggered_by_vblank) {
+	// By default, we want to schedule render immediately, later in this function we
+	// might adjust that and move the render later, based on render timing statistics.
 	double delay_s = 0;
-	ps->next_render = 0;
-	if (!ps->frame_pacing || !ps->redirected) {
-		// Not doing frame pacing, schedule a render immediately, if not already
-		// scheduled.
-		// If not redirected, we schedule immediately to have a chance to
-		// redirect. We won't have frame or render timing information anyway.
-		if (!ev_is_active(&ps->draw_timer)) {
-			// We don't know the msc, so we set it to 1, because 0 is a
-			// special value
-			ps->target_msc = 1;
-			goto schedule;
-		}
-		return;
-	}
-	struct timespec render_time;
-	bool completed =
-	    ps->backend_data->ops->last_render_time(ps->backend_data, &render_time);
-	if (!completed || ev_is_active(&ps->draw_timer)) {
-		// There is already a render underway (either just scheduled, or is
-		// rendered but awaiting completion), don't schedule another one.
-		if (ps->target_msc <= ps->last_msc) {
-			log_debug("Target frame %ld is in the past, but we are still "
-			          "rendering",
-			          ps->target_msc);
-			// We missed our target, push it back one frame
-			ps->target_msc = ps->last_msc + 1;
-		}
-		log_trace("Still rendering for target frame %ld, not scheduling another "
-		          "render",
-		          ps->target_msc);
-		return;
-	}
-	if (ps->target_msc > ps->last_msc) {
-		// Render for the target frame is completed, but is yet to be displayed.
-		// Don't schedule another render.
-		log_trace("Target frame %ld is in the future, and we have already "
-		          "rendered, last msc: %d",
-		          ps->target_msc, (int)ps->last_msc);
-		return;
-	}
-
+	unsigned int divisor = 0;
 	struct timespec now;
 	clock_gettime(CLOCK_MONOTONIC, &now);
 	auto now_us = (uint64_t)now.tv_sec * 1000000 + (uint64_t)now.tv_nsec / 1000;
-	if (triggered_by_vblank) {
-		log_trace("vblank schedule delay: %ld us", now_us - ps->last_msc_instant);
+
+	ps->next_render = now_us;
+
+	if (!ps->frame_pacing || !ps->redirected) {
+		// If not doing frame pacing, schedule a render immediately; if
+		// not redirected, we schedule immediately to have a chance to
+		// redirect. We won't have frame or render timing information
+		// anyway.
+		assert(!ev_is_active(&ps->draw_timer));
+		goto schedule;
 	}
 
-	int render_time_us =
-	    (int)(render_time.tv_sec * 1000000L + render_time.tv_nsec / 1000L);
-	if (ps->target_msc == ps->last_msc) {
-		// The frame has just been displayed, record its render time;
-		if (ps->did_render) {
-			log_trace("Last render call took: %d (gpu) + %d (cpu) us, "
-			          "last_msc: %" PRIu64,
-			          render_time_us, (int)ps->last_schedule_delay, ps->last_msc);
-			render_statistics_add_render_time_sample(
-			    &ps->render_stats, render_time_us + (int)ps->last_schedule_delay);
-		}
-		ps->target_msc = 0;
-		ps->did_render = false;
-		ps->last_schedule_delay = 0;
-	}
-
-	unsigned int divisor = 0;
-	auto render_budget = render_statistics_get_budget(&ps->render_stats, &divisor);
+	// if ps->o.debug_options.smart_frame_pacing is false, we won't have any render
+	// time or vblank interval estimates, so we would naturally fallback to schedule
+	// render immediately.
+	auto render_budget = render_statistics_get_budget(&ps->render_stats);
 	auto frame_time = render_statistics_get_vblank_time(&ps->render_stats);
 	if (frame_time == 0) {
 		// We don't have enough data for render time estimates, maybe there's
 		// no frame rendered yet, or the backend doesn't support render timing
 		// information, schedule render immediately.
-		ps->target_msc = ps->last_msc + 1;
+		log_verbose("Not enough data for render time estimates.");
 		goto schedule;
 	}
 
-	auto const deadline = ps->last_msc_instant + (unsigned long)divisor * frame_time;
+	if (render_budget >= frame_time) {
+		// If the estimated render time is already longer than the estimated
+		// vblank interval, there is no way we can make it. Instead of always
+		// dropping frames, we try desperately to catch up and schedule a
+		// render immediately.
+		log_verbose("Render budget: %u us >= frame time: %" PRIu32 " us",
+		            render_budget, frame_time);
+		goto schedule;
+	}
+
+	auto target_frame = (now_us + render_budget - ps->last_msc_instant) / frame_time + 1;
+	auto const deadline = ps->last_msc_instant + target_frame * frame_time;
 	unsigned int available = 0;
 	if (deadline > now_us) {
 		available = (unsigned int)(deadline - now_us);
 	}
 
-	ps->target_msc = ps->last_msc + divisor;
 	if (available > render_budget) {
 		delay_s = (double)(available - render_budget) / 1000000.0;
 		ps->next_render = deadline - render_budget;
-	} else {
-		delay_s = 0;
-		ps->next_render = now_us;
 	}
+
 	if (delay_s > 1) {
 		log_warn("Delay too long: %f s, render_budget: %d us, frame_time: "
 		         "%" PRIu32 " us, now_us: %" PRIu64 " us, next_msc: %" PRIu64 " u"
@@ -286,38 +394,32 @@ void schedule_render(session_t *ps, bool triggered_by_vblank) {
 		         delay_s, render_budget, frame_time, now_us, deadline);
 	}
 
