Re: [mm] 3484b2de949: -56.2% vm-scalability.throughput, +9.3% turbostat.Pkg_W

[Mel Gorman <mgorman@suse.de>]

On Tue, Aug 19, 2014 at 12:32:52PM +0800, Fengguang Wu wrote:
> Hi Mel,
> 
> We noticed the below vm-scalability performance/power regressions on
> commit 3484b2de9499df23c4604a513b36f96326ae81ad ("mm: rearrange zone
> fields into read-only, page alloc, statistics and page reclaim lines").
> 
> 24b7e5819ad5cbe  3484b2de9499df23c4604a513  testbox/testcase/testparams
> ---------------  -------------------------  ---------------------------
>      %stddev        %change               %stddev
>             \          |                 /
>       9.95 ± 2%     +69.1%      16.83 ± 5%  brickland3/vm-scalability/300s-lru-file-mmap-read
>       2.32 ± 6%    +229.4%       7.63 ± 5%  brickland3/vm-scalability/300s-lru-file-readonce
>      12.27 ± 3%     +99.4%      24.46 ± 5%  TOTAL vm-scalability.stddev
> 
> 24b7e5819ad5cbe  3484b2de9499df23c4604a513  
> ---------------  -------------------------  
>   13882598 ± 0%     -35.8%    8915310 ± 1%  brickland3/vm-scalability/300s-lru-file-mmap-read
>   36379953 ± 1%     -64.0%   13093373 ± 0%  brickland3/vm-scalability/300s-lru-file-readonce
>   50262551 ± 0%     -56.2%   22008683 ± 0%  TOTAL vm-scalability.throughput
> 


What units are these? It's completely unclear what is good and bad from the
figures. 300s-lru-file-mmap-read appears multiple times in this report,
each with different numbers beside them but little clue as to what they
mean or what I'm meant to be looking for :(

This is the same patch that was reported as having a performance gain in
another set of tests from lkp so am a little confused.

More importantly, as this patch is primary abougt cache misses it should
be very unlikely that it makes a noticable difference to IO as the
relative cost of a cache miss is so low. Similarly any difference it
makes to reclaim activity is likely to be a coincidence or due to test
variance.

-- 
Mel Gorman
SUSE Labs