linux-kernel - Re: Read I/O starvation with writeback RAID controller

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Message-ID: <alpine.LNX.2.01.1302222133130.25382@nerf07.vanv.qr>
Date:	Fri, 22 Feb 2013 21:35:12 +0100 (CET)
From:	Jan Engelhardt <jengelh@...i.de>
To:	Martin Svec <martin.svec@...er.cz>
cc:	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>,
	linux-scsi <linux-scsi@...r.kernel.org>,
	target-devel <target-devel@...r.kernel.org>,
	linux-kernel@...r.kernel.org
Subject: Re: Read I/O starvation with writeback RAID controller


On Friday 2013-02-22 20:28, Martin Svec wrote:
>
> Yes, I've already tried the ROW scheduler. It helped for some low iodepths
> depending on quantum settings but generally didn't solve the problem. I think
> the key issue is that none of the schedulers can throttle I/O according to e.g.
> average request roundtrip time. Shaohua Li is right here:
> https://lkml.org/lkml/2012/12/11/598 -- as long as there's free room in
> device's queue they blindly dispatch requests to it.
>
> Which is exactly what I see in deadline scheduler fifo queues: There're no read
> requests to be scheduled between writes because all readers are starving. So
> the scheduler keeps dispatching writes using all the remaining capacity of
> device queue. Which in turn worses the read starvation. Bigger queue depth and
> bigger writeback cache means higher chance for read starvation even from a
> single writer.

Sounds just like the bufferbloat problem in networking.
Waiting for codel for the block layer  :)
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