Author: mjg
Date: Sun Apr  8 16:34:10 2018
New Revision: 332285
URL: https://svnweb.freebsd.org/changeset/base/332285

Log:
  locks: tweak backoff a little bit
  
  Previous limits were chosen when locking primitives had spurious lock
  accesses.
  
  Flipping the starting point to 1 (or rather 2 as the first call shifts it)
  provides a modest win when mild contention is seen while not hurting worse
  cases. Tested on a bunch of one, two and four socket old and new systems
  (Westmere, Skylake, Threadreaper and others) by doing concurrent page faults,
  buildkernel/buildworld and other stuff (although not all systems got all the
  tests).
  
  Another thing is the upper limit. It is semi-arbitrarily chosen as it was
  getting out of hand for slightly less small systems (e.g. a 128-thread one).
  
  Note that backoff is fundamentally a speculative bandaid and this change just
  makes it fit a little bit better. It remains completely oblivious to the
  hardware topology or the contention pattern. This is being experimented with.

Modified:
  head/sys/kern/subr_lock.c

Modified: head/sys/kern/subr_lock.c
==============================================================================
--- head/sys/kern/subr_lock.c   Sun Apr  8 16:29:24 2018        (r332284)
+++ head/sys/kern/subr_lock.c   Sun Apr  8 16:34:10 2018        (r332285)
@@ -156,8 +156,10 @@ void
 lock_delay_default_init(struct lock_delay_config *lc)
 {
 
-       lc->base = lock_roundup_2(mp_ncpus) / 4;
-       lc->max = lc->base * 1024;
+       lc->base = 1;
+       lc->max = lock_roundup_2(mp_ncpus) * 256;
+       if (lc->max > 32678)
+               lc->max = 32678;
 }
 
 #ifdef DDB
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