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    SubjectRe: [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 <>
    >>> 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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