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Performance Guidelines
This document describes various guidelines to follow to ensure good and consistent performance of GitLab.
Workflow
The process of solving performance problems is roughly as follows:
- Make sure there's an issue open somewhere (for example, on the GitLab CE issue tracker), and create one if there is not. See #15607 for an example.
- Measure the performance of the code in a production environment such as GitLab.com (see the Tooling section below). Performance should be measured over a period of at least 24 hours.
- Add your findings based on the measurement period (screenshots of graphs, timings, etc) to the issue mentioned in step 1.
- Solve the problem.
- Create a merge request, assign the "Performance" label and follow the performance review process.
- Once a change has been deployed make sure to again measure for at least 24 hours to see if your changes have any impact on the production environment.
- Repeat until you're done.
When providing timings make sure to provide:
- The 95th percentile
- The 99th percentile
- The mean
When providing screenshots of graphs, make sure that both the X and Y axes and the legend are clearly visible. If you happen to have access to GitLab.com's own monitoring tools you should also provide a link to any relevant graphs/dashboards.
Tooling
GitLab provides built-in tools to help improve performance and availability:
- Profiling.
- Distributed Tracing
- GitLab Performance Monitoring.
- Request Profiling.
- QueryRecoder for preventing
N+1
regressions. - Chaos endpoints for testing failure scenarios. Intended mainly for testing availability.
- Service measurement for measuring and logging service execution.
GitLab team members can use GitLab.com's performance monitoring systems located at
dashboards.gitlab.net
, this requires you to log in using your
@gitlab.com
email address. Non-GitLab team-members are advised to set up their
own Prometheus and Grafana stack.
Benchmarks
Benchmarks are almost always useless. Benchmarks usually only test small bits of code in isolation and often only measure the best case scenario. On top of that, benchmarks for libraries (such as a Gem) tend to be biased in favour of the library. After all there's little benefit to an author publishing a benchmark that shows they perform worse than their competitors.
Benchmarks are only really useful when you need a rough (emphasis on "rough") understanding of the impact of your changes. For example, if a certain method is slow a benchmark can be used to see if the changes you're making have any impact on the method's performance. However, even when a benchmark shows your changes improve performance there's no guarantee the performance also improves in a production environment.
When writing benchmarks you should almost always use
benchmark-ips. Ruby's Benchmark
module that comes with the standard library is rarely useful as it runs either a
single iteration (when using Benchmark.bm
) or two iterations (when using
Benchmark.bmbm
). Running this few iterations means external factors, such as a
video streaming in the background, can very easily skew the benchmark
statistics.
Another problem with the Benchmark
module is that it displays timings, not
iterations. This means that if a piece of code completes in a very short period
of time it can be very difficult to compare the timings before and after a
certain change. This in turn leads to patterns such as the following:
Benchmark.bmbm(10) do |bench|
bench.report 'do something' do
100.times do
... work here ...
end
end
end
This however leads to the question: how many iterations should we run to get meaningful statistics?
The benchmark-ips
gem takes care of all this and much more. You should therefore use it instead of the Benchmark
module.
The GitLab Gemfile also contains the benchmark-memory
gem, which works similarly to the benchmark
and benchmark-ips
gems. However, benchmark-memory
instead returns the memory size, objects, and strings allocated and retained during the benchmark.
In short:
- Don't trust benchmarks you find on the internet.
- Never make claims based on just benchmarks, always measure in production to confirm your findings.
- X being N times faster than Y is meaningless if you don't know what impact it has on your production environment.
- A production environment is the only benchmark that always tells the truth (unless your performance monitoring systems are not set up correctly).
- If you must write a benchmark use the benchmark-ips Gem instead of Ruby's
Benchmark
module.
Profiling with Stackprof
By collecting snapshots of process state at regular intervals, profiling allows you to see where time is spent in a process. The Stackprof gem is included in GitLab, allowing you to profile which code is running on CPU in detail.
It's important to note that profiling an application alters its performance. Different profiling strategies have different overheads. Stackprof is a sampling profiler. It samples stack traces from running threads at a configurable frequency (for example, 100hz, that is 100 stacks per second). This type of profiling has quite a low (albeit non-zero) overhead and is generally considered to be safe for production.
