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

0 files changed, 0 insertions, 0 deletions