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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
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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