From mboxrd@z Thu Jan  1 00:00:00 1970
From: Fengguang Wu <fengguang.wu-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>
Subject: Re: [RFC] writeback and cgroup
Date: Sun, 22 Apr 2012 22:26:48 +0800
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To: Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
Cc: Jens Axboe <axboe-tSWWG44O7X1aa/9Udqfwiw@public.gmane.org>, ctalbott-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org, Jan Kara <jack-AlSwsSmVLrQ@public.gmane.org>, rni-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org, andrea-oIIqvOZpAevzfdHfmsDf5w@public.gmane.org, containers-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, sjayaraman-IBi9RG/b67k@public.gmane.org, lsf-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org, linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org, jmoyer-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org, linux-fsdevel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Vivek Goyal <vgoyal-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
List-Id: containers.vger.kernel.org

On Fri, Apr 20, 2012 at 02:33:01PM -0700, Tejun Heo wrote:
> On Fri, Apr 20, 2012 at 03:29:30PM -0400, Vivek Goyal wrote:
> > I am personally is not too excited about the case of putting async IO
> > in separate groups due to the reason that async IO of one group will
> > start impacting latencies of sync IO of another group and in practice
> > it might not be desirable. But there are others who have use cases for
> > separate async IO queue. So as long as switch is there to change the
> > behavior, I am not too worried.
> 
> Why not just fix cfq so that it prefers groups w/ sync IOs?

There may be a sync+async group in front, but when switch into it, it
decides to give its async queue a run. That's not necessarily a bad
decision, but we do lose some control here.