On 2014-06-30 19:44:47 +0200, Andres Freund wrote: > On 2014-06-30 13:15:23 -0400, Robert Haas wrote: > > People who write code that uses atomics are not > > likely to think about how those algorithms will actually perform when > > those atomics are merely emulated, and I suspect that means that in > > practice platforms that have only emulated atomics are going to > > regress significantly vs. the status quo today. > > I think you're overstating the likely performance penalty for > nonparallel platforms/workloads here quite a bit. The platforms without > changes of gettings atomics implemented aren't ones where that's a > likely thing. Yes, you'll see a regression if you run a readonly pgbench > over a 4 node NUMA system - but it's not large. You won't see much of > improved performance in comparison to 9.4, but I think that's primarily > it.
To quantify this, on my 2 socket xeon E5520 workstation - which is too small to heavily show the contention problems in pgbench -S - the numbers are: pgbench -M prepared -c 16 -j 16 -T 10 -S (best of 5, noticeably variability) master: 152354.294117 lwlocks-atomics: 170528.784958 lwlocks-atomics-spin: 159416.202578 pgbench -M prepared -c 1 -j 1 -T 10 -S (best of 5, noticeably variability) master: 16273.702191 lwlocks-atomics: 16768.712433 lwlocks-atomics-spin: 16744.913083 So, there really isn't a problem with the emulation. It's not actually that surprising - the absolute number of atomic ops is prety much the same. Where we earlier took a spinlock to increment shared we now still take one. I expect that repeating this on the 4 socket machine will show a large gap between lwlocks-atomics and the other two. The other two will be about the same, with lwlocks-atomics-spin remaining a bit better than master. 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