linux-kernel - Re: [RFC PATCH] block: Fix bio merge induced high I/O latency

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Message-ID: <20090119182336.GS30821@kernel.dk>
Date:	Mon, 19 Jan 2009 19:23:37 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Nikanth K <nikanth@...il.com>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Andrea Arcangeli <andrea@...e.de>, akpm@...ux-foundation.org,
	Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, ltt-dev@...ts.casi.polymtl.ca
Subject: Re: [RFC PATCH] block: Fix bio merge induced high I/O latency

On Mon, Jan 19 2009, Nikanth K wrote:
> On Sun, Jan 18, 2009 at 12:34 AM, Jens Axboe <jens.axboe@...cle.com> wrote:
> 
> >
> > As a quick test, could you try and increase the slice_idle to eg 20ms?
> > Sometimes I've seen timing being slightly off, which makes us miss the
> > sync window for the ls (in your case) process. Then you get a mix of
> > async and sync IO all the time, which very much slows down the sync
> > process.
> >
> 
> Do you mean to say that 'ls' could not submit another request until
> the previous sync request completes, but its idle window gets disabled
> as it takes way too long to complete during heavy load? But when there

'ls' would never submit a new request before the previous one completes,
such is the nature of sync processes. That's the whole reason we have
the idle window.

> are requests in the driver, wont the idling be disabled anyway? Or did
> you mean to increase slice_sync?

No, idling is on a per-cfqq (process) basis. I did not mean to increase
slice_sync, that wont help at all. It's the window between submissions
of requests that I wanted to test being larger, but apparently that
wasn't the case here.

-- 
Jens Axboe

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