-	log_trace("Delay: %.6lf s, last_msc: %" PRIu64 ", render_budget: %d, frame_time: "
-	          "%" PRIu32 ", now_us: %" PRIu64 ", next_msc: %" PRIu64 ", "
-	          "target_msc: %" PRIu64 ", divisor: %d",
-	          delay_s, ps->last_msc_instant, render_budget, frame_time, now_us,
-	          deadline, ps->target_msc, divisor);
+	log_verbose("Delay: %.6lf s, last_msc: %" PRIu64 ", render_budget: %d, "
+	            "frame_time: %" PRIu32 ", now_us: %" PRIu64 ", next_render: %" PRIu64
+	            ", next_msc: %" PRIu64 ", divisor: "
+	            "%d",
+	            delay_s, ps->last_msc_instant, render_budget, frame_time, now_us,
+	            ps->next_render, deadline, divisor);
 
 schedule:
+	// If the backend is not busy, we just need to schedule the render at the
+	// specified time; otherwise we need to wait for the next vblank event and
+	// reschedule.
+	ps->last_schedule_delay = 0;
 	assert(!ev_is_active(&ps->draw_timer));
 	ev_timer_set(&ps->draw_timer, delay_s, 0);
 	ev_timer_start(ps->loop, &ps->draw_timer);
 }
 
 void queue_redraw(session_t *ps) {
-	if (ps->screen_is_off) {
-		// The screen is off, if there is a draw queued for the next frame (i.e.
-		// ps->redraw_needed == true), it won't be triggered until the screen is
-		// on again, because the abnormal Present events we will receive from the
-		// X server when the screen is off. Yet we need the draw_callback to be
-		// called as soon as possible so the screen can be unredirected.
-		// So here we unconditionally start the draw timer.
-		ev_timer_stop(ps->loop, &ps->draw_timer);
-		ev_timer_set(&ps->draw_timer, 0, 0);
-		ev_timer_start(ps->loop, &ps->draw_timer);
+	log_verbose("Queue redraw, render_queued: %d, backend_busy: %d",
+	            ps->render_queued, ps->backend_busy);
+
+	if (ps->render_queued) {
 		return;
 	}
-	// Whether we have already rendered for the current frame.
-	// If frame pacing is not enabled, pretend this is false.
-	// If --benchmark is used, redraw is always queued
-	if (!ps->redraw_needed && !ps->o.benchmark) {
-		schedule_render(ps, false);
-	}
-	ps->redraw_needed = true;
+	ps->render_queued = true;
+	schedule_render(ps, false);
 }
 
 /**
@@ -1013,19 +1115,6 @@ static bool paint_preprocess(session_t *ps, bool *fade_running, bool *animation,
 		// If there's no window to paint, and the screen isn't redirected,
 		// don't redirect it.
 		unredir_possible = true;
-	} else if (ps->screen_is_off) {
-		// Screen is off, unredirect
-		// We do this unconditionally disregarding "unredir_if_possible"
-		// because it's important for correctness, because we need to
-		// workaround problems X server has around screen off.
-		//
-		// Known problems:
-		//   1. Sometimes OpenGL front buffer can lose content, and if we
-		//      are doing partial updates (i.e. use-damage = true), the
-		//      result will be wrong.
-		//   2. For frame pacing, X server sends bogus
-		//      PresentCompleteNotify events when screen is off.
-		unredir_possible = true;
 	}
 	if (unredir_possible) {
 		if (ps->redirected) {
@@ -1397,7 +1486,7 @@ static bool redirect_start(session_t *ps) {
 		pixman_region32_init(&ps->damage_ring[i]);
 	}
 
-	ps->frame_pacing = !ps->o.no_frame_pacing;
+	ps->frame_pacing = !ps->o.no_frame_pacing && ps->o.vsync;
 	if ((ps->o.legacy_backends || ps->o.benchmark || !ps->backend_data->ops->last_render_time) &&
 	    ps->frame_pacing) {
 		// Disable frame pacing if we are using a legacy backend or if we are in
@@ -1406,25 +1495,31 @@ static bool redirect_start(session_t *ps) {
 		ps->frame_pacing = false;
 	}
 
-	if (ps->present_exists && ps->frame_pacing) {
-		ps->present_event_id = x_new_id(&ps->c);
-		auto select_input = xcb_present_select_input(
-		    ps->c.c, ps->present_event_id, session_get_target_window(ps),
-		    XCB_PRESENT_EVENT_MASK_COMPLETE_NOTIFY);
-		auto notify_msc = xcb_present_notify_msc(
-		    ps->c.c, session_get_target_window(ps), 0, 0, 1, 0);
-		set_cant_fail_cookie(&ps->c, select_input);
-		set_cant_fail_cookie(&ps->c, notify_msc);
-		ps->present_event = xcb_register_for_special_xge(
-		    ps->c.c, &xcb_present_id, ps->present_event_id, NULL);
+	// Re-detect driver since we now have a backend
+	ps->drivers = detect_driver(ps->c.c, ps->backend_data, ps->c.screen_info->root);
+	apply_driver_workarounds(ps, ps->drivers);
 
+	if (ps->present_exists && ps->frame_pacing) {
 		// Initialize rendering and frame timing statistics, and frame pacing
 		// states.
 		ps->last_msc_instant = 0;
 		ps->last_msc = 0;
 		ps->last_schedule_delay = 0;
-		ps->target_msc = 0;
 		render_statistics_reset(&ps->render_stats);
+		enum vblank_scheduler_type scheduler_type =
+		    choose_vblank_scheduler(ps->drivers);
+		if (ps->o.debug_options.force_vblank_scheduler != LAST_VBLANK_SCHEDULER) {
+			scheduler_type =
+			    (enum vblank_scheduler_type)ps->o.debug_options.force_vblank_scheduler;
+		}
+		log_info("Using vblank scheduler: %s.", vblank_scheduler_str[scheduler_type]);
+		ps->vblank_scheduler = vblank_scheduler_new(
+		    ps->loop, &ps->c, session_get_target_window(ps), scheduler_type);
+		if (!ps->vblank_scheduler) {
+			return false;
+		}
+		vblank_scheduler_schedule(ps->vblank_scheduler,
+		                          collect_vblank_interval_statistics, ps);
 	} else if (ps->frame_pacing) {
 		log_error("Present extension is not supported, frame pacing disabled.");
 		ps->frame_pacing = false;
@@ -1436,10 +1531,6 @@ static bool redirect_start(session_t *ps) {
 	ps->redirected = true;
 	ps->first_frame = true;
 
-	// Re-detect driver since we now have a backend
-	ps->drivers = detect_driver(ps->c.c, ps->backend_data, ps->c.screen_info->root);
-	apply_driver_workarounds(ps, ps->drivers);
-
 	root_damaged(ps);
 
 	// Repaint the whole screen
@@ -1472,12 +1563,9 @@ static void unredirect(session_t *ps) {
 	free(ps->damage_ring);
 	ps->damage_ring = ps->damage = NULL;
 
-	if (ps->present_event_id) {
-		xcb_present_select_input(ps->c.c, ps->present_event_id,
-		                         session_get_target_window(ps), 0);
-		ps->present_event_id = XCB_NONE;
-		xcb_unregister_for_special_event(ps->c.c, ps->present_event);
-		ps->present_event = NULL;
+	if (ps->vblank_scheduler) {
+		vblank_scheduler_free(ps->vblank_scheduler);
+		ps->vblank_scheduler = NULL;
 	}
 
