Re: Design notes for BufMgrLock rewrite

Simon Riggs <simon(at)2ndquadrant(dot)com> writes:
> ...but do you agree with my comments on the lack of scalability in cache
> miss situations?

No. Grabbing a lock during a cache miss is the least of your worries;
you're going to do I/O, or at least a kernel call, so it hardly matters
as long as you're not holding the lock for a time that's long in
comparison to that overhead.

The only test case I've seen that exposes a significant amount of bufmgr
contention is one that involves zero I/O (100% cache hit rate), so that
the fraction of time spent holding the BufMgrLock is a significant part
of the total time. As soon as you move off 100%, the bufmgr isn't the
critical path anymore. So I think the fact that this redesign is able
to reduce the contention at all in that case is just gravy. (It does
reduce contention because ReleaseBuffer doesn't take a global lock
anymore, and because BufMappingLock and BufFreelistLock are separate
locks.)

If testing shows that we still have contention issues with this design
then we can try subdividing the BufFreelistLock --- but right now my
guess is that we'd just be giving up more cache management efficiency
in return for not much.

regards, tom lane