Date: Sat, 28 May 2011 21:03:42 +0200 From: Pieter de Goeje <pieter@degoeje.nl> To: svn-src-all@freebsd.org Cc: svn-src-head@freebsd.org, Matthew D Fleming <mdf@freebsd.org>, src-committers@freebsd.org Subject: Re: svn commit: r221853 - in head/sys: dev/md dev/null sys vm Message-ID: <201105282103.43370.pieter@degoeje.nl> In-Reply-To: <201105131848.p4DIm1j7079495@svn.freebsd.org> References: <201105131848.p4DIm1j7079495@svn.freebsd.org>
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On Friday 13 May 2011 20:48:01 Matthew D Fleming wrote: > Author: mdf > Date: Fri May 13 18:48:00 2011 > New Revision: 221853 > URL: http://svn.freebsd.org/changeset/base/221853 > > Log: > Usa a globally visible region of zeros for both /dev/zero and the md > device. There are likely other kernel uses of "blob of zeros" than can > be converted. > > Reviewed by: alc > MFC after: 1 week > This change seems to reduce /dev/zero performance by 68% as measured by this command: dd if=/dev/zero of=/dev/null bs=64k count=100000. x dd-8-stable + dd-9-current +-------------------------------------------------------------------------+ |+ | |+ | |+ | |+ x x| |+ x x x| |A |MA_|| +-------------------------------------------------------------------------+ N Min Max Median Avg Stddev x 5 1.2573578e+10 1.3156063e+10 1.2827355e+10 1.290079e+10 2.4951207e+08 + 5 4.1271391e+09 4.1453925e+09 4.1295157e+09 4.1328097e+09 7487363.6 Difference at 95.0% confidence -8.76798e+09 +/- 2.57431e+08 -67.9647% +/- 1.99547% (Student's t, pooled s = 1.76511e+08) This particular measurement was against 8-stable but the results are the same for -current just before this commit. Basically througput drops from ~13GB/sec to 4GB/sec. Hardware is a Phenom II X4 945 with 8GB of 800Mhz DDR2 memory. FreeBSD/amd64 is installed. This processor has 6MB of L3 cache. To me it looks like it's not able to cache the zeroes anymore. Is this intentional? I tried to change ZERO_REGION_SIZE back to 64K but that didn't help. Regards, Pieter de Goeje
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