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

[Greg KH <gregkh@linuxfoundation.org>]

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.

Please do so.

thanks,

greg k-h