debian-mirror-gitlab/doc/development/performance.md

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:

1. Make sure there's an issue open somewhere (e.g., on the GitLab CE issue tracker), create one if there isn't. See #15607 for an example.
2. 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.
3. Add your findings based on the measurement period (screenshots of graphs, timings, etc) to the issue mentioned in step 1.
4. Solve the problem.
5. Create a merge request, assign the "performance" label and ping the right people (e.g. @yorickpeterse and @joshfng).
6. 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.
7. 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 two built-in tools to aid the process of improving performance:

• Sherlock
• GitLab Performance Monitoring

GitLab employees can use GitLab.com's performance monitoring systems located at http://performance.gitlab.net, this requires you to log in using your @gitlab.com Email address. Non-GitLab employees are advised to set up their own InfluxDB + 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 (e.g., 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 (e.g. 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 basically takes care of all this and much more, and as a result of this should be used instead of the Benchmark module.

In short:

1. Don't trust benchmarks you find on the internet.
2. Never make claims based on just benchmarks, always measure in production to confirm your findings.
3. X being N times faster than Y is meaningless if you don't know what impact it will actually have on your production environment.
4. A production environment is the only benchmark that always tells the truth (unless your performance monitoring systems are not set up correctly).
5. If you must write a benchmark use the benchmark-ips Gem instead of Ruby's Benchmark module.

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:

1. 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 (e.g., the total time spent in a web request).
2. Ask others (preferably in the form of an issue).

Some examples of changes that aren't 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 (e.g. 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:

1. An error won't prevent the request from completing.
2. The process being slow won't affect the loading time of a page.
3. In case of a failure it's easy to re-try the process (Sidekiq takes care of this automatically).
4. By isolating the code from a web request it will hopefully 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 will 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 won't end up retrieving 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

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.

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 this will only allocate the "foo" String once:

10.times do
  'foo'.freeze
end

Blindly adding a .freeze call to every String is an anti-pattern that should be avoided unless one can prove (using production data) the call actually has a positive impact on performance.

This feature of Ruby wasn't really meant to make things faster directly, instead it was meant to reduce the number of allocations. 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 there's no guarantee it will.

Another common flavour of this is to not only freeze a String, but also assign it to a constant, 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'

Moving Allocations to Constants

Storing an object as a constant so you only allocate it once may improve performance, but there's no guarantee this will. 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.