Date: Tue, 17 Apr 2007 17:50:37 +0100 (BST) From: Robert Watson <rwatson@FreeBSD.org> To: Kevin Way <kevin@insidesystems.net> Cc: freebsd-performance@freebsd.org Subject: Re: Anyone interested in improving postgresql scaling? Message-ID: <20070417174825.X42234@fledge.watson.org> In-Reply-To: <461B0CD0.8090404@insidesystems.net> References: <461B0CD0.8090404@insidesystems.net>
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On Tue, 10 Apr 2007, Kevin Way wrote: > Kris Kennaway wrote: >> If so, then your task is the following: >> >> Make SYSV semaphores less dumb about process wakeups. Currently whenever >> the semaphore state changes, all processes sleeping on the semaphore are >> woken, even if we only have released enough resources for one waiting >> process to claim. i.e. there is a thundering herd wakeup situation which >> destroys performance at high loads. Fixing this will involve replacing the >> wakeup() calls with appropriate amounts of wakeup_one(). > Could this cause problem cause a situation where an 8-Core system was 50-75% > slower than an otherwise equivalent 2-Core system? > > I have a graph of my sysbench/pgsql results here: > > http://blog.insidesystems.net/files/sysctl-pgsq-amd64-wtf.png > > As the graph shows, the 8-core system is about half the speed of the 2-core > system at 2 simultaneous threads, and it decays down to approximately 1/4 > the speed of the 2-core system as the # of threads hits 5. > > All other (non-pgsql, non-sysv) tests came back approximately as expected, > but I'm left wondering if I did something wrong, or if 8 cpus are slower > than 2, when it comes to Postgres on currently available FreeBSD. > > Kevin Way Inside Systems, Inc. > > (Full detail of what I did is available at: > http://blog.insidesystems.net/articles/2007/04/09/what-did-i-do-wrong ) Would you be able to re-run these tests trying a recent 7.x kernel? If so, make sure you build a non-debugging kernel, and try two variants: one with SCHED_4BSD, and one with SCHED_ULE. Another experiment that would be interesting is to try using device.hints to disable various numbers of CPUs on your 8-core system, and see how the performance graph changes as the number of enabled CPUs changes. Robert N M Watson Computer Laboratory University of Cambridge
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