svn commit: r221853 - in head/sys: dev/md dev/null sys vm

[mdf at FreeBSD.org]

On Sat, May 28, 2011 at 12:03 PM, Pieter de Goeje <pieter at degoeje.nl> wrote:
> 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.

Hmm.  I don't have access to my FreeBSD box over the weekend, but I'll
run this on my box when I get back to work.

Meanwhile you could try setting ZERO_REGION_SIZE to PAGE_SIZE and I
think that will restore things to the original performance.

Cheers,
matthew