Date: Tue, 09 May 2006 19:38:38 -0600 From: Scott Long <scottl@samsco.org> To: Sven Petai <hadara@bsd.ee> Cc: Julian Elischer <julian@elischer.org>, freebsd-performance@freebsd.org, current@freebsd.org, David Xu <davidxu@freebsd.org>, Kris Kennaway <kris@obsecurity.org> Subject: Re: Fine-grained locking for POSIX local sockets (UNIX domain sockets ) Message-ID: <4461441E.60304@samsco.org> In-Reply-To: <200605100418.54475.hadara@bsd.ee> References: <20060506150622.C17611@fledge.watson.org> <200605091818.12676.hadara@bsd.ee> <4460D2CA.5090808@elischer.org> <200605100418.54475.hadara@bsd.ee>
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Sven Petai wrote: > On Tuesday 09 May 2006 20:35, Julian Elischer wrote: > >>Sven Petai wrote: >> >>are there any patches that take the gettimeofday() calls and replace >>them with something that is cheap >>such as only doing every 10th one and just returning the last value ++ 1 >>uSec for the other ones.. >> >>a ktrace of Mysql shows a LOT of gettimeofday() calls. >> > > > well I have actually done that although in a very hackish way that > is suitable *only* for benchmarking, but my goal really was > just to find out if gettimeofday really is the bottleneck. > > Basically I just preloaded my version of "buffering" time() over libc's. > That function asked kernel for time only after every 5000 invocations. > I could get away with that for benchmarking since > about 99,99% of gettimeofday() calls from mysqld really come through > time() which has anyway only 1 second resolution and mysqld seems to use > the value for purposes that are not really all that critical, like > stats and safeguard timers. > It's also called far more than 5000 times in a second while benchmark is > running so it should return correct values too. > > Anyway, the results really astonished me, what I got for reducing > gettimeofday() calls by ~96% was performance *decrease* of about 4-5%. > I thought something was wrong with my measurement techniques, so I > wrote the buffering time() function that used TSC directly into mysqld, > replaced all the time() calls with that, so that number of gettimeofday() > syscalls ktrace reported was down to only the initial one, but still got the > same 5% decrease. > > I just can't explain it, but can't find any mistakes in methods too. > > The library I used is available @ > http://bsd.ee/~hadara/debug/mysql4/time_lib_hack.c Were you testing on SMP, and if so, was Hyperthreading enabled? Scott
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