 	// Must call XSync() here
@@ -1487,92 +1575,12 @@ static void unredirect(session_t *ps) {
 	log_debug("Screen unredirected.");
 }
 
-static void
-handle_present_complete_notify(session_t *ps, xcb_present_complete_notify_event_t *cne) {
-	if (cne->kind != XCB_PRESENT_COMPLETE_KIND_NOTIFY_MSC) {
-		return;
-	}
-
-	bool event_is_invalid = false;
-	if (ps->frame_pacing) {
-		auto next_msc = cne->msc + 1;
-		if (cne->msc <= ps->last_msc || cne->ust == 0) {
-			// X sometimes sends duplicate/bogus MSC events, don't
-			// use the msc value. Also ignore these events.
-			//
-			// See:
-			// https://gitlab.freedesktop.org/xorg/xserver/-/issues/1418
-			next_msc = ps->last_msc + 1;
-			event_is_invalid = true;
-		}
-		auto cookie = xcb_present_notify_msc(
-		    ps->c.c, session_get_target_window(ps), 0, next_msc, 0, 0);
-		set_cant_fail_cookie(&ps->c, cookie);
-	}
-	if (event_is_invalid) {
-		return;
-	}
-
-	struct timespec now;
-	clock_gettime(CLOCK_MONOTONIC, &now);
-	uint64_t now_usec = (uint64_t)(now.tv_sec * 1000000 + now.tv_nsec / 1000);
-	uint64_t drift;
-	if (cne->ust > now_usec) {
-		drift = cne->ust - now_usec;
-	} else {
-		drift = now_usec - cne->ust;
-	}
-
-	if (ps->last_msc_instant != 0) {
-		auto frame_count = cne->msc - ps->last_msc;
-		int frame_time = (int)((cne->ust - ps->last_msc_instant) / frame_count);
-		render_statistics_add_vblank_time_sample(&ps->render_stats, frame_time);
-		log_trace("Frame count %lu, frame time: %d us, rolling average: %u us, "
-		          "msc: %" PRIu64 ", offset: %d us",
-		          frame_count, frame_time,
-		          render_statistics_get_vblank_time(&ps->render_stats), cne->ust,
-		          (int)drift);
-	} else if (drift > 1000000 && ps->frame_pacing) {
-		// This is the first MSC event we receive, let's check if the timestamps
-		// align with the monotonic clock. If not, disable frame pacing because we
-		// can't schedule frames reliably.
-		log_error("Temporal anomaly detected, frame pacing disabled. (Are we "
-		          "running inside a time namespace?), %" PRIu64 " %" PRIu64,
-		          now_usec, ps->last_msc_instant);
-		ps->frame_pacing = false;
-	}
-	ps->last_msc_instant = cne->ust;
-	ps->last_msc = cne->msc;
-	if (ps->redraw_needed) {
-		schedule_render(ps, true);
-	}
-}
-
-static void handle_present_events(session_t *ps) {
-	if (!ps->present_event) {
-		// Screen not redirected
-		return;
-	}
-	xcb_present_generic_event_t *ev;
-	while ((ev = (void *)xcb_poll_for_special_event(ps->c.c, ps->present_event))) {
-		if (ev->event != ps->present_event_id) {
-			// This event doesn't have the right event context, it's not meant
-			// for us.
-			goto next;
-		}
-
-		// We only subscribed to the complete notify event.
-		assert(ev->evtype == XCB_PRESENT_EVENT_COMPLETE_NOTIFY);
-		handle_present_complete_notify(ps, (void *)ev);
-	next:
-		free(ev);
-	}
-}
-
 // Handle queued events before we go to sleep
 static void handle_queued_x_events(EV_P attr_unused, ev_prepare *w, int revents attr_unused) {
 	session_t *ps = session_ptr(w, event_check);
-	handle_present_events(ps);
+	if (ps->vblank_scheduler) {
+		vblank_handle_x_events(ps->vblank_scheduler);
+	}
 
 	xcb_generic_event_t *ev;
 	while ((ev = xcb_poll_for_queued_event(ps->c.c))) {
@@ -1694,6 +1702,9 @@ static void handle_pending_updates(EV_P_ struct session *ps) {
 }
 
 static void draw_callback_impl(EV_P_ session_t *ps, int revents attr_unused) {
+	assert(!ps->backend_busy);
+	assert(ps->render_queued);
+
 	struct timespec now;
 	int64_t draw_callback_enter_us;
 	clock_gettime(CLOCK_MONOTONIC, &now);
@@ -1779,17 +1790,18 @@ static void draw_callback_impl(EV_P_ session_t *ps, int revents attr_unused) {
 	log_trace("paint_preprocess took: %" PRIi64 " us",
 	          after_preprocess_us - after_handle_pending_updates_us);
 
-	// If the screen is unredirected, free all_damage to stop painting
+	// If the screen is unredirected, we don't render anything.
+	bool did_render = false;
 	if (ps->redirected && ps->o.stoppaint_force != ON) {
 		static int paint = 0;
 
-		log_trace("Render start, frame %d", paint);
+		log_verbose("Render start, frame %d", paint);
 		if (!ps->o.legacy_backends) {
-			paint_all_new(ps, bottom);
+			did_render = paint_all_new(ps, bottom);
 		} else {
 			paint_all(ps, bottom);
 		}
-		log_trace("Render end");
+		log_verbose("Render end");
 
 		ps->first_frame = false;
 		paint++;
@@ -1798,16 +1810,36 @@ static void draw_callback_impl(EV_P_ session_t *ps, int revents attr_unused) {
 		}
 	}
 
+	// With frame pacing, we set backend_busy to true after the end of
+	// vblank. Without frame pacing, we won't be receiving vblank events, so
+	// we set backend_busy to false here, right after we issue the render
+	// commands.
+	// The other case is if we decided there is no change to render, in that
+	// case no render command is issued, so we also set backend_busy to
+	// false.
+	ps->backend_busy = (ps->frame_pacing && did_render);
+	ps->next_render = 0;
+
 	if (!fade_running) {
 		ps->fade_time = 0L;
 	}
 
+	ps->render_queued = false;
+
 	// TODO(yshui) Investigate how big the X critical section needs to be. There are
 	// suggestions that rendering should be in the critical section as well.
 
