Shrink compression codec buffers to prevent memory bloat#184
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CompPacketCodec's in_buf and out_buf grow to accommodate large packets but never shrink, effectively leaking memory for the connection lifetime. With 20+ compressed connections this wastes 200-500 MB of RSS. After each decode/encode cycle, if a buffer is fully drained and its capacity exceeds 4 MiB, replace it with a fresh BytesMut::new(). This is zero-cost for normal-sized packets and only triggers for genuinely bloated buffers. Relates to blackbeam/mysql_async#349
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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Problem
CompPacketCodec'sin_bufandout_buf(BytesMut) grow to accommodate large compressed/decompressed packets but never shrink back. Over time every compressed connection accumulates megabytes of idle capacity in these buffers. With 20+ compressed connections this easily wastes 200–500 MB of RSS.Relates to blackbeam/mysql_async#349.
Fix
After each decode/encode cycle, if a buffer is fully drained (
is_empty()) and its allocated capacity exceeds a 4 MiB threshold, replace it with a freshBytesMut::new().This is:
Production status
We've been running an equivalent patch (with a 1 MiB threshold) in production for several months against MySQL 5.1.53 with compression enabled, ~20 concurrent connections. Memory usage dropped from ~400 MB to ~40 MB with no issues.
Open to feedback
If you'd prefer this threshold to be configurable (e.g. via a field on
CompPacketCodecor a connection option), happy to rework. The hardcoded constant felt like the least invasive starting point.