On 2013-08-19 15:17:44 -0700, Jeff Janes wrote: > On Wed, Aug 7, 2013 at 7:40 AM, Merlin Moncure <mmonc...@gmail.com> wrote: > > > I agree; at least then it's not unambiguously better. if you (in > > effect) swap all contention on allocation from a lwlock to a spinlock > > it's not clear if you're improving things; it would have to be proven > > and I'm trying to keep things simple. > > > > Attached is a scaled down version of the patch that keeps the freelist > > lock but still removes the spinlock during the clock sweep. This > > still hits the major objectives of reducing the chance of scheduling > > out while holding the BufFreelistLock and mitigating the worst case > > impact of doing so if it does happen. An even more scaled down > > version would keep the current logic exactly as is except for > > replacing buffer lock in the clock sweep with a trylock (which is > > IMNSHO a no-brainer). > > Since usage_count is unsigned, are you sure that changing the tests > from "buf->usage_count == 0" to "buf->usage_count <= 0" accomplishes > what you need it to? If usage_count gets decremented when it already > zero, it will wrap around to 65,535, at least on some compilers some > of the time, won't it?
Overflow of *unsigned* variables is actually defined and will always wrap around. It's signed variables which don't have such a clear behaviour. Greetings, Andres Freund -- Andres Freund http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
