Re: [PATCH 0/2] generic_file_buffered_read improvements

[Dave Chinner <david@fromorbit.com>]

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