--- stage: none group: unassigned info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#assignments --- # Performance Guidelines This document describes various guidelines to follow to ensure good and consistent performance of GitLab. Refer to the [Index](#performance-documentation) section below to navigate to Performance-related pages. ## Performance Documentation - General: - [Solving performance issues](#workflow) - [Handbook performance page](https://about.gitlab.com/handbook/engineering/performance/) - [Merge request performance guidelines](../development/merge_request_performance_guidelines.md) - Backend: - [Tooling](#tooling) - Database: - [Query performance guidelines](../development/query_performance.md) - [Pagination performance guidelines](../development/database/pagination_performance_guidelines.md) - [Keyset pagination performance](../development/database/keyset_pagination.md#performance) - [Troubleshooting import/export performance issues](../development/import_export.md#troubleshooting-performance-issues) - [Pipelines performance in the `gitlab` project](../development/pipelines.md#performance) - Frontend: - [Performance guidelines](../development/fe_guide/performance.md) - [Performance dashboards and monitoring guidelines](../development/new_fe_guide/development/performance.md) - [Browser performance testing guidelines](../user/project/merge_requests/browser_performance_testing.md) - [`gdk measure` and `gdk measure-workflow`](https://gitlab.com/gitlab-org/gitlab-development-kit/-/blob/main/doc/gdk_commands.md#measure-performance) - QA: - [Load performance testing](../user/project/merge_requests/load_performance_testing.md) - [GitLab Performance Tool project](https://gitlab.com/gitlab-org/quality/performance) - [Review apps performance metrics](../development/testing_guide/review_apps.md#performance-metrics) - Monitoring & Overview: - [GitLab performance monitoring](../administration/monitoring/performance/index.md) - [Development department performance indicators](https://about.gitlab.com/handbook/engineering/development/performance-indicators/) - [Service measurement](../development/service_measurement.md) - Self-managed administration and customer-focused: - [File system performance benchmarking](../administration/operations/filesystem_benchmarking.md) - [Sidekiq performance troubleshooting](../administration/troubleshooting/sidekiq.md) ## Workflow The process of solving performance problems is roughly as follows: 1. 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](https://gitlab.com/gitlab-org/gitlab-foss/-/issues/15607) for an example. 1. Measure the performance of the code in a production environment such as GitLab.com (see the [Tooling](#tooling) section below). Performance should be measured over a period of _at least_ 24 hours. 1. Add your findings based on the measurement period (screenshots of graphs, timings, etc) to the issue mentioned in step 1. 1. Solve the problem. 1. Create a merge request, assign the "Performance" label and follow the [performance review process](merge_request_performance_guidelines.md). 1. 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. 1. 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](profiling.md). - [Distributed Tracing](distributed_tracing.md) - [GitLab Performance Monitoring](../administration/monitoring/performance/index.md). - [QueryRecoder](query_recorder.md) for preventing `N+1` regressions. - [Chaos endpoints](chaos_endpoints.md) for testing failure scenarios. Intended mainly for testing availability. - [Service measurement](service_measurement.md) for measuring and logging service execution. GitLab team members can use [GitLab.com's performance monitoring systems](https://about.gitlab.com/handbook/engineering/monitoring/) located at [`dashboards.gitlab.net`](https://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](https://github.com/evanphx/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: ```ruby 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`](https://github.com/evanphx/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`](https://github.com/michaelherold/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](https://github.com/tmm1/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: ```ruby StackProf.run(mode: :wall, out: 'tmp/stackprof-profiling.dump') do #... end ``` This creates a `.dump` file that you can [read](#reading-a-stackprof-profile). For all available options, see the [Stackprof documentation](https://github.com/tmm1/stackprof#all-options). ### Performance bar With the [Performance bar](../administration/monitoring/performance/performance_bar.md), you have the option to profile a request using Stackprof and immediately output the results to a [Speedscope flamegraph](profiling.md#speedscope-flamegraphs). ### 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: ```shell $ 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 to `false`. - `STACKPROF_MODE`: See [sampling modes](https://github.com/tmm1/stackprof#sampling). Defaults to `cpu`. - `STACKPROF_INTERVAL`: Sampling interval. Unit semantics depend on `STACKPROF_MODE`. For `object` mode this is a per-event interval (every `nth` event is sampled) and defaults to `100`. For other modes such as `cpu` this is a frequency interval and defaults to `10100` μs (99hz). - `STACKPROF_FILE_PREFIX`: File path prefix where profiles are stored. Defaults to `$TMPDIR` (often corresponds to `/tmp`). - `STACKPROF_TIMEOUT_S`: Profiling timeout in seconds. Profiling will automatically stop after this time has elapsed. Defaults to `30`. - `STACKPROF_RAW`: Whether to collect raw samples or only aggregates. Raw samples are needed to generate flame graphs, but they do have a higher memory and disk overhead. Defaults to `true`. Once enabled, profiling can be triggered by sending a `SIGUSR2` signal to the Ruby process. The process begins sampling stacks. Profiling can be stopped by sending another `SIGUSR2`. Alternatively, it stops automatically after the timeout. Once profiling stops, the profile is written out to disk at `$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](#reading-a-stackprof-profile). 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: ```shell 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](https://kcachegrind.github.io/): ```shell 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: ```shell # 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](https://github.com/brendangregg/FlameGraph): ```shell stackprof --stackcollapse /tmp/group_member_policy_spec.rb.dump | flamegraph.pl > flamegraph.svg ``` It's also possible to view flame graphs through [speedscope](https://github.com/jlfwong/speedscope). You can do this when using the [performance bar](profiling.md#speedscope-flamegraphs) and when [profiling code blocks](https://github.com/jlfwong/speedscope/wiki/Importing-from-stackprof-(ruby)). This option isn't supported by `bin/rspec-stackprof`. You can profile speciific methods by using `--method method_name`: ```shell $ 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 95 ( 16.4%) block in 86 ( 14.9%) block in 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`](https://github.com/foraker/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: ```shell 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: ```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](https://gitlab-org.gitlab.io/rspec_profiling_stats/). 