diff --git a/ChangeLog b/ChangeLog
index a460911428..303f381408 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,12 @@
+Mon Jul 18 09:36:25 2005  Tanaka Akira  <akr@m17n.org>
+
+	* rubyio.h (FMODE_WSPLIT, FMODE_WSPLIT_INITIALIZED): new constant.
+
+	* io.c (wsplit_p): new function.
+	  (io_fflush): split writing data by PIPE_BUF if wsplit_p is true in
+	  multi-threaded mode.
+	  (io_fwrite): ditto.
+
 Mon Jul 18 05:00:00 2005  NARUSE, Yui  <naruse@ruby-lang.org>
 
         * ext/nkf/nkf-utf8/nkf.c: import nkf.c 1.73
diff --git a/io.c b/io.c
index 6546e75212..116dc5c55f 100644
--- a/io.c
+++ b/io.c
@@ -372,11 +372,29 @@ io_alloc(klass)
     return (VALUE)io;
 }
 
+static int
+wsplit_p(OpenFile *fptr)
+{
+    int r;
+    if (!(fptr->mode & FMODE_WSPLIT_INITIALIZED)) {
+        struct stat buf;
+        if (fstat(fptr->fd, &buf) == 0 &&
+            !(S_ISREG(buf.st_mode) ||
+              S_ISDIR(buf.st_mode)) &&
+            (r = fcntl(fptr->fd, F_GETFL)) != -1 &&
+            !(r & O_NONBLOCK)) {
+            fptr->mode |= FMODE_WSPLIT;
+        }
+        fptr->mode |= FMODE_WSPLIT_INITIALIZED;
+    }
+    return fptr->mode & FMODE_WSPLIT;
+}
+
 static int
 io_fflush(fptr)
     OpenFile *fptr;
 {
-    int r;
+    int r, l;
     int wbuf_off, wbuf_len;
 
     rb_io_check_closed(fptr);
@@ -390,8 +408,15 @@ io_fflush(fptr)
         return 0;
     wbuf_off = fptr->wbuf_off;
     wbuf_len = fptr->wbuf_len;
+    l = fptr->wbuf_len;
+    if (PIPE_BUF < l &&
+        !rb_thread_critical &&
+        !rb_thread_alone() &&
+        wsplit_p(fptr)) {
+        l = PIPE_BUF;
+    }
     TRAP_BEG;
-    r = write(fptr->fd, fptr->wbuf+fptr->wbuf_off, fptr->wbuf_len);
+    r = write(fptr->fd, fptr->wbuf+fptr->wbuf_off, l);
     TRAP_END; /* xxx: signal handler may modify wbuf */
     if (r == fptr->wbuf_len) {
         fptr->wbuf_off = 0;
@@ -504,7 +529,7 @@ io_fwrite(str, fptr)
     VALUE str;
     OpenFile *fptr;
 {
-    long len, n, r, offset = 0;
+    long len, n, r, l, offset = 0;
 
     len = RSTRING(str)->len;
     if ((n = len) <= 0) return n;
@@ -537,8 +562,15 @@ io_fwrite(str, fptr)
 	    rb_io_check_closed(fptr);
 	}
       retry:
+        l = n;
+        if (PIPE_BUF < l &&
+            !rb_thread_critical &&
+            !rb_thread_alone() &&
+            wsplit_p(fptr)) {
+            l = PIPE_BUF;
+        }
         TRAP_BEG;
-	r = write(fptr->fd, RSTRING(str)->ptr+offset, n);
+	r = write(fptr->fd, RSTRING(str)->ptr+offset, l);
         TRAP_END; /* xxx: signal handler may modify given string. */
         if (r == n) return len;
         if (0 <= r) {
diff --git a/rubyio.h b/rubyio.h
index a041d90ebc..cdfd8c797f 100644
--- a/rubyio.h
+++ b/rubyio.h
@@ -48,6 +48,8 @@ typedef struct OpenFile {
 #define FMODE_SYNC      8
 #define FMODE_TTY      16
 #define FMODE_DUPLEX   32
+#define FMODE_WSPLIT  0x200
+#define FMODE_WSPLIT_INITIALIZED  0x400
 
 #define GetOpenFile(obj,fp) rb_io_check_closed((fp) = RFILE(rb_io_taint_check(obj))->fptr)