Date: Mon, 26 Sep 2005 14:02:59 -0400 From: John Baldwin <jhb@FreeBSD.org> To: freebsd-amd64@freebsd.org Subject: Re: Patch to use fence instructions Message-ID: <200509261403.00274.jhb@FreeBSD.org> In-Reply-To: <43381EF8.2060308@gneto.com> References: <200509201616.22475.jhb@FreeBSD.org> <200509211507.04755.jhb@FreeBSD.org> <43381EF8.2060308@gneto.com>
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On Monday 26 September 2005 12:16 pm, Martin Nilsson wrote: > John Baldwin wrote: > > On Wednesday 21 September 2005 07:45 am, Martin Nilsson wrote: > >>John Baldwin wrote: > >>>This patch changes the atomic operations and bus space barriers to use > >>>the x86 fence instructions. Please test, thanks! > >>> > >>>http://www.FreeBSD.org/~jhb/patches/amd64_fences.patch > >> > >>What kind of performance improvements can we expect with this patch? > >>Is it worthwile to compare performance on dualcore Pentium D with > >>sysbench before and after this patch? Does it affect threads & mutex > >>performance? > >> > >>Sysbench is a benchmark specially made to determine lowlevel performance > >>important for MySQL and be found here: http://sysbench.sourceforge.net/ > > > > I'm not sure what improvements it would provide (I don't have any amd64 > > hardware to test on anyway). I believe that in some microbenchmarks bde@ > > found that just using lfence or sfence was only about half the cost of > > using the 'lock' prefix. Thus, things like atomic_store_rel (used in > > mutexes) might perform better. > > I have tested the patch but I'm not able to see any difference with the > mutex & threads tests in sysbench. On the other hand I'm not seeing any > regressions either and everything seems to work OK. Can you suggest a > better low-level test? Robert Watson (rwatson@) has a kernel module (or maybe a hack) for benchmarking our in-kernel primitives. Running that might be a good micro-benchmark. -- John Baldwin <jhb@FreeBSD.org> <>< http://www.FreeBSD.org/~jhb/ "Power Users Use the Power to Serve" = http://www.FreeBSD.org
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