 	// Queue redraw if animation is running. This should be picked up by next present
 	// event.
-	ps->redraw_needed = animation;
+	if (animation) {
+		queue_redraw(ps);
+	}
+	if (ps->vblank_scheduler) {
+		// Even if we might not want to render during next vblank, we want to keep
+		// `backend_busy` up to date, so when the next render comes, we can
+		// immediately know if we can render.
+		vblank_scheduler_schedule(ps->vblank_scheduler, check_render_finish, ps);
+	}
 }
 
 static void draw_callback(EV_P_ ev_timer *w, int revents) {
@@ -1974,7 +2006,6 @@ static session_t *session_init(int argc, char **argv, Display *dpy,
 	    .randr_exists = 0,
 	    .randr_event = 0,
 	    .randr_error = 0,
-	    .present_event_id = XCB_NONE,
 	    .glx_exists = false,
 	    .glx_event = 0,
 	    .glx_error = 0,
@@ -2109,17 +2140,8 @@ static session_t *session_init(int argc, char **argv, Display *dpy,
 
 	ext_info = xcb_get_extension_data(ps->c.c, &xcb_dpms_id);
 	ps->dpms_exists = ext_info && ext_info->present;
-	if (ps->dpms_exists) {
-		auto r = xcb_dpms_info_reply(ps->c.c, xcb_dpms_info(ps->c.c), NULL);
-		if (!r) {
-			log_fatal("Failed to query DPMS info");
-			goto err;
-		}
-		ps->screen_is_off = dpms_screen_is_off(r);
-		// Check screen status every half second
-		ev_timer_init(&ps->dpms_check_timer, check_dpms_status, 0, 0.5);
-		ev_timer_start(ps->loop, &ps->dpms_check_timer);
-		free(r);
+	if (!ps->dpms_exists) {
+		log_warn("No DPMS extension");
 	}
 
 	// Parse configuration file
@@ -2727,7 +2749,6 @@ static void session_destroy(session_t *ps) {
 	// Stop libev event handlers
 	ev_timer_stop(ps->loop, &ps->unredir_timer);
 	ev_timer_stop(ps->loop, &ps->fade_timer);
-	ev_timer_stop(ps->loop, &ps->dpms_check_timer);
 	ev_timer_stop(ps->loop, &ps->draw_timer);
 	ev_prepare_stop(ps->loop, &ps->event_check);
 	ev_signal_stop(ps->loop, &ps->usr1_signal);
diff --git a/src/statistics.c b/src/statistics.c
index 5e3d9249..11c7466e 100644
--- a/src/statistics.c
+++ b/src/statistics.c
@@ -55,26 +55,15 @@ void render_statistics_add_render_time_sample(struct render_statistics *rs, int
 /// A `divisor` is also returned, indicating the target framerate. The divisor is
 /// the number of vblanks we should wait between each frame. A divisor of 1 means
 /// full framerate, 2 means half framerate, etc.
-unsigned int
-render_statistics_get_budget(struct render_statistics *rs, unsigned int *divisor) {
+unsigned int render_statistics_get_budget(struct render_statistics *rs) {
 	if (rs->render_times.nelem < rs->render_times.window_size) {
 		// No valid render time estimates yet. Assume maximum budget.
-		*divisor = 1;
 		return UINT_MAX;
 	}
 
 	// N-th percentile of render times, see render_statistics_init for N.
 	auto render_time_percentile =
 	    rolling_quantile_estimate(&rs->render_time_quantile, &rs->render_times);
-	auto vblank_time_us = render_statistics_get_vblank_time(rs);
-	if (vblank_time_us == 0) {
-		// We don't have a good estimate of the vblank time yet, so we
-		// assume we can finish in one vblank.
-		*divisor = 1;
-	} else {
-		*divisor =
-		    (unsigned int)(render_time_percentile / rs->vblank_time_us.mean + 1);
-	}
 	return (unsigned int)render_time_percentile;
 }
 
diff --git a/src/statistics.h b/src/statistics.h
index 67d02119..a1114861 100644
--- a/src/statistics.h
+++ b/src/statistics.h
@@ -23,11 +23,8 @@ void render_statistics_add_vblank_time_sample(struct render_statistics *rs, int
 void render_statistics_add_render_time_sample(struct render_statistics *rs, int time_us);
 
 /// How much time budget we should give to the backend for rendering, in microseconds.
-///
-/// A `divisor` is also returned, indicating the target framerate. The divisor is
-/// the number of vblanks we should wait between each frame. A divisor of 1 means
-/// full framerate, 2 means half framerate, etc.
-unsigned int
-render_statistics_get_budget(struct render_statistics *rs, unsigned int *divisor);
+unsigned int render_statistics_get_budget(struct render_statistics *rs);
 
+/// Return the measured vblank interval in microseconds. Returns 0 if not enough
+/// samples have been collected yet.
 unsigned int render_statistics_get_vblank_time(struct render_statistics *rs);
diff --git a/src/utils.h b/src/utils.h
index fc6dd306..446fba8b 100644
--- a/src/utils.h
+++ b/src/utils.h
@@ -125,14 +125,22 @@ safe_isnan(double a) {
  * @param max maximum value
  * @return normalized value
  */
-static inline int attr_const normalize_i_range(int i, int min, int max) {
-	if (i > max)
+static inline int attr_const attr_unused normalize_i_range(int i, int min, int max) {
+	if (i > max) {
 		return max;
-	if (i < min)
+	}
+	if (i < min) {
 		return min;
+	}
 	return i;
 }
 
+/// Generic integer abs()
+#define iabs(val)                                                                        \
+	({                                                                               \
+		__auto_type __tmp = (val);                                               \
+		__tmp > 0 ? __tmp : -__tmp;                                              \
+	})
 #define min2(a, b) ((a) > (b) ? (b) : (a))
 #define max2(a, b) ((a) > (b) ? (a) : (b))
 #define min3(a, b, c) min2(a, min2(b, c))
@@ -149,10 +157,12 @@ static inline int attr_const normalize_i_range(int i, int min, int max) {
  * @return normalized value
  */
 static inline double attr_const normalize_d_range(double d, double min, double max) {
-	if (d > max)
+	if (d > max) {
 		return max;
-	if (d < min)
+	}
+	if (d < min) {
 		return min;
+	}
 	return d;
 }
 
