Add PREK_MAX_CONCURRENCY environment variable by drichardson · Pull Request #1697 · j178/prek

drichardson · 2026-02-27T00:56:04Z

Summary

Add PREK_MAX_CONCURRENCY environment variable to cap the maximum number of concurrent hooks
Value is floored at 1; values above CPU count are allowed (useful for I/O-bound hooks)
Invalid values show a user-visible warning and fall back to CPU count
PREK_NO_CONCURRENCY takes precedence when both are set

Motivation

When ulimit -n is low, concurrent hook execution can exhaust file descriptors. This provides an environment variable to limit concurrency.

For #1696

Test plan

mise run lint — no warnings
mise run test — all tests pass
Unit tests for resolve_concurrency: valid value, above CPU count, zero floors to 1, invalid string, empty string, unset, no_concurrency, no_concurrency overrides max

prek-ci-bot · 2026-02-27T01:03:17Z

📦 Cargo Bloat Comparison

Binary size change: -0.42% (23.9 MiB → 23.8 MiB)

Expand for cargo-bloat output

Head Branch Results

 File  .text    Size             Crate Name
 0.3%   0.8% 80.2KiB             prek? <prek::cli::Command as clap_builder::derive::Subcommand>::augment_subcommands
 0.3%   0.7% 71.3KiB              prek prek::languages::<impl prek::config::Language>::run::{{closure}}::{{closure}}
 0.3%   0.6% 65.6KiB              prek prek::languages::<impl prek::config::Language>::run::{{closure}}::{{closure}}
 0.2%   0.5% 51.2KiB annotate_snippets annotate_snippets::renderer::render::render
 0.2%   0.5% 50.5KiB              prek prek::languages::<impl prek::config::Language>::install::{{closure}}
 0.2%   0.4% 41.2KiB              prek prek::cli::run::run::run::{{closure}}
 0.2%   0.4% 39.2KiB              prek prek::run::{{closure}}
 0.1%   0.3% 31.1KiB             prek? <prek::cli::RunArgs as clap_builder::derive::Args>::augment_args
 0.1%   0.3% 28.5KiB      serde_saphyr saphyr_parser_bw::scanner::Scanner<T>::fetch_more_tokens
 0.1%   0.2% 24.7KiB             prek? <prek::config::_::<impl serde_core::de::Deserialize for prek::config::Config>::deserialize::__Visitor as serde_core::de::Visitor>::visit_map
 0.1%   0.2% 22.8KiB      serde_saphyr saphyr_parser_bw::scanner::Scanner<T>::fetch_more_tokens
 0.1%   0.2% 22.7KiB              prek <prek::languages::ruby::ruby::Ruby as prek::languages::LanguageImpl>::install::{{closure}}
 0.1%   0.2% 22.5KiB              prek prek::hooks::meta_hooks::MetaHooks::run::{{closure}}
 0.1%   0.2% 21.1KiB      clap_builder clap_builder::parser::parser::Parser::get_matches_with
 0.1%   0.2% 20.5KiB              prek prek::hooks::meta_hooks::MetaHooks::run::{{closure}}
 0.1%   0.2% 20.0KiB               std core::ptr::drop_in_place<prek::languages::<impl prek::config::Language>::install::{{closure}}>
 0.1%   0.2% 20.0KiB   cargo_metadata? <cargo_metadata::_::<impl serde_core::de::Deserialize for cargo_metadata::Package>::deserialize::__Visitor as serde_core::de::Visitor>::visit_map
 0.1%   0.2% 19.5KiB              prek prek::archive::unzip::{{closure}}
 0.1%   0.2% 18.7KiB     serde_saphyr? <serde_saphyr::de::YamlDeserializer as serde_core::de::Deserializer>::deserialize_map
 0.1%   0.2% 18.6KiB     serde_saphyr? <serde_saphyr::de::YamlDeserializer as serde_core::de::Deserializer>::deserialize_map
38.3%  91.7%  9.1MiB                   And 21095 smaller methods. Use -n N to show more.
41.8% 100.0% 10.0MiB                   .text section size, the file size is 23.8MiB

