Re: Design notes for BufMgrLock rewrite

On Mon, 2005-02-21 at 18:45 -0500, Tom Lane wrote:
> 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 I/O does alleviate contention to a certain extent, but if you have a
well laid out system that can soak up the I/O you're throwing AND you
have multiple CPUs trying to get at blocks, then you have contention.

The other problem is the OS cache. A PostgreSQL cache miss isn't
necessarily an I/O. If PostgreSQL more easily supported very large
shared_buffers then I would be more in agreement.

> 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.)

Let's talk about Mark's TPC-C like tests. As soon as the cache is full,
the response times go to hell. (see
http://www.osdl.org/projects/dbt2dev/results/dev4-010/264/)
Once the cache is full, each dirty cache miss costs two BufMgrLock
calls. On larger caches, very roughly 80% of the cache is dirty, so the
overall rise in contention is around 1.6 times what it was before. I see
that as a possible indicator of the effects of BufMgrLock contention.

(It does
> reduce contention because ReleaseBuffer doesn't take a global lock
> anymore, and because BufMappingLock and BufFreelistLock are separate
> locks.)

Yes, understood.

> 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.

OK to that.

[and please remember, all, that I'm discussing the very highest end of
performance architecture...]

Best Regards, Simon Riggs