@@ -162,7 +172,7 @@ static inline double attr_const normalize_d_range(double d, double min, double m
  * @param d double value to normalize
  * @return normalized value
  */
-static inline double attr_const normalize_d(double d) {
+static inline double attr_const attr_unused normalize_d(double d) {
 	return normalize_d_range(d, 0.0, 1.0);
 }
 
diff --git a/src/vblank.c b/src/vblank.c
new file mode 100644
index 00000000..ff83a9a7
--- /dev/null
+++ b/src/vblank.c
@@ -0,0 +1,541 @@
+#include <assert.h>
+
+#include <ev.h>
+#include <inttypes.h>
+#include <stdatomic.h>
+#include <string.h>
+#include <time.h>
+#include <xcb/xcb.h>
+#include <xcb/xproto.h>
+#include "config.h"
+
+#ifdef CONFIG_OPENGL
+// Enable sgi_video_sync_vblank_scheduler
+#include <GL/glx.h>
+#include <X11/X.h>
+#include <X11/Xlib-xcb.h>
+#include <X11/Xlib.h>
+#include <X11/Xutil.h>
+#include <pthread.h>
+
+#include "backend/gl/glx.h"
+#endif
+
+#include "compiler.h"
+#include "list.h"        // for container_of
+#include "log.h"
+#include "vblank.h"
+#include "x.h"
+
+struct vblank_closure {
+	vblank_callback_t fn;
+	void *user_data;
+};
+
+#define VBLANK_WIND_DOWN 4
+
+struct vblank_scheduler {
+	struct x_connection *c;
+	size_t callback_capacity, callback_count;
+	struct vblank_closure *callbacks;
+	struct ev_loop *loop;
+	/// Request extra vblank events even when no callbacks are scheduled.
+	/// This is because when callbacks are scheduled too close to a vblank,
+	/// we might send PresentNotifyMsc request too late and miss the vblank event.
+	/// So we request extra vblank events right after the last vblank event
+	/// to make sure this doesn't happen.
+	unsigned int wind_down;
+	xcb_window_t target_window;
+	enum vblank_scheduler_type type;
+	bool vblank_event_requested;
+};
+
+struct present_vblank_scheduler {
+	struct vblank_scheduler base;
+
+	uint64_t last_msc;
+	/// The timestamp for the end of last vblank.
+	uint64_t last_ust;
+	ev_timer callback_timer;
+	xcb_present_event_t event_id;
+	xcb_special_event_t *event;
+};
+
+struct vblank_scheduler_ops {
+	size_t size;
+	void (*init)(struct vblank_scheduler *self);
+	void (*deinit)(struct vblank_scheduler *self);
+	void (*schedule)(struct vblank_scheduler *self);
+	bool (*handle_x_events)(struct vblank_scheduler *self);
+};
+
+static void
+vblank_scheduler_invoke_callbacks(struct vblank_scheduler *self, struct vblank_event *event);
+
+#ifdef CONFIG_OPENGL
+struct sgi_video_sync_vblank_scheduler {
+	struct vblank_scheduler base;
+
+	// Since glXWaitVideoSyncSGI blocks, we need to run it in a separate thread.
+	// ... and all the thread shenanigans that come with it.
+	_Atomic unsigned int last_msc;
+	_Atomic uint64_t last_ust;
+	ev_async notify;
+	pthread_t sync_thread;
+	bool running, error;
+
+	/// Protects `running`, `error` and `base.vblank_event_requested`
+	pthread_mutex_t vblank_requested_mtx;
+	pthread_cond_t vblank_requested_cnd;
+};
+
+struct sgi_video_sync_thread_args {
+	struct sgi_video_sync_vblank_scheduler *self;
+	int start_status;
+	pthread_mutex_t start_mtx;
+	pthread_cond_t start_cnd;
+};
+
+static bool check_sgi_video_sync_extension(Display *dpy, int screen) {
+	const char *glx_ext = glXQueryExtensionsString(dpy, screen);
+	const char *needle = "GLX_SGI_video_sync";
+	char *found = strstr(glx_ext, needle);
+	if (!found) {
+		return false;
+	}
+	if (found != glx_ext && found[-1] != ' ') {
+		return false;
+	}
+	if (found[strlen(needle)] != ' ' && found[strlen(needle)] != '\0') {
+		return false;
+	}
+
+	glXWaitVideoSyncSGI = (PFNGLXWAITVIDEOSYNCSGIPROC)(void *)glXGetProcAddress(
+	    (const GLubyte *)"glXWaitVideoSyncSGI");
+	if (!glXWaitVideoSyncSGI) {
+		return false;
+	}
+	return true;
+}
+
+static void *sgi_video_sync_thread(void *data) {
+	auto args = (struct sgi_video_sync_thread_args *)data;
+	auto self = args->self;
+	Display *dpy = XOpenDisplay(NULL);
+	int error_code = 0;
+	if (!dpy) {
+		error_code = 1;
+		goto start_failed;
+	}
+	Window root = DefaultRootWindow(dpy), dummy = None;
+	int screen = DefaultScreen(dpy);
+	int ncfg = 0;
+	GLXFBConfig *cfg_ = glXChooseFBConfig(
+	    dpy, screen,
+	    (int[]){GLX_RENDER_TYPE, GLX_RGBA_BIT, GLX_DRAWABLE_TYPE, GLX_WINDOW_BIT, 0},
+	    &ncfg);
+	GLXContext ctx = NULL;
+	GLXDrawable drawable = None;
+
+	if (!cfg_) {
+		error_code = 2;
+		goto start_failed;
+	}
+	GLXFBConfig cfg = cfg_[0];
+	XFree(cfg_);
+
+	XVisualInfo *vi = glXGetVisualFromFBConfig(dpy, cfg);
+	if (!vi) {
+		error_code = 3;
+		goto start_failed;
+	}
+
+	Visual *visual = vi->visual;
+	const int depth = vi->depth;
+	XFree(vi);
+
+	Colormap colormap = XCreateColormap(dpy, root, visual, AllocNone);
+	XSetWindowAttributes attributes;
+	attributes.colormap = colormap;
+
+	dummy = XCreateWindow(dpy, root, 0, 0, 1, 1, 0, depth, InputOutput, visual,
+	                      CWColormap, &attributes);
+	XFreeColormap(dpy, colormap);
+	if (dummy == None) {
+		error_code = 4;
+		goto start_failed;
+	}
+
+	drawable = glXCreateWindow(dpy, cfg, dummy, NULL);
+	if (drawable == None) {
+		error_code = 5;
+		goto start_failed;
+	}
+
+	ctx = glXCreateNewContext(dpy, cfg, GLX_RGBA_TYPE, 0, true);
+	if (ctx == NULL) {
+		error_code = 6;
+		goto start_failed;
+	}
+
+	if (!glXMakeContextCurrent(dpy, drawable, drawable, ctx)) {
+		error_code = 7;
+		goto start_failed;
+	}
+
+	if (!check_sgi_video_sync_extension(dpy, screen)) {
+		error_code = 8;
+		goto start_failed;
+	}
+
+	pthread_mutex_lock(&args->start_mtx);
+	args->start_status = 0;
+	pthread_cond_signal(&args->start_cnd);
+	pthread_mutex_unlock(&args->start_mtx);
+
+	pthread_mutex_lock(&self->vblank_requested_mtx);
+	while (self->running) {
+		if (!self->base.vblank_event_requested) {
+			pthread_cond_wait(&self->vblank_requested_cnd,
+			                  &self->vblank_requested_mtx);
+			continue;
+		}
+		pthread_mutex_unlock(&self->vblank_requested_mtx);
+
+		unsigned int last_msc;
+		glXWaitVideoSyncSGI(1, 0, &last_msc);
+
+		struct timespec now;
+		clock_gettime(CLOCK_MONOTONIC, &now);
+		atomic_store(&self->last_msc, last_msc);
+		atomic_store(&self->last_ust,
+		             (uint64_t)(now.tv_sec * 1000000 + now.tv_nsec / 1000));
+		ev_async_send(self->base.loop, &self->notify);
+		pthread_mutex_lock(&self->vblank_requested_mtx);
+	}
+	pthread_mutex_unlock(&self->vblank_requested_mtx);
+	goto cleanup;
+
+start_failed:
+	pthread_mutex_lock(&args->start_mtx);
+	args->start_status = error_code;
+	pthread_cond_signal(&args->start_cnd);
+	pthread_mutex_unlock(&args->start_mtx);
+
+cleanup:
+	if (dpy) {
+		glXMakeCurrent(dpy, None, NULL);
+		if (ctx) {
