linux-kernel - Re: [PATCH v3 14/16] Gut bio_add

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Date:	Fri, 25 May 2012 14:09:44 -0700
From:	Kent Overstreet <koverstreet@...gle.com>
To:	Mike Snitzer <snitzer@...hat.com>
Cc:	linux-kernel@...r.kernel.org, linux-bcache@...r.kernel.org,
	dm-devel@...hat.com, linux-fsdevel@...r.kernel.org,
	axboe@...nel.dk, yehuda@...newdream.net, mpatocka@...hat.com,
	vgoyal@...hat.com, bharrosh@...asas.com, tj@...nel.org,
	sage@...dream.net, agk@...hat.com, drbd-dev@...ts.linbit.com,
	Dave Chinner <dchinner@...hat.com>, tytso@...gle.com
Subject: Re: [PATCH v3 14/16] Gut bio_add_page()

On Fri, May 25, 2012 at 04:46:51PM -0400, Mike Snitzer wrote:
> I'd love to see the merge_bvec stuff go away but it does serve a
> purpose: filesystems benefit from accurately building up much larger
> bios (based on underlying device limits).  XFS has leveraged this for
> some time and ext4 adopted this (commit bd2d0210cf) because of the
> performance advantage.

That commit only talks about skipping buffer heads, from the patch
description I don't see how merge_bvec_fn would have anything to do with
what it's after.

> So if you don't have a mechanism for the filesystem's IO to have
> accurate understanding of the limits of the device the filesystem is
> built on (merge_bvec was the mechanism) and are leaning on late
> splitting does filesystem performance suffer?

So is the issue that it may take longer for an IO to complete, or is it
CPU utilization/scalability?

If it's the former, we've got a real problem. If it's the latter - it
might be a problem in the interim (I don't expect generic_make_request()
to be splitting bios in the common case long term), but I doubt it's
going to be much of an issue.

> Would be nice to see before and after XFS and ext4 benchmarks against a
> RAID device (level 5 or 6).  I'm especially interested to get Dave
> Chinner's and Ted's insight here.

Yeah.

I can't remember who it was, but Ted knows someone who was able to
benchmark on a 48 core system. I don't think we need numbers from a 48
core machine for these patches, but whatever workloads they were testing
that were problematic CPU wise would be useful to test.
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