47 lines
2.2 KiB
Markdown
47 lines
2.2 KiB
Markdown
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# Switching to Puma
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## Puma
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GitLab plans to use [Puma](https://github.com/puma/puma) to replace
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[Unicorn](https://bogomips.org/unicorn/).
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## Why switch to Puma?
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Puma has a multi-thread architecture which uses less memory than a multi-process
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application server like Unicorn.
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Most Rails applications requests normally include a proportion of I/O wait time.
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During I/O wait time MRI Ruby will release the GVL (Global VM Lock) to other threads.
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Multi-threaded Puma can therefore still serve more requests than a single process.
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## Performance caveat when using Puma with Rugged
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For deployments where NFS is used to store Git repository, we allow GitLab to use
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[Direct Git Access](../gitaly/#direct-git-access-in-gitlab-rails) to improve performance via usage of [Rugged](https://github.com/libgit2/rugged).
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Rugged usage is automatically enabled if Direct Git Access is present, unless it
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is disabled by [feature flags](../../development/gitaly.md#legacy-rugged-code).
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MRI Ruby uses a GVL. This allows MRI Ruby to be multi-threaded, but running at
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most on a single core. Since Rugged can use a thread for long periods of
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time (due to intensive I/O operations of Git access), this can starve other threads
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that might be processing requests. This is not a case for Unicorn or Puma running
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in a single thread mode, as concurrently at most one request is being processed.
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We are actively working on removing Rugged usage. Even though performance without Rugged
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is acceptable today, in some cases it might be still beneficial to run with it.
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Given the caveat of running Rugged with multi-threaded Puma, and acceptable
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performance of Gitaly, we are disabling Rugged usage if Puma multi-threaded is
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used (when Puma is configured to run with more than one thread).
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This default behavior may not be the optimal configuration in some situations. If Rugged
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plays an important role in your deployment, we suggest you benchmark to find the
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optimal configuration:
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- The safest option is to start with single-threaded Puma. When working with
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Rugged, single-threaded Puma does work the same as Unicorn.
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- To force Rugged auto detect with multi-threaded Puma, you can use [feature
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flags](../../development/gitaly.md#legacy-rugged-code).
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