Sourced from regex's changelog.

1.9.5 (2023-09-02)

This is a patch release that hopefully mostly fixes a performance bug that occurs when sharing a regex across multiple threads.

Issue #934 explains this in more detail. It is also noted in the crate documentation. The bug can appear when sharing a regex across multiple threads simultaneously, as might be the case when using a regex from a OnceLock, lazy_static or similar primitive. Usually high contention only results when using many threads to execute searches on small haystacks.

One can avoid the contention problem entirely through one of two methods. The first is to use lower level APIs from regex-automata that require passing state explicitly, such as meta::Regex::search_with. The second is to clone a regex and send it to other threads explicitly. This will not use any additional memory usage compared to sharing the regex. The only downside of this approach is that it may be less convenient, for example, it won't work with things like OnceLock or lazy_static or once_cell.

With that said, as of this release, the contention performance problems have been greatly reduced. This was achieved by changing the free-list so that it was sharded across threads, and that ensuring each sharded mutex occupies a single cache line to mitigate false sharing. So while contention may still impact performance in some cases, it should be a lot better now.

Because of the changes to how the free-list works, please report any issues you find with this release. That not only includes search time regressions but also significant regressions in memory usage. Reporting improvements is also welcome as well! If possible, provide a reproduction.

Bug fixes:

• [BUG #934](rust-lang/regex#934): Fix a performance bug where high contention on a single regex led to massive slow downs.

• 554469b 1.9.5
• 48e09a8 deps: bump regex-automata to 0.3.8
• 894dcbe regex-automata-0.3.8
• 135e11b changelog: 1.9.5
• f578d74 automata: reduce regex contention somewhat
• 9a505a1 deps: bump to memchr 2.6
• 15cdc64 cli: remove use of deprecated API
• 329c6a3 ci: use dtolnay@master instead of @​v1
• 2637e11 ci: remove stale comment
• See full diff in compare view

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