linux-kernel - Re: CFQ: async queue blocks the whole system

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Message-ID: <4DF1E235.8010104@kernel.dk>
Date:	Fri, 10 Jun 2011 11:21:57 +0200
From:	Jens Axboe <axboe@...nel.dk>
To:	Vivek Goyal <vgoyal@...hat.com>
CC:	Tao Ma <tm@....ma>, Shaohua Li <shaohua.li@...el.com>,
	linux-kernel@...r.kernel.org
Subject: Re: CFQ: async queue blocks the whole system

On 2011-06-10 11:20, Vivek Goyal wrote:
> On Fri, Jun 10, 2011 at 11:02:11AM +0800, Tao Ma wrote:
> 
> [..]
>>> I don't think we can give a deadline for async request, because we
>>> still want to give sync high priority. We can give async some slices,
>>> so for a workload of small number of async requests and large number
>>> sync requests, we don't starve async too much. But for a workload with
>>> large number of sync/async requests, async will be starved for sure
>>> and we can't solve this in cfq.
>> OK, so if you guys thinks a 500 seconds wait is good for an async write
>> to complete, fine, then we have to switch to deadline.
> 
> I don't think that starving WRITES completely is a good idea. Especially
> given the fact that you were not able to dispatch WRITES for 500 seconds.
> This needs fixing.
> 
> Its not about giving hard deadline to WRITES, but making sure we don't
> starve them completely and they also make some progress.

Agree, we need to have some sort of forward progress guarantee at least.
Starving buffered writes indefinitely is surely a BUG. And it must be a
regression from not that long ago.

Trying to catch up with this thread...

-- 
Jens Axboe

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