Nick Price
ee44a57e34
umtxq_hash() multiplies the key by 0x9E370001 and keeps the high bits. That constant is 0x9E37 * 2^16 + 1, so it degenerates for keys whose spacing carries trailing zero bits: at a 64 KiB stride it puts 128 of 512 parked waiters onto a single chain mutex, and at 16 KiB and up it uses only a handful of the 512 chains. Base-system consumers never hit this because libthr places its own wait words 128 bytes apart, but a Linux-ABI runtime waiting on addresses it allocates itself lands squarely on the floor. Switch to 0x61C88647, which leaves at most 3 waiters per chain at the same stride; Linux made this exact change in 2016, after judging the sparse constants "actively bad for hashing". Approved by: adrian (mentor) Reviewed by: kib, adrian, emaste Differential Revision: https://reviews.freebsd.org/D58337 Signed-off-by: Nick Price <[email protected]>
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