umtx: use a distribution-fair multiplier for the chain hashHEADmain

  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 <nprice@FreeBSD.org>

0 files changed, 0 insertions, 0 deletions