A profiler can be a very useful tool during development, even if it does run in an unrepresentative environment. In particular, a method is not necessarily troublesome just because it's executed many times, or takes a long time to execute. Profiles are tools you can use to better understand what is happening in an application - using that information wisely is up to you!
There are multiple ways to create a profile with Stackprof.
Wrapping a code block
To profile a specific code block, you can wrap that block in a Stackprof.run
call:
StackProf.run(mode: :wall, out: 'tmp/stackprof-profiling.dump') do
#...
end
This creates a .dump
file that you can read.
For all available options, see the Stackprof documentation.
Performance bar
With the Performance bar, you have the option to profile a request using Stackprof and immediately output the results to a Speedscope flamegraph.
RSpec profiling with Stackprof
To create a profile from a spec, identify (or create) a spec that
exercises the troublesome code path, then run it using the bin/rspec-stackprof
helper, for example:
$ LIMIT=10 bin/rspec-stackprof spec/policies/project_policy_spec.rb
8/8 |====== 100 ======>| Time: 00:00:18
Finished in 18.19 seconds (files took 4.8 seconds to load)
8 examples, 0 failures
==================================
Mode: wall(1000)
Samples: 17033 (5.59% miss rate)
GC: 1901 (11.16%)
==================================
TOTAL (pct) SAMPLES (pct) FRAME
6000 (35.2%) 2566 (15.1%) Sprockets::Cache::FileStore#get
2018 (11.8%) 888 (5.2%) ActiveRecord::ConnectionAdapters::PostgreSQLAdapter#exec_no_cache
1338 (7.9%) 640 (3.8%) ActiveRecord::ConnectionAdapters::PostgreSQL::DatabaseStatements#execute
3125 (18.3%) 394 (2.3%) Sprockets::Cache::FileStore#safe_open
913 (5.4%) 301 (1.8%) ActiveRecord::ConnectionAdapters::PostgreSQLAdapter#exec_cache
288 (1.7%) 288 (1.7%) ActiveRecord::Attribute#initialize
246 (1.4%) 246 (1.4%) Sprockets::Cache::FileStore#safe_stat
295 (1.7%) 193 (1.1%) block (2 levels) in class_attribute
187 (1.1%) 187 (1.1%) block (4 levels) in class_attribute
You can limit the specs that are run by passing any arguments RSpec
would
normally take.
Using Stackprof in production
Stackprof can also be used to profile production workloads.
In order to enable production profiling for Ruby processes, you can set the STACKPROF_ENABLED
environment variable to true
.
The following configuration options can be configured:
STACKPROF_ENABLED
: Enables Stackprof signal handler on SIGUSR2 signal. Defaults tofalse
.STACKPROF_MODE
: See sampling modes. Defaults tocpu
.STACKPROF_INTERVAL
: Sampling interval. Unit semantics depend onSTACKPROF_MODE
. Forobject
mode this is a per-event interval (everynth
event is sampled) and defaults to100
. For other modes such ascpu
this is a frequency interval and defaults to10100
μs (99hz).STACKPROF_FILE_PREFIX
: File path prefix where profiles are stored. Defaults to$TMPDIR
(often corresponds to/tmp
).STACKPROF_TIMEOUT_S
: Profiling timeout in seconds. Profiling will automatically stop after this time has elapsed. Defaults to30
.STACKPROF_RAW
: Whether to collect raw samples or only aggregates. Raw samples are needed to generate flame graphs, but they do have a higher memory and disk overhead. Defaults totrue
.
Once enabled, profiling can be triggered by sending a SIGUSR2
signal to the
Ruby process. The process begins sampling stacks. Profiling can be stopped
by sending another SIGUSR2
. Alternatively, it stops automatically after
the timeout.
Once profiling stops, the profile is written out to disk at
$STACKPROF_FILE_PREFIX/stackprof.$PID.$RAND.profile
. It can then be inspected
further through the stackprof
command line tool, as described in the
Reading a Stackprof profile section.
Currently supported profiling targets are:
- Puma worker
- Sidekiq
NOTE: The Puma master process is not supported. Sending SIGUSR2 to it triggers restarts. In the case of Puma, take care to only send the signal to Puma workers.
This can be done via pkill -USR2 puma:
. The :
distinguishes between puma 4.3.3.gitlab.2 ...
