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    SubjectRe: [PATCH] block: Make cfq_target_latency tunable through sysfs.
    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@taobao.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.

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