linux-kernel - Re: [PATCH] block: Make cfq_target

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Date:	Thu, 29 Mar 2012 12:38:06 +0200
From:	Jens Axboe <axboe@...nel.dk>
To:	Tao Ma <tm@....ma>
CC:	Greg KH <gregkh@...uxfoundation.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] block: Make cfq_target_latency tunable through sysfs.

On 03/29/2012 02:36 AM, Tao Ma wrote:
> On 03/29/2012 08:32 AM, Greg KH wrote:
>> On Tue, Mar 27, 2012 at 04:16:53PM +0800, Tao Ma wrote:
>>> From: Tao Ma <boyu.mt@...bao.com>
>>>
>>> In cfq, when we calculate a time slice for a process(or a cfqq to
>>> be precise), we have to consider the cfq_target_latency so that all the
>>> sync request have an estimated latency(300ms) and it is controlled by
>>> cfq_target_latency. But in some hadoop test, we have found that if
>>> there are many processes doing sequential read(24 for example), the
>>> throughput is bad because every process can only work for about 25ms
>>> and the cfqq is switched. That leads to a higher disk seek. We can
>>> achive the good throughput by setting low_latency=0, but then some
>>> read's latency is too much for the application.
>>>
>>> So this patch makes cfq_target_latency tunable through sysfs so that
>>> we can tune it and find some magic number which is not bad for both
>>> the throughput and the read latency.
>>
>> If you add/modify sysfs files, you HAVE to also have a matching change
>> to Documentation/ABI.
> OK, I will add it in the next round. Great thanks.

If you do that, we can queue up the patch. I'm also a bit nervous about
adding new sysfs files, but target latency is generic enough that it
definitely makes sense to add.

-- 
Jens Axboe

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