(the master process) from puma: cluster worker 0: ...
(the
worker processes), selecting the latter.
For Sidekiq, the signal can be sent to the sidekiq-cluster
process via pkill -USR2 bin/sidekiq-cluster
, which forwards the signal to all Sidekiq
children. Alternatively, you can also select a specific PID of interest.
Reading a Stackprof profile
The output is sorted by the Samples
column by default. This is the number of samples taken where
the method is the one currently executed. The Total
column shows the number of samples taken where
the method (or any of the methods it calls) is executed.
To create a graphical view of the call stack:
stackprof tmp/project_policy_spec.rb.dump --graphviz > project_policy_spec.dot
dot -Tsvg project_policy_spec.dot > project_policy_spec.svg
To load the profile in KCachegrind:
stackprof tmp/project_policy_spec.rb.dump --callgrind > project_policy_spec.callgrind
kcachegrind project_policy_spec.callgrind # Linux
qcachegrind project_policy_spec.callgrind # Mac
You can also generate and view the resultant flame graph. To view a flame graph that
bin/rspec-stackprof
creates, you must set the RAW
environment variable to true
when running
bin/rspec-stackprof
.
It might take a while to generate based on the output file size:
# Generate
stackprof --flamegraph tmp/group_member_policy_spec.rb.dump > group_member_policy_spec.flame
# View
stackprof --flamegraph-viewer=group_member_policy_spec.flame
To export the flame graph to an SVG file, use Brendan Gregg's FlameGraph tool:
stackprof --stackcollapse /tmp/group_member_policy_spec.rb.dump | flamegraph.pl > flamegraph.svg
It's also possible to view flame graphs through speedscope.
You can do this when using the performance bar
and when profiling code blocks.
This option isn't supported by bin/rspec-stackprof
.
You can profile speciific methods by using --method method_name
:
$ stackprof tmp/project_policy_spec.rb.dump --method access_allowed_to
ProjectPolicy#access_allowed_to? (/Users/royzwambag/work/gitlab-development-kit/gitlab/app/policies/project_policy.rb:793)
samples: 0 self (0.0%) / 578 total (0.7%)
callers:
397 ( 68.7%) block (2 levels) in <class:ProjectPolicy>
95 ( 16.4%) block in <class:ProjectPolicy>
86 ( 14.9%) block in <class:ProjectPolicy>
callees (578 total):
399 ( 69.0%) ProjectPolicy#team_access_level
141 ( 24.4%) Project::GeneratedAssociationMethods#project_feature
30 ( 5.2%) DeclarativePolicy::Base#can?
8 ( 1.4%) Featurable#access_level
code:
| 793 | def access_allowed_to?(feature)
141 (0.2%) | 794 | return false unless project.project_feature
| 795 |
8 (0.0%) | 796 | case project.project_feature.access_level(feature)
| 797 | when ProjectFeature::DISABLED
| 798 | false
| 799 | when ProjectFeature::PRIVATE
429 (0.5%) | 800 | can?(:read_all_resources) || team_access_level >= ProjectFeature.required_minimum_access_level(feature)
| 801 | else
When using Stackprof to profile specs, the profile includes the work done by the test suite and the application code. You can therefore use these profiles to investigate slow tests as well. However, for smaller runs (like this example), this means that the cost of setting up the test suite tends to dominate.
RSpec profiling
The GitLab development environment also includes the
rspec_profiling
gem, which is used
to collect data on spec execution times. This is useful for analyzing the
performance of the test suite itself, or seeing how the performance of a spec
may have changed over time.
To activate profiling in your local environment, run the following:
export RSPEC_PROFILING=yes
rake rspec_profiling:install
This creates an SQLite3 database in tmp/rspec_profiling
, into which statistics
are saved every time you run specs with the RSPEC_PROFILING
environment
variable set.
Ad-hoc investigation of the collected results can be performed in an interactive shell:
$ rake rspec_profiling:console
irb(main):001:0> results.count
=> 231
irb(main):002:0> results.last.attributes.keys
=> ["id", "commit", "date", "file", "line_number", "description", "time", "status", "exception", "query_count", "query_time", "request_count", "request_time", "created_at", "updated_at"]
irb(main):003:0> results.where(status: "passed").average(:time).to_s
=> "0.211340155844156"
These results can also be placed into a PostgreSQL database by setting the
RSPEC_PROFILING_POSTGRES_URL
variable. This is used to profile the test suite
when running in the CI environment.
