Re: [PATCH] block: Make cfq_target_latency tunable through sysfs.

[Tao Ma]

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@xxxxxxxxxx>
>>
>> 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.

Tao
> 
> Please do so.
> 
> thanks,
> 
> greg k-h
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