linux-kernel - Re: [LKP] [mm] b72fd1470c9: -41.7% perf-profile.cpu-cycles.get_page_from_freelist.__alloc_pages_nodemask.alloc_pages_current.__page_cache_alloc.pagecache_get

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Date:	Thu, 31 Jul 2014 10:32:41 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	Aaron Lu <aaron.lu@...el.com>
Cc:	Stephen Rothwell <sfr@...b.auug.org.au>,
	LKML <linux-kernel@...r.kernel.org>, lkp@...org
Subject: Re: [LKP] [mm] b72fd1470c9: -41.7%
 perf-profile.cpu-cycles.get_page_from_freelist.__alloc_pages_nodemask.alloc_pages_current.__page_cache_alloc.pagecache_get_page

On Thu, Jul 31, 2014 at 05:01:30PM +0800, Aaron Lu wrote:
> On Thu, Jul 31, 2014 at 09:48:32AM +0100, Mel Gorman wrote:
> > On Thu, Jul 31, 2014 at 01:50:35PM +0800, Aaron Lu wrote:
> > > FYI, we noticed the below changes on
> > > 
> > > git://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git master
> > > commit b72fd1470c9735f53485d089aa918dc327a86077 ("mm: rearrange zone fields into read-only, page alloc, statistics and page reclaim lines")
> > > 
> > > test case: lkp-st02/dd-write/5m-11HDD-JBOD-cfq-xfs-10dd
> > > 
> > > e28c951ff01a805  b72fd1470c9735f53485d089a  
> > > ---------------  -------------------------  
> > >       1.06 ~ 6%     -41.7%       0.62 ~ 3%  TOTAL perf-profile.cpu-cycles.get_page_from_freelist.__alloc_pages_nodemask.alloc_pages_current.__page_cache_alloc.pagecache_get_page
> > >       1.34 ~ 2%     -19.8%       1.07 ~ 2%  TOTAL perf-profile.cpu-cycles.__block_write_begin.xfs_vm_write_begin.generic_perform_write.xfs_file_buffered_aio_write.xfs_file_write_iter
> > >       1.19 ~ 5%     -12.1%       1.05 ~ 4%  TOTAL perf-profile.cpu-cycles.copy_from_user_atomic_iovec.iov_iter_copy_from_user_atomic.generic_perform_write.xfs_file_buffered_aio_write.xfs_file_write_iter
> > >       2.78 ~ 1%     -16.3%       2.32 ~ 4%  TOTAL perf-profile.cpu-cycles.__clear_user.read_zero.read_zero.vfs_read.sys_read
> > >   2.96e+09 ~ 4%      -5.2%  2.806e+09 ~ 0%  TOTAL perf-stat.cache-misses
> > >   3.86e+12 ~ 5%      -5.2%  3.658e+12 ~ 1%  TOTAL perf-stat.ref-cycles
> > > 
> > > Legend:
> > > 	~XX%    - stddev percent
> > > 	[+-]XX% - change percent
> > > 
> > 
> > I'm not exactly sure what I'm reading here. I think it is reporting on cpu
> > cycles and cache misses used in various kernel functions. It's not clear what
> > the units are but it looks like percentages of overall cycles spent in the
> > reported functions. That may or may not be good depending on whether there
> > is a higher cost elsewhere pushing the percentages down but that detail
> > is not in the report. It looks like this is reporting that fewer clock
> > cycles are being spent and incurring fewer cache misses. What is the problem?
> 
> LKP does not report problems only, it will also report commits that make
> things better :-)
> 
> From the perf-stat.cache-misses, I think it is indicating your commit
> does something for good.
> 

Hooray! Thanks for the good news :D

-- 
Mel Gorman
SUSE Labs
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