Kernel memory allocator change (cvs commit: src/sys/vm uma_core.c uma_int.h (fwd)

[Robert Watson]

I've finally committed the UMA critical section per-cpu cache 
synchronization change to the CVS HEAD.  Two notes:

- This is intended to make things slightly faster.  If you notice a
   consistent slow-down, ideally characterizable with a highly
   reproduceable benchmark, I'd like to hear about it.  "Yes, it did get
   faster" is also good, as is "it didn't get slower".

- We've done quite a bit of testing with these patches in place.  However,
   that doesn't preclude a stability proble.  For those running regular
   stability testing on HEAD, running with this change would be a good
   thing.

Thanks,

Robert N M Watson

---------- Forwarded message ----------
Date: Fri, 29 Apr 2005 18:56:36 +0000 (UTC)
From: Robert Watson <rwatson at FreeBSD.org>
To: src-committers at FreeBSD.org, cvs-src at FreeBSD.org, cvs-all at FreeBSD.org
Subject: cvs commit: src/sys/vm uma_core.c uma_int.h

rwatson     2005-04-29 18:56:36 UTC

   FreeBSD src repository

   Modified files:
     sys/vm               uma_core.c uma_int.h
   Log:
   Modify UMA to use critical sections to protect per-CPU caches, rather than
   mutexes, which offers lower overhead on both UP and SMP.  When allocating
   from or freeing to the per-cpu cache, without INVARIANTS enabled, we now
   no longer perform any mutex operations, which offers a 1%-3% performance
   improvement in a variety of micro-benchmarks.  We rely on critical
   sections to prevent (a) preemption resulting in reentrant access to UMA on
   a single CPU, and (b) migration of the thread during access.  In the event
   we need to go back to the zone for a new bucket, we release the critical
   section to acquire the global zone mutex, and must re-acquire the critical
   section and re-evaluate which cache we are accessing in case migration has
   occured, or circumstances have changed in the current cache.

   Per-CPU cache statistics are now gathered lock-free by the sysctl, which
   can result in small races in statistics reporting for caches.

   Reviewed by:    bmilekic, jeff (somewhat)
   Tested by:      rwatson, kris, gnn, scottl, mike at sentex dot net, others

   Revision  Changes    Path
   1.119     +120 -103  src/sys/vm/uma_core.c
   1.30      +0 -10     src/sys/vm/uma_int.h