+			glXDestroyContext(dpy, ctx);
+		}
+		if (drawable) {
+			glXDestroyWindow(dpy, drawable);
+		}
+		if (dummy) {
+			XDestroyWindow(dpy, dummy);
+		}
+		XCloseDisplay(dpy);
+	}
+	return NULL;
+}
+
+static void sgi_video_sync_scheduler_schedule(struct vblank_scheduler *base) {
+	auto self = (struct sgi_video_sync_vblank_scheduler *)base;
+	log_verbose("Requesting vblank event for msc %d", self->last_msc + 1);
+	pthread_mutex_lock(&self->vblank_requested_mtx);
+	assert(!base->vblank_event_requested);
+	base->vblank_event_requested = true;
+	pthread_cond_signal(&self->vblank_requested_cnd);
+	pthread_mutex_unlock(&self->vblank_requested_mtx);
+}
+
+static void
+sgi_video_sync_scheduler_callback(EV_P attr_unused, ev_async *w, int attr_unused revents) {
+	auto sched = container_of(w, struct sgi_video_sync_vblank_scheduler, notify);
+	auto event = (struct vblank_event){
+	    .msc = atomic_load(&sched->last_msc),
+	    .ust = atomic_load(&sched->last_ust),
+	};
+	sched->base.vblank_event_requested = false;
+	log_verbose("Received vblank event for msc %lu", event.msc);
+	vblank_scheduler_invoke_callbacks(&sched->base, &event);
+}
+
+static void sgi_video_sync_scheduler_init(struct vblank_scheduler *base) {
+	auto self = (struct sgi_video_sync_vblank_scheduler *)base;
+	auto args = (struct sgi_video_sync_thread_args){
+	    .self = self,
+	    .start_status = -1,
+	};
+	pthread_mutex_init(&args.start_mtx, NULL);
+	pthread_cond_init(&args.start_cnd, NULL);
+
+	base->type = VBLANK_SCHEDULER_SGI_VIDEO_SYNC;
+	ev_async_init(&self->notify, sgi_video_sync_scheduler_callback);
+	ev_async_start(base->loop, &self->notify);
+	pthread_mutex_init(&self->vblank_requested_mtx, NULL);
+	pthread_cond_init(&self->vblank_requested_cnd, NULL);
+
+	self->running = true;
+	pthread_create(&self->sync_thread, NULL, sgi_video_sync_thread, &args);
+
+	pthread_mutex_lock(&args.start_mtx);
+	while (args.start_status == -1) {
+		pthread_cond_wait(&args.start_cnd, &args.start_mtx);
+	}
+	if (args.start_status != 0) {
+		log_fatal("Failed to start sgi_video_sync_thread, error code: %d",
+		          args.start_status);
+		abort();
+	}
+	pthread_mutex_destroy(&args.start_mtx);
+	pthread_cond_destroy(&args.start_cnd);
+	log_info("Started sgi_video_sync_thread");
+}
+
+static void sgi_video_sync_scheduler_deinit(struct vblank_scheduler *base) {
+	auto self = (struct sgi_video_sync_vblank_scheduler *)base;
+	ev_async_stop(base->loop, &self->notify);
+	pthread_mutex_lock(&self->vblank_requested_mtx);
+	self->running = false;
+	pthread_cond_signal(&self->vblank_requested_cnd);
+	pthread_mutex_unlock(&self->vblank_requested_mtx);
+
+	pthread_join(self->sync_thread, NULL);
+
+	pthread_mutex_destroy(&self->vblank_requested_mtx);
+	pthread_cond_destroy(&self->vblank_requested_cnd);
+}
+#endif
+
+static void present_vblank_scheduler_schedule(struct vblank_scheduler *base) {
+	auto self = (struct present_vblank_scheduler *)base;
+	log_verbose("Requesting vblank event for window 0x%08x, msc %" PRIu64,
+	            base->target_window, self->last_msc + 1);
+	assert(!base->vblank_event_requested);
+	x_request_vblank_event(base->c, base->target_window, self->last_msc + 1);
+	base->vblank_event_requested = true;
+}
+
+static void present_vblank_callback(EV_P attr_unused, ev_timer *w, int attr_unused revents) {
+	auto sched = container_of(w, struct present_vblank_scheduler, callback_timer);
+	auto event = (struct vblank_event){
+	    .msc = sched->last_msc,
+	    .ust = sched->last_ust,
+	};
+	sched->base.vblank_event_requested = false;
+	vblank_scheduler_invoke_callbacks(&sched->base, &event);
+}
+
+static void present_vblank_scheduler_init(struct vblank_scheduler *base) {
+	auto self = (struct present_vblank_scheduler *)base;
+	base->type = VBLANK_SCHEDULER_PRESENT;
+	ev_timer_init(&self->callback_timer, present_vblank_callback, 0, 0);
+
+	self->event_id = x_new_id(base->c);
+	auto select_input =
+	    xcb_present_select_input(base->c->c, self->event_id, base->target_window,
+	                             XCB_PRESENT_EVENT_MASK_COMPLETE_NOTIFY);
+	set_cant_fail_cookie(base->c, select_input);
+	self->event =
+	    xcb_register_for_special_xge(base->c->c, &xcb_present_id, self->event_id, NULL);
+}
+
+static void present_vblank_scheduler_deinit(struct vblank_scheduler *base) {
+	auto self = (struct present_vblank_scheduler *)base;
+	ev_timer_stop(base->loop, &self->callback_timer);
+	auto select_input =
+	    xcb_present_select_input(base->c->c, self->event_id, base->target_window, 0);
+	set_cant_fail_cookie(base->c, select_input);
+	xcb_unregister_for_special_event(base->c->c, self->event);
+}
+
+/// Handle PresentCompleteNotify events
+///
+/// Schedule the registered callback to be called when the current vblank ends.
+static void handle_present_complete_notify(struct present_vblank_scheduler *self,
+                                           xcb_present_complete_notify_event_t *cne) {
+	assert(self->base.type == VBLANK_SCHEDULER_PRESENT);
+
+	if (cne->kind != XCB_PRESENT_COMPLETE_KIND_NOTIFY_MSC) {
+		return;
+	}
+
+	assert(self->base.vblank_event_requested);
+
+	// X sometimes sends duplicate/bogus MSC events, when screen has just been turned
+	// off. Don't use the msc value in these events. We treat this as not receiving a
+	// vblank event at all, and try to get a new one.
+	//
+	// See:
+	// https://gitlab.freedesktop.org/xorg/xserver/-/issues/1418
+	bool event_is_invalid = cne->msc <= self->last_msc || cne->ust == 0;
+	if (event_is_invalid) {
+		log_debug("Invalid PresentCompleteNotify event, %" PRIu64 " %" PRIu64,
+		          cne->msc, cne->ust);
+		x_request_vblank_event(self->base.c, cne->window, self->last_msc + 1);
+		return;
+	}
+
+	self->last_ust = cne->ust;
+	self->last_msc = cne->msc;
+
+	struct timespec now;
+	clock_gettime(CLOCK_MONOTONIC, &now);
+	auto now_us = (unsigned long)(now.tv_sec * 1000000L + now.tv_nsec / 1000);
+	double delay_sec = 0.0;
+	if (now_us < cne->ust) {
+		log_trace("The end of this vblank is %lu us into the "
+		          "future",
+		          cne->ust - now_us);
+		delay_sec = (double)(cne->ust - now_us) / 1000000.0;
+	}
+	// Wait until the end of the current vblank to invoke callbacks. If we
+	// call it too early, it can mistakenly think the render missed the
+	// vblank, and doesn't schedule render for the next vblank, causing frame
+	// drops.
+	assert(!ev_is_active(&self->callback_timer));
+	ev_timer_set(&self->callback_timer, delay_sec, 0);
+	ev_timer_start(self->base.loop, &self->callback_timer);
+}
+
+static bool handle_present_events(struct vblank_scheduler *base) {
