Protect against HashMap rehash spikes in UnresolvedSamples #737
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While adding support for in-kernel unwinding with sample rates as high as 4KHz, sample dropping became an issue. Looking closer using a `perf record` of compiler running on 16 CPU, 4KHz samples, rehashing was taking over 260ms in the worst case (see "Stack Chart" and highlighting rehash): https://share.firefox.dev/3Y32wsT This changes so that UnresolvedStacks uses a BTreeMap for the interning storage, which has a much more bounded O(log n) worst case insertion. An LRU hash map serves on top as an O(1) fast path. The new implementation is roughly on par and perhaps slightly faster, and processing time spikes are almost gone: https://share.firefox.dev/4pMM9wy Footnote, the `stacks` array itself also suffers from O(n) realloc behavior, but that doesn't seem to be problematic *yet*, so I'll leave this for when it's found to be a problem in the future.
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While adding support for in-kernel unwinding with sample rates as high as 4KHz, sample dropping became an issue. Looking closer using a
perf recordof compiler running on 16 CPU, 4KHz samples, rehashing was taking over 260ms in the worst case (see "Stack Chart" and highlighting rehash): https://share.firefox.dev/3Y32wsTThis changes so that UnresolvedStacks uses a BTreeMap for the interning storage, which has a much more bounded O(log n) worst case insertion. An LRU hash map serves on top as an O(1) fast path.
The new implementation is roughly on par and perhaps slightly faster, and processing time spikes are almost gone:
https://share.firefox.dev/4pMM9wy
Footnote, the
stacksarray itself also suffers from O(n) realloc behavior, but that doesn't seem to be problematic yet, so I'll leave this for when it's found to be a problem in the future.