We store these results also when running nightly scheduled CI jobs on the
default branch on gitlab.com
. Statistics of these profiling data are
available online. For
example, you can find which tests take longest to run or which execute the most
queries. This can be handy for optimizing our tests or identifying performance
issues in our code.
Memory optimization
We can use a set of different techniques, often in combination, to track down memory issues:
- Leaving the code intact and wrapping a profiler around it.
- Use memory allocation counters for requests and services.
- Monitor memory usage of the process while disabling/enabling different parts of the code we suspect could be problematic.
Memory allocations
Ruby shipped with GitLab includes a special patch to allow tracing memory allocations. This patch is available by default for Omnibus, CNG, GitLab CI, GCK and can additionally be enabled for GDK.
This patch provides the following metrics that make it easier to understand efficiency of memory use for a given code path:
mem_total_bytes
: the number of bytes consumed both due to new objects being allocated into existing object slots plus additional memory allocated for large objects (that is,mem_bytes + slot_size * mem_objects
).mem_bytes
: the number of bytes allocated bymalloc
for objects that did not fit into an existing object slot.mem_objects
: the number of objects allocated.mem_mallocs
: the number ofmalloc
calls.
The number of objects and bytes allocated impact how often GC cycles happen. Fewer object allocations result in a significantly more responsive application.
It is advised that web server requests do not allocate more than 100k mem_objects
and 100M mem_bytes
. You can view the current usage on GitLab.com.
Checking memory pressure of own code
There are two ways of measuring your own code:
- Review
api_json.log
,development_json.log
,sidekiq.log
that includes memory allocation counters. - Use
Gitlab::Memory::Instrumentation.with_memory_allocations
for a given code block and log it. - Use Measuring module
{"time":"2021-02-15T11:20:40.821Z","severity":"INFO","duration_s":0.27412,"db_duration_s":0.05755,"view_duration_s":0.21657,"status":201,"method":"POST","path":"/api/v4/projects/user/1","mem_objects":86705,"mem_bytes":4277179,"mem_mallocs":22693,"correlation_id":"...}
Different types of allocations
The mem_*
values represent different aspects of how objects and memory are allocated in Ruby:
-
The following example will create around of
1000
ofmem_objects
since strings can be frozen, and while the underlying string object remains the same, we still need to allocate 1000 references to this string:Gitlab::Memory::Instrumentation.with_memory_allocations do 1_000.times { '0123456789' } end => {:mem_objects=>1001, :mem_bytes=>0, :mem_mallocs=>0}
-
The following example will create around of
1000
ofmem_objects
, as strings are created dynamically. Each of them will not allocate additional memory, as they fit into Ruby slot of 40 bytes:Gitlab::Memory::Instrumentation.with_memory_allocations do s = '0' 1_000.times { s * 23 } end => {:mem_objects=>1002, :mem_bytes=>0, :mem_mallocs=>0}
-
The following example will create around of
1000
ofmem_objects
, as strings are created dynamically. Each of them will allocate additional memory as strings are larger than Ruby slot of 40 bytes:Gitlab::Memory::Instrumentation.with_memory_allocations do s = '0' 1_000.times { s * 24 } end => {:mem_objects=>1002, :mem_bytes=>32000, :mem_mallocs=>1000}
-
The following example will allocate over 40kB of data, and perform only a single memory allocation. The existing object will be reallocated/resized on subsequent iterations:
Gitlab::Memory::Instrumentation.with_memory_allocations do str = '' append = '0123456789012345678901234567890123456789' # 40 bytes 1_000.times { str.concat(append) } end => {:mem_objects=>3, :mem_bytes=>49152, :mem_mallocs=>1}
-
The following example will create over 1k of objects, perform over 1k of allocations, each time mutating the object. This does result in copying a lot of data and perform a lot of memory allocations (as represented by
mem_bytes
counter) indicating very inefficient method of appending string:Gitlab::Memory::Instrumentation.with_memory_allocations do str = '' append = '0123456789012345678901234567890123456789' # 40 bytes 1_000.times { str += append } end => {:mem_objects=>1003, :mem_bytes=>21968752, :mem_mallocs=>1000}
Using Memory Profiler
We can use memory_profiler
for profiling.
