linux-kernel - Re: ext3 IO latency measurements (was: Linux 2.6.29)

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Message-ID: <20090327213806.GG31071@duck.suse.cz>
Date:	Fri, 27 Mar 2009 22:38:06 +0100
From:	Jan Kara <jack@...e.cz>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>, Theodore Tso <tytso@....edu>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Arjan van de Ven <arjan@...radead.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Nick Piggin <npiggin@...e.de>,
	Jens Axboe <jens.axboe@...cle.com>,
	David Rees <drees76@...il.com>, Jesper Krogh <jesper@...gh.cc>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Roland McGrath <roland@...hat.com>
Subject: Re: ext3 IO latency measurements (was: Linux 2.6.29)

On Thu 26-03-09 15:57:25, Andrew Morton wrote:
> On Thu, 26 Mar 2009 15:39:53 -0700 (PDT) Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> 
> > On Thu, 26 Mar 2009, Jan Kara wrote:
> > >
> > >   Reads are measurably better with the patch - the test with cat you
> > > describe below took ~0.5s per file without the patch and always less than
> > > 0.02s with the patch. So it seems to help something.
> > 
> > That would seem to be a _huge_ improvement.
> 
> It's strange that we still don't have an ext3_writepages().  Open a
> transaction, do a large pile of writes, close the transaction again. 
> We don't even have a data=writeback writepages() implementation, which
> should be fairly simple.
  Doable but not fairly simple ;) Firstly you have to restart a transaction
when you've used up all the credits you originally started with (easy),
secondly ext3 uses lock order PageLock -> "transaction start" which is
unusable for the scheme you suggest. So we'd have to revert that - which
needs larger audit of our locking scheme and that's probably the reason
why noone has done it yet.

> Bizarre.
> 
> Mingming had a shot at it a few years ago and I think Badari did as
> well, but I guess it didn't work out.
> 
> Falling back to generic_writepages() on our main local fs is a bit lame.

									Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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