Re: [PATCH 0/7 v1] Speed up page cache truncation

From: Dave Chinner <david@fromorbit.com>
To: Jan Kara <jack@suse.cz>
Cc: Dave Hansen <dave.hansen@intel.com>,
	Andi Kleen <ak@linux.intel.com>,
	linux-mm@kvack.org, Mel Gorman <mgorman@suse.de>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH 0/7 v1] Speed up page cache truncation
Date: Thu, 12 Oct 2017 08:24:01 +1100	[thread overview]
Message-ID: <20171011212401.GM15067@dastard> (raw)
In-Reply-To: <20171011210613.GQ3667@quack2.suse.cz>

On Wed, Oct 11, 2017 at 11:06:13PM +0200, Jan Kara wrote:
> On Wed 11-10-17 10:34:47, Dave Hansen wrote:
> > On 10/11/2017 01:06 AM, Jan Kara wrote:
> > >>> when rebasing our enterprise distro to a newer kernel (from 4.4 to 4.12) we
> > >>> have noticed a regression in bonnie++ benchmark when deleting files.
> > >>> Eventually we have tracked this down to a fact that page cache truncation got
> > >>> slower by about 10%. There were both gains and losses in the above interval of
> > >>> kernels but we have been able to identify that commit 83929372f629 "filemap:
> > >>> prepare find and delete operations for huge pages" caused about 10% regression
> > >>> on its own.
> > >> It's odd that just checking if some pages are huge should be that
> > >> expensive, but ok ..
> > > Yeah, I was surprised as well but profiles were pretty clear on this - part
> > > of the slowdown was caused by loads of page->_compound_head (PageTail()
> > > and page_compound() use that) which we previously didn't have to load at
> > > all, part was in hpage_nr_pages() function and its use.
> > 
> > Well, page->_compound_head is part of the same cacheline as the rest of
> > the page, and the page is surely getting touched during truncation at
> > _some_ point.  The hpage_nr_pages() might cause the cacheline to get
> > loaded earlier than before, but I can't imagine that it's that expensive.
> 
> Then my intuition matches yours ;) but profiles disagree.

Do you get the same benefit across different filesystems?

> That being said
> I'm not really expert in CPU microoptimizations and profiling so feel free
> to gather perf profiles yourself before and after commit 83929372f629 and
> get better explanation of where the cost is - I would be really curious
> what you come up with because the explanation I have disagrees with my
> intuition as well...

When I see this sort of stuff my immediate thought is "what is the
change in the icache footprint of the hot codepath"? There's a
few IO benchmarks (e.g. IOZone) that are l1/l2 cache footprint
sensitive on XFS, and can see up to 10% differences in performance
from kernel build to kernel build that have no code changes in the
IO paths or l1/l2 dcache footprint.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

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