The memory_profiler
gem is already present in the GitLab Gemfile
. It's also available in the performance bar
for the current URL.
To use the memory profiler directly in your code, use require
to add it:
require 'memory_profiler'
report = MemoryProfiler.report do
# Code you want to profile
end
output = File.open('/tmp/profile.txt','w')
report.pretty_print(output)
The report shows the retained and allocated memory grouped by gem, file, location, and class. The memory profiler also performs a string analysis that shows how often a string is allocated and retained.
Retained versus allocated
- Retained memory: long-lived memory use and object count retained due to the execution of the code block. This has a direct impact on memory and the garbage collector.
- Allocated memory: all object allocation and memory allocation during the code block. This might have minimal impact on memory, but substantial impact on performance. The more objects you allocate, the more work is being done and the slower the application is.
As a general rule, retained is always smaller than or equal to allocated.
The actual RSS cost is always slightly higher as MRI heaps are not squashed to size and memory fragments.
Rbtrace
One of the reasons of the increased memory footprint could be Ruby memory fragmentation.
To diagnose it, you can visualize Ruby heap as described in this post by Aaron Patterson.
To start, you want to dump the heap of the process you're investigating to a JSON file.
You need to run the command inside the process you're exploring, you may do that with rbtrace
.
rbtrace
is already present in GitLab Gemfile
, you just need to require it.
It could be achieved running webserver or Sidekiq with the environment variable set to ENABLE_RBTRACE=1
.
To get the heap dump:
bundle exec rbtrace -p <PID> -e 'File.open("heap.json", "wb") { |t| ObjectSpace.dump_all(output: t) }'
Having the JSON, you finally could render a picture using the script provided by Aaron or similar:
ruby heapviz.rb heap.json
Fragmented Ruby heap snapshot could look like this:
Memory fragmentation could be reduced by tuning GC parameters as described in this post. This should be considered as a tradeoff, as it may affect overall performance of memory allocation and GC cycles.
Derailed Benchmarks
derailed_benchmarks
is a gem
described as "A series of things you can use to benchmark a Rails or Ruby app."
We include derailed_benchmarks
in our Gemfile
.
We run derailed exec perf:mem
in every pipeline with a test
stage, in a job
called memory-on-boot
. (Read an example job..)
You may find the results:
- On the merge request Overview tab, in the merge request reports area, in the Metrics Reports dropdown list.
- In the
memory-on-boot
artifacts for a full report and a dependency breakdown.
derailed_benchmarks
also provides other methods to investigate memory. To learn more,
refer to the gem documentation.
Most of the methods (derailed exec perf:*
) attempt to boot your Rails app in a
production
environment and run benchmarks against it.
It is possible both in GDK and GCK:
- For GDK, follow the the instructions on the gem page. You must do similar for Redis configurations to avoid errors.
- GCK includes
production
configuration sections out of the box.
Importance of Changes
When working on performance improvements, it's important to always ask yourself the question "How important is it to improve the performance of this piece of code?". Not every piece of code is equally important and it would be a waste to spend a week trying to improve something that only impacts a tiny fraction of our users. For example, spending a week trying to squeeze 10 milliseconds out of a method is a waste of time when you could have spent a week squeezing out 10 seconds elsewhere.
There is no clear set of steps that you can follow to determine if a certain piece of code is worth optimizing. The only two things you can do are:
- Think about what the code does, how it's used, how many times it's called and how much time is spent in it relative to the total execution time (for example, the total time spent in a web request).
- Ask others (preferably in the form of an issue).
Some examples of changes that are not really important/worth the effort:
- Replacing double quotes with single quotes.
- Replacing usage of Array with Set when the list of values is very small.
- Replacing library A with library B when both only take up 0.1% of the total execution time.
- Calling
freeze
on every string (see String Freezing).
Slow Operations & Sidekiq
Slow operations, like merging branches, or operations that are prone to errors (using external APIs) should be performed in a Sidekiq worker instead of directly in a web request as much as possible. This has numerous benefits such as:
- An error doesn't prevent the request from completing.
- The process being slow doesn't affect the loading time of a page.
- In case of a failure you can retry the process (Sidekiq takes care of this automatically).