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](https://gitlab.com/gitlab-org/gitlab/-/issues/296530). This patch is available by default for [Omnibus](https://gitlab.com/gitlab-org/omnibus-gitlab/-/merge_requests/4948), [CNG](https://gitlab.com/gitlab-org/build/CNG/-/merge_requests/591), [GitLab CI](https://gitlab.com/gitlab-org/gitlab-build-images/-/merge_requests/355), [GCK](https://gitlab.com/gitlab-org/gitlab-compose-kit/-/merge_requests/149) and can additionally be enabled for [GDK](https://gitlab.com/gitlab-org/gitlab-development-kit/-/blob/main/doc/advanced.md#apply-custom-patches-for-ruby). 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 by `malloc` for objects that did not fit into an existing object slot. - `mem_objects`: the number of objects allocated. - `mem_mallocs`: the number of `malloc` 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](https://log.gprd.gitlab.net/goto/3a9678bb595e3f89a0c7b5c61bcc47b9). #### Checking memory pressure of own code There are two ways of measuring your own code: 1. Review `api_json.log`, `development_json.log`, `sidekiq.log` that includes memory allocation counters. 1. Use `Gitlab::Memory::Instrumentation.with_memory_allocations` for a given code block and log it. 1. Use [Measuring module](service_measurement.md) ```json {"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` of `mem_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: ```ruby 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` of `mem_objects`, as strings are created dynamically. Each of them will not allocate additional memory, as they fit into Ruby slot of 40 bytes: ```ruby 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` of `mem_objects`, as strings are created dynamically. Each of them will allocate additional memory as strings are larger than Ruby slot of 40 bytes: ```ruby 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: ```ruby 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: ```ruby 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`](https://github.com/SamSaffron/memory_profiler) gem is already present in the GitLab `Gemfile`. It's also available in the [performance bar](../administration/monitoring/performance/performance_bar.md) for the current URL. To use the memory profiler directly in your code, use `require` to add it: ```ruby 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](https://tenderlovemaking.com/2017/09/27/visualizing-your-ruby-heap.html). 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: ```ruby bundle exec rbtrace -p -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](https://gist.github.com/tenderlove/f28373d56fdd03d8b514af7191611b88) or similar: ```shell ruby heapviz.rb heap.json ``` Fragmented Ruby heap snapshot could look like this: ![Ruby heap fragmentation](img/memory_ruby_heap_fragmentation.png) Memory fragmentation could be reduced by tuning GC parameters [as described in this post](https://www.speedshop.co/2017/12/04/malloc-doubles-ruby-memory.html). 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](https://github.com/zombocom/derailed_benchmarks) 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.](https://gitlab.com/gitlab-org/gitlab/-/jobs/2144695684).) You may find the results: - On the merge request **Overview** tab, in the merge request reports area, in the **Metrics Reports** [dropdown list](../ci/metrics_reports.md). - 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](https://github.com/zombocom/derailed_benchmarks#running-derailed-exec). 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](https://github.com/zombocom/derailed_benchmarks#running-in-production-locally) on the gem page. You must do similar for Redis configurations to avoid errors. - GCK includes `production` configuration sections [out of the box](https://gitlab.com/gitlab-org/gitlab-compose-kit#running-production-like). ## 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 (for example, the total time spent in a web request). 1. 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](#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: 1. An error doesn't prevent the request from completing. 1. The process being slow doesn't affect the loading time of a page. 1. In case of a failure you can retry the process (Sidekiq takes care of this automatically). 1. 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: ```ruby if repository.exists? repository.branch_names.each do |name| ... end end ``` You can just write: ```ruby 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: ```ruby 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: ```ruby 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](#using-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: ```ruby # 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: ```ruby 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](https://gitlab.com/gitlab-org/gitlab/-/tree/master/lib/banzai), 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: ```shell bin/rake benchmark:banzai ``` This command generates output like this: ```plaintext --> 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`](https://gitlab.com/gitlab-org/gitlab/-/blob/master/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: ```ruby 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): ```shell $ ps -o rss -p RSS 783436 ``` And here is an excerpt of what the garbage collector was doing: ```ruby 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: ```ruby File.new('file').each { |line| puts line } ``` Alternatively, you can read individual lines explicitly using `IO.readline` or `IO.gets` functions: ```ruby 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](https://en.wikipedia.org/wiki/Anti-pattern) 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: ```ruby 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: ```ruby 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](mass_insert.md) functionality. Assuming you are working with ActiveRecord models, you might also find these links helpful: - [Insert records in batches](insert_into_tables_in_batches.md) - [BulkInsert gem](https://github.com/jamis/bulk_insert) - [ActiveRecord::PgGenerateSeries gem](https://github.com/ryu39/active_record-pg_generate_series) ### Examples You may find some useful examples in [this snippet](https://gitlab.com/gitlab-org/gitlab-foss/snippets/33946).