+	auto self = (struct present_vblank_scheduler *)base;
+	xcb_present_generic_event_t *ev;
+	while ((ev = (void *)xcb_poll_for_special_event(base->c->c, self->event))) {
+		if (ev->event != self->event_id) {
+			// This event doesn't have the right event context, it's not meant
+			// for us.
+			goto next;
+		}
+
+		// We only subscribed to the complete notify event.
+		assert(ev->evtype == XCB_PRESENT_EVENT_COMPLETE_NOTIFY);
+		handle_present_complete_notify(self, (void *)ev);
+	next:
+		free(ev);
+	}
+	return true;
+}
+
+static const struct vblank_scheduler_ops vblank_scheduler_ops[LAST_VBLANK_SCHEDULER] = {
+    [VBLANK_SCHEDULER_PRESENT] =
+        {
+            .size = sizeof(struct present_vblank_scheduler),
+            .init = present_vblank_scheduler_init,
+            .deinit = present_vblank_scheduler_deinit,
+            .schedule = present_vblank_scheduler_schedule,
+            .handle_x_events = handle_present_events,
+        },
+#ifdef CONFIG_OPENGL
+    [VBLANK_SCHEDULER_SGI_VIDEO_SYNC] =
+        {
+            .size = sizeof(struct sgi_video_sync_vblank_scheduler),
+            .init = sgi_video_sync_scheduler_init,
+            .deinit = sgi_video_sync_scheduler_deinit,
+            .schedule = sgi_video_sync_scheduler_schedule,
+            .handle_x_events = NULL,
+        },
+#endif
+};
+
+static void vblank_scheduler_schedule_internal(struct vblank_scheduler *self) {
+	assert(self->type < LAST_VBLANK_SCHEDULER);
+	auto fn = vblank_scheduler_ops[self->type].schedule;
+	assert(fn != NULL);
+	fn(self);
+}
+
+bool vblank_scheduler_schedule(struct vblank_scheduler *self,
+                               vblank_callback_t vblank_callback, void *user_data) {
+	if (self->callback_count == 0 && self->wind_down == 0) {
+		vblank_scheduler_schedule_internal(self);
+	}
+	if (self->callback_count == self->callback_capacity) {
+		size_t new_capacity =
+		    self->callback_capacity ? self->callback_capacity * 2 : 1;
+		void *new_buffer =
+		    realloc(self->callbacks, new_capacity * sizeof(*self->callbacks));
+		if (!new_buffer) {
+			return false;
+		}
+		self->callbacks = new_buffer;
+		self->callback_capacity = new_capacity;
+	}
+	self->callbacks[self->callback_count++] = (struct vblank_closure){
+	    .fn = vblank_callback,
+	    .user_data = user_data,
+	};
+	return true;
+}
+
+static void
+vblank_scheduler_invoke_callbacks(struct vblank_scheduler *self, struct vblank_event *event) {
+	// callbacks might be added during callback invocation, so we need to
+	// copy the callback_count.
+	size_t count = self->callback_count, write_head = 0;
+	if (count == 0) {
+		self->wind_down--;
+	} else {
+		self->wind_down = VBLANK_WIND_DOWN;
+	}
+	for (size_t i = 0; i < count; i++) {
+		auto action = self->callbacks[i].fn(event, self->callbacks[i].user_data);
+		switch (action) {
+		case VBLANK_CALLBACK_AGAIN:
+			if (i != write_head) {
+				self->callbacks[write_head] = self->callbacks[i];
+			}
+			write_head++;
+		case VBLANK_CALLBACK_DONE:
+		default:        // nothing to do
+			break;
+		}
+	}
+	memset(self->callbacks + write_head, 0,
+	       (count - write_head) * sizeof(*self->callbacks));
+	assert(count == self->callback_count && "callbacks should not be added when "
+	                                        "callbacks are being invoked.");
+	self->callback_count = write_head;
+	if (self->callback_count || self->wind_down) {
+		vblank_scheduler_schedule_internal(self);
+	}
+}
+
+void vblank_scheduler_free(struct vblank_scheduler *self) {
+	assert(self->type < LAST_VBLANK_SCHEDULER);
+	auto fn = vblank_scheduler_ops[self->type].deinit;
+	if (fn != NULL) {
+		fn(self);
+	}
+	free(self->callbacks);
+	free(self);
+}
+
+struct vblank_scheduler *
+vblank_scheduler_new(struct ev_loop *loop, struct x_connection *c,
+                     xcb_window_t target_window, enum vblank_scheduler_type type) {
+	size_t object_size = vblank_scheduler_ops[type].size;
+	auto init_fn = vblank_scheduler_ops[type].init;
+	if (!object_size || !init_fn) {
+		log_error("Unsupported or invalid vblank scheduler type: %d", type);
+		return NULL;
+	}
+
+	assert(object_size >= sizeof(struct vblank_scheduler));
+	struct vblank_scheduler *self = calloc(1, object_size);
+	self->target_window = target_window;
+	self->c = c;
+	self->loop = loop;
+	init_fn(self);
+	return self;
+}
+
+bool vblank_handle_x_events(struct vblank_scheduler *self) {
+	assert(self->type < LAST_VBLANK_SCHEDULER);
+	auto fn = vblank_scheduler_ops[self->type].handle_x_events;
+	if (fn != NULL) {
+		return fn(self);
+	}
+	return true;
+}
\ No newline at end of file
diff --git a/src/vblank.h b/src/vblank.h
new file mode 100644
index 00000000..8050f487
--- /dev/null
+++ b/src/vblank.h
@@ -0,0 +1,47 @@
+#pragma once
+
+#include <stdbool.h>
+#include <stdint.h>
+#include <xcb/present.h>
+#include <xcb/xcb.h>
+
+#include <ev.h>
+#include <xcb/xproto.h>
+
+#include "config.h"
+#include "x.h"
+
+/// An object that schedule vblank events.
+struct vblank_scheduler;
+
+struct vblank_event {
+	uint64_t msc;
+	uint64_t ust;
+};
+
+enum vblank_callback_action {
+	/// The callback should be called again in the next vblank.
+	VBLANK_CALLBACK_AGAIN,
+	/// The callback is done and should not be called again.
+	VBLANK_CALLBACK_DONE,
+};
+
+typedef enum vblank_callback_action (*vblank_callback_t)(struct vblank_event *event,
+                                                         void *user_data);
+
+/// Schedule a vblank event.
+///
+/// Schedule for `cb` to be called when the current vblank ends. If this is called
+/// from a callback function for the current vblank, the newly scheduled callback
+/// will be called in the next vblank.
+///
+/// Returns whether the scheduling is successful. Scheduling can fail if there
+/// is not enough memory.
+bool vblank_scheduler_schedule(struct vblank_scheduler *self, vblank_callback_t cb,
+                               void *user_data);
+struct vblank_scheduler *
+vblank_scheduler_new(struct ev_loop *loop, struct x_connection *c,
+                     xcb_window_t target_window, enum vblank_scheduler_type type);
+void vblank_scheduler_free(struct vblank_scheduler *);
+
+bool vblank_handle_x_events(struct vblank_scheduler *self);
diff --git a/src/x.c b/src/x.c
index 1840ebb1..06c8b939 100644
--- a/src/x.c
+++ b/src/x.c
@@ -9,7 +9,9 @@
 #include <pixman.h>
 #include <xcb/composite.h>
 #include <xcb/damage.h>
+#include <xcb/dpms.h>
 #include <xcb/glx.h>
+#include <xcb/present.h>
 #include <xcb/randr.h>
 #include <xcb/render.h>
 #include <xcb/sync.h>
@@ -777,6 +779,31 @@ err:
 	return false;
 }
 