- By isolating the code from a web request it should be easier to test and maintain.
It's especially important to use Sidekiq as much as possible when dealing with Git operations as these operations can take quite some time to complete depending on the performance of the underlying storage system.
Git Operations
Care should be taken to not run unnecessary Git operations. For example,
retrieving the list of branch names using Repository#branch_names
can be done
without an explicit check if a repository exists or not. In other words, instead
of this:
if repository.exists?
repository.branch_names.each do |name|
...
end
end
You can just write:
repository.branch_names.each do |name|
...
end
Caching
Operations that often return the same result should be cached using Redis, in particular Git operations. When caching data in Redis, make sure the cache is flushed whenever needed. For example, a cache for the list of tags should be flushed whenever a new tag is pushed or a tag is removed.
When adding cache expiration code for repositories, this code should be placed
in one of the before/after hooks residing in the Repository class. For example,
if a cache should be flushed after importing a repository this code should be
added to Repository#after_import
. This ensures the cache logic stays within
the Repository class instead of leaking into other classes.
When caching data, make sure to also memoize the result in an instance variable. While retrieving data from Redis is much faster than raw Git operations, it still has overhead. By caching the result in an instance variable, repeated calls to the same method don't retrieve data from Redis upon every call. When memoizing cached data in an instance variable, make sure to also reset the instance variable when flushing the cache. An example:
def first_branch
@first_branch ||= cache.fetch(:first_branch) { branches.first }
end
def expire_first_branch_cache
cache.expire(:first_branch)
@first_branch = nil
end
String Freezing
In recent Ruby versions calling .freeze
on a String leads to it being allocated
only once and re-used. For example, on Ruby 2.3 or later this only allocates the
"foo" String once:
10.times do
'foo'.freeze
end
Depending on the size of the String and how frequently it would be allocated
(before the .freeze
call was added), this may make things faster, but
this isn't guaranteed.
Freezing strings saves memory, as every allocated string uses at least one RVALUE_SIZE
bytes (40
bytes on x64) of memory.
You can use the memory profiler
to see which strings are allocated often and could potentially benefit from a .freeze
.
Strings are frozen by default in Ruby 3.0. To prepare our codebase for this eventuality, we are adding the following header to all Ruby files:
# frozen_string_literal: true
This may cause test failures in the code that expects to be able to manipulate
strings. Instead of using dup
, use the unary plus to get an unfrozen string:
test = +"hello"
test += " world"
When adding new Ruby files, please check that you can add the above header, as omitting it may lead to style check failures.
Banzai pipelines and filters
When writing or updating Banzai filters and pipelines, it can be difficult to understand what the performance of the filter is, and what effect it might have on the overall pipeline performance.
To perform benchmarks run:
bin/rake benchmark:banzai
This command generates output like this:
--> Benchmarking Full, Wiki, and Plain pipelines
Calculating -------------------------------------
Full pipeline 1.000 i/100ms
Wiki pipeline 1.000 i/100ms
Plain pipeline 1.000 i/100ms
-------------------------------------------------
Full pipeline 3.357 (±29.8%) i/s - 31.000
Wiki pipeline 2.893 (±34.6%) i/s - 25.000 in 10.677014s
Plain pipeline 15.447 (±32.4%) i/s - 119.000
Comparison:
Plain pipeline: 15.4 i/s
Full pipeline: 3.4 i/s - 4.60x slower
Wiki pipeline: 2.9 i/s - 5.34x slower
.
--> Benchmarking FullPipeline filters
Calculating -------------------------------------
Markdown 24.000 i/100ms
Plantuml 8.000 i/100ms
SpacedLink 22.000 i/100ms
...
TaskList 49.000 i/100ms
InlineDiff 9.000 i/100ms
SetDirection 369.000 i/100ms
-------------------------------------------------
Markdown 237.796 (±16.4%) i/s - 2.304k
Plantuml 80.415 (±36.1%) i/s - 520.000
SpacedLink 168.188 (±10.1%) i/s - 1.672k
...
