linux-kernel - Re: [ext3][kernels >= 2.6.20.7 at least] KDE going comatose when FS is under heavy write load (massive starvation)

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Date:	Sat, 28 Apr 2007 19:11:37 +0200 (CEST)
From:	Mikulas Patocka <mikulas@...ax.karlin.mff.cuni.cz>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Mike Galbraith <efault@....de>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jens Axboe <jens.axboe@...cle.com>
Subject: Re: [ext3][kernels >= 2.6.20.7 at least] KDE going comatose when FS
 is under heavy write load (massive starvation)

> So perhaps if there's any privileged reads going on then we should limit
> writes to a depth of 2 at most, with some timeout mechanism that would

SCSI has a "high priority" bit in the command block, so you can just set 
it --- but I am not sure how well do disks support it.

Mikulas

> gradually allow the deepening of the hardware queue, as long as no
> highprio reads come inbetween? With 2 pending requests and even assuming
> worst-case seeks the user-visible latency would be on the order of 20-30
> msecs, which is at the edge of human perception. The problem comes when
> a hardware queue of 32-64 entries starves that one highprio read which
> then results in a 2+ seconds latency.
>
> 	Ingo
>
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