From: Dave Chinner <david@fromorbit.com>
To: Kent Overstreet <kent.overstreet@gmail.com>
Cc: Carlos Maiolino <cmaiolino@redhat.com>,
linux-fsdevel@vger.kernel.org, viro@zeniv.linux.org.uk
Subject: Re: [PATCH 0/2] generic_file_buffered_read improvements
Date: Sat, 18 Aug 2018 08:42:00 +1000 [thread overview]
Message-ID: <20180817224159.GE31495@dastard> (raw)
In-Reply-To: <20180816100531.GC24640@kmo-pixel>
On Thu, Aug 16, 2018 at 06:05:31AM -0400, Kent Overstreet wrote:
> On Thu, Aug 16, 2018 at 09:57:40AM +0200, Carlos Maiolino wrote:
> > On Wed, Aug 15, 2018 at 07:26:30PM -0400, Kent Overstreet wrote:
> > > Small patch series to
> > > - firstly, refactor generic_file_buffered_read enough that it can be modified
> > > in more interesting ways without going insane, and then
> > >
> > > - secondly, change it to use find_get_pages_contig() to batch up the page
> > > operations, and then copy data to userspace in a separate loop that touches
> > > no other shared cachelines.
> > >
> > > I've been seeing profiles where the radix tree lookups in the buffered read path
> > > are a shockingly large portion of the profile (around 25%, if memory serves) -
> > > that's what this patch series is addressing. I've benchmarked small block reads
> > > as well, performance there is unaffected or slightly improved (it's within the
> > > margin of error).
> > >
> >
> > /me didn't review the patches, but...
> >
> > Could you share how you benchmarked it? Despite the fact I'm curious about it,
> > it's going to be interesting the 'proof' of such improvement.
>
> I tried coming up with a microbenchmark and gave up because it was getting too
> ridiculous - you need something _modifying_ the page cache radix tree for the
> contention to show up. That's usually going to be page reclaim, which means you
> can't just populate the page cache and run your benchmark, you have to be
> reading stuff in and evicting stuff. Which means you need waaay higher end IO
> devices than what I have at home for it to show up.
POSIX_FADV_DONTNEED will remove clean pages from the radix tree on
command. So essentially you could just hammer on a single file from
lots of CPUs with buffered reads and POSIX_FADV_DONTNEED to stress
the radix tree.
Cheers,
Dave.
--
Dave Chinner
david@fromorbit.com
prev parent reply other threads:[~2018-08-18 1:47 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-08-15 23:26 [PATCH 0/2] generic_file_buffered_read improvements Kent Overstreet
2018-08-15 23:26 ` [PATCH 1/2] fs: Break generic_file_buffered_read up into multiple functions Kent Overstreet
2018-08-15 23:26 ` [PATCH 2/2] fs: generic_file_buffered_read() now uses find_get_pages_contig Kent Overstreet
2018-08-16 14:56 ` kbuild test robot
2018-08-16 7:57 ` [PATCH 0/2] generic_file_buffered_read improvements Carlos Maiolino
2018-08-16 10:05 ` Kent Overstreet
2018-08-17 22:42 ` Dave Chinner [this message]
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