+void x_request_vblank_event(struct x_connection *c, xcb_window_t window, uint64_t msc) {
+	auto cookie = xcb_present_notify_msc(c->c, window, 0, msc, 1, 0);
+	set_cant_fail_cookie(c, cookie);
+}
+
+static inline bool dpms_screen_is_off(xcb_dpms_info_reply_t *info) {
+	// state is a bool indicating whether dpms is enabled
+	return info->state && (info->power_level != XCB_DPMS_DPMS_MODE_ON);
+}
+
+bool x_check_dpms_status(struct x_connection *c, bool *screen_is_off) {
+	auto r = xcb_dpms_info_reply(c->c, xcb_dpms_info(c->c), NULL);
+	if (!r) {
+		log_error("Failed to query DPMS status.");
+		return false;
+	}
+	auto now_screen_is_off = dpms_screen_is_off(r);
+	if (*screen_is_off != now_screen_is_off) {
+		log_debug("Screen is now %s", now_screen_is_off ? "off" : "on");
+		*screen_is_off = now_screen_is_off;
+	}
+	free(r);
+	return true;
+}
+
 /**
  * Convert a struct conv to a X picture convolution filter, normalizing the kernel
  * in the process. Allow the caller to specify the element at the center of the kernel,
diff --git a/src/x.h b/src/x.h
index b5bd1a59..df45b5cc 100644
--- a/src/x.h
+++ b/src/x.h
@@ -419,3 +419,11 @@ void x_update_monitors(struct x_connection *, struct x_monitors *);
 void x_free_monitor_info(struct x_monitors *);
 
 uint32_t attr_deprecated xcb_generate_id(xcb_connection_t *c);
+
+/// Ask X server to send us a notification for the next end of vblank.
+void x_request_vblank_event(struct x_connection *c, xcb_window_t window, uint64_t msc);
+
+/// Update screen_is_off to reflect the current DPMS state.
+///
+/// Returns true if the DPMS state was successfully queried, false otherwise.
+bool x_check_dpms_status(struct x_connection *c, bool *screen_is_off);