Base Branch Results

 File  .text    Size             Crate Name
 0.3%   0.7% 71.3KiB              prek prek::languages::<impl prek::config::Language>::run::{{closure}}::{{closure}}
 0.3%   0.7% 66.4KiB             prek? <prek::cli::Command as clap_builder::derive::Subcommand>::augment_subcommands
 0.3%   0.6% 65.6KiB              prek prek::languages::<impl prek::config::Language>::run::{{closure}}::{{closure}}
 0.2%   0.5% 51.2KiB annotate_snippets annotate_snippets::renderer::render::render
 0.2%   0.5% 50.5KiB              prek prek::languages::<impl prek::config::Language>::install::{{closure}}
 0.2%   0.4% 41.2KiB              prek prek::cli::run::run::run::{{closure}}
 0.2%   0.4% 38.8KiB              prek prek::run::{{closure}}
 0.1%   0.3% 32.0KiB             prek? <prek::cli::RunArgs as clap_builder::derive::Args>::augment_args
 0.1%   0.3% 28.5KiB      serde_saphyr saphyr_parser_bw::scanner::Scanner<T>::fetch_more_tokens
 0.1%   0.2% 24.7KiB             prek? <prek::config::_::<impl serde_core::de::Deserialize for prek::config::Config>::deserialize::__Visitor as serde_core::de::Visitor>::visit_map
 0.1%   0.2% 22.8KiB      serde_saphyr saphyr_parser_bw::scanner::Scanner<T>::fetch_more_tokens
 0.1%   0.2% 22.7KiB              prek <prek::languages::ruby::ruby::Ruby as prek::languages::LanguageImpl>::install::{{closure}}
 0.1%   0.2% 22.7KiB              prek prek::hooks::meta_hooks::MetaHooks::run::{{closure}}
 0.1%   0.2% 22.4KiB              prek prek::hooks::meta_hooks::MetaHooks::run::{{closure}}
 0.1%   0.2% 21.1KiB      clap_builder clap_builder::parser::parser::Parser::get_matches_with
 0.1%   0.2% 20.0KiB               std core::ptr::drop_in_place<prek::languages::<impl prek::config::Language>::install::{{closure}}>
 0.1%   0.2% 19.5KiB              prek prek::archive::unzip::{{closure}}
 0.1%   0.2% 19.4KiB   cargo_metadata? <cargo_metadata::_::<impl serde_core::de::Deserialize for cargo_metadata::Package>::deserialize::__Visitor as serde_core::de::Visitor>::visit_map
 0.1%   0.2% 18.7KiB     serde_saphyr? <serde_saphyr::de::YamlDeserializer as serde_core::de::Deserializer>::deserialize_map
 0.1%   0.2% 18.6KiB     serde_saphyr? <serde_saphyr::de::YamlDeserializer as serde_core::de::Deserializer>::deserialize_map
38.3%  91.8%  9.1MiB                   And 21088 smaller methods. Use -n N to show more.
41.8% 100.0% 10.0MiB                   .text section size, the file size is 23.9MiB

Allow users to cap the maximum number of concurrent hooks via the PREK_MAX_CONCURRENCY environment variable. The value is clamped between 1 and the available CPU core count. This is useful when ulimit -n is low and concurrent hook execution can exhaust file descriptors. Unlike ulimit, an environment variable can be set in CI configs, .env files, and wrapper scripts without requiring shell builtins. Closes j178#1696 Assisted by AI Co-Authored-By: Claude <[email protected]>

codecov · 2026-02-27T02:39:06Z

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.59%. Comparing base (f7b0c00) to head (735bf9b).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1697      +/-   ##
==========================================
+ Coverage   91.57%   91.59%   +0.01%     
==========================================
  Files          96       96              
  Lines       18842    18880      +38     
==========================================
+ Hits        17255    17293      +38     
  Misses       1587     1587

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j178 · 2026-02-27T04:01:49Z

Thanks!

drichardson force-pushed the add-prek-max-concurrency branch 10 times, most recently from 3318291 to eae84c2 Compare February 27, 2026 01:28

drichardson force-pushed the add-prek-max-concurrency branch from eae84c2 to 735bf9b Compare February 27, 2026 01:34

drichardson marked this pull request as ready for review February 27, 2026 01:44

drichardson requested a review from j178 as a code owner February 27, 2026 01:44

j178 merged commit 91a1fae into j178:master Feb 27, 2026
49 checks passed

drichardson mentioned this pull request Feb 27, 2026

priority: parallel hooks will end-up triggering Too many open files (os error 24) #1696

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BrewTestBot mentioned this pull request Feb 28, 2026

prek 0.3.4 Homebrew/homebrew-core#269872

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Add PREK_MAX_CONCURRENCY environment variable#1697

Add PREK_MAX_CONCURRENCY environment variable#1697
j178 merged 1 commit intoj178:masterfrom
drichardson:add-prek-max-concurrency

drichardson commented Feb 27, 2026 •

edited by j178

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Summary

Motivation

Test plan

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drichardson commented Feb 27, 2026 •

edited by j178

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