linux-kernel - Re: 2.6.36 io bring the system to its knees

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Message-ID: <AANLkTim-A7DLOOw4myQU3Lfip+ZEE32F2Ap_PJXuxG6G@mail.gmail.com>
Date:	Fri, 29 Oct 2010 21:33:56 +0600
From:	Aidar Kultayev <the.aidar@...il.com>
To:	"Ted Ts'o" <tytso@....edu>, Pekka Enberg <penberg@...nel.org>,
	Chris Mason <chris.mason@...cle.com>,
	Ingo Molnar <mingo@...e.hu>,
	Aidar Kultayev <the.aidar@...il.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jens Axboe <axboe@...nel.dk>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Nick Piggin <npiggin@...e.de>,
	Arjan van de Ven <arjan@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: 2.6.36 io bring the system to its knees

puling the git now - I will try whatever you throw at me.

On Fri, Oct 29, 2010 at 8:52 PM, Ted Ts'o <tytso@....edu> wrote:
> On Thu, Oct 28, 2010 at 08:57:49PM +0300, Pekka Enberg wrote:
>> Don't we need to call ext4_should_writeback_data() before we drop the
>> lock? It pokes at ->i_mode which needs ->i_mutex AFAICT.
>
> No, it should be fine.  It's not like a file is going to change from
> being a regular file to a directory or vice versa.  :-)
>
> From a quick inspection it looks OK, but I haven't had the time to
> look more closely to be 100% sure, and of course I haven't run it
> through a battery of regression tests.  For normal usage it should be
> fine though.
>
> Aidar, if you'd be willing to try it with this patch applied, and with
> the file system mounted data=writeback, and then let me know what the
> latencytop reports, that would be useful.  I'm fairly sure that fixing
> llseek() probably won't make that much difference, since it will
> probably spread things out to other places, but it would be good to
> make the experiment.
>
> We will probably also need to use the uninitialized bit for protecting
> data from showing up after a crash for extent-based files, and turning
> on data=writeback is a good way to simulate that.  (Sorry, no way
> we're going to make a change like that this merge cycle, but that
> might be something we could do for 2.6.38.)  But I am curious to see
> what are the next things that come up as being problematic after that.
>
> Thanks,
>
>                                        - Ted
>
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