TaskList 101.145 (± 6.9%) i/s - 1.029k
InlineDiff 52.925 (±15.1%) i/s - 522.000
SetDirection 3.728k (±17.2%) i/s - 34.317k in 10.617882s
Comparison:
Suggestion: 739616.9 i/s
Kroki: 306449.0 i/s - 2.41x slower
InlineGrafanaMetrics: 156535.6 i/s - 4.72x slower
SetDirection: 3728.3 i/s - 198.38x slower
...
UserReference: 2.1 i/s - 360365.80x slower
ExternalLink: 1.6 i/s - 470400.67x slower
ProjectReference: 0.7 i/s - 1128756.09x slower
.
--> Benchmarking PlainMarkdownPipeline filters
Calculating -------------------------------------
Markdown 19.000 i/100ms
-------------------------------------------------
Markdown 241.476 (±15.3%) i/s - 2.356k
This can give you an idea how various filters perform, and which ones might be performing the slowest.
The test data has a lot to do with how well a filter performs. If there is nothing in the test data
that specifically triggers the filter, it might look like it's running incredibly fast.
Make sure that you have relevant test data for your filter in the
spec/fixtures/markdown.md.erb
file.
Reading from files and other data sources
Ruby offers several convenience functions that deal with file contents specifically
or I/O streams in general. Functions such as IO.read
and IO.readlines
make
it easy to read data into memory, but they can be inefficient when the
data grows large. Because these functions read the entire contents of a data
source into memory, memory use grows by at least the size of the data source.
In the case of readlines
, it grows even further, due to extra bookkeeping
the Ruby VM has to perform to represent each line.
Consider the following program, which reads a text file that is 750MB on disk:
File.readlines('large_file.txt').each do |line|
puts line
end
Here is a process memory reading from while the program was running, showing how we indeed kept the entire file in memory (RSS reported in kilobytes):
$ ps -o rss -p <pid>
RSS
783436
And here is an excerpt of what the garbage collector was doing:
pp GC.stat
{
:heap_live_slots=>2346848,
:malloc_increase_bytes=>30895288,
...
}
We can see that heap_live_slots
(the number of reachable objects) jumped to ~2.3M,
which is roughly two orders of magnitude more compared to reading the file line by
line instead. It was not just the raw memory usage that increased, but also how the garbage collector (GC)
responded to this change in anticipation of future memory use. We can see that malloc_increase_bytes
jumped
to ~30MB, which compares to just ~4kB for a "fresh" Ruby program. This figure specifies how
much additional heap space the Ruby GC claims from the operating system next time it runs out of memory.
Not only did we occupy more memory, we also changed the behavior of the application
to increase memory use at a faster rate.
The IO.read
function exhibits similar behavior, with the difference that no extra memory is
allocated for each line object.
Recommendations
Instead of reading data sources into memory in full, it is better to read them line by line
instead. This is not always an option, for instance when you need to convert a YAML file
into a Ruby Hash
, but whenever you have data where each row represents some entity that
can be processed and then discarded, you can use the following approaches.
First, replace calls to readlines.each
with either each
or each_line
.
The each_line
and each
functions read the data source line by line without keeping
already visited lines in memory:
File.new('file').each { |line| puts line }
Alternatively, you can read individual lines explicitly using IO.readline
or IO.gets
functions:
while line = file.readline
# process line
end
This might be preferable if there is a condition that allows exiting the loop early, saving not just memory but also unnecessary time spent in CPU and I/O for processing lines you're not interested in.
Anti-Patterns
This is a collection of anti-patterns that should be avoided unless these changes have a measurable, significant, and positive impact on production environments.
Moving Allocations to Constants
Storing an object as a constant so you only allocate it once may improve performance, but this is not guaranteed. Looking up constants has an impact on runtime performance, and as such, using a constant instead of referencing an object directly may even slow code down. For example:
SOME_CONSTANT = 'foo'.freeze
9000.times do
SOME_CONSTANT
end
The only reason you should be doing this is to prevent somebody from mutating the global String. However, since you can just re-assign constants in Ruby there's nothing stopping somebody from doing this elsewhere in the code:
SOME_CONSTANT = 'bar'
How to seed a database with millions of rows
You might want millions of project rows in your local database, for example, in order to compare relative query performance, or to reproduce a bug. You could do this by hand with SQL commands or using Mass Inserting Rails Models functionality.
Assuming you are working with ActiveRecord models, you might also find these links helpful:
Examples
You may find some useful examples in this snippet.