Re: [PATCH 8/8] block: hook up writeback throttling

[Jens Axboe]

On 11/08/2016 06:42 AM, Jan Kara wrote:
> On Tue 01-11-16 15:08:51, Jens Axboe wrote:
> > Enable throttling of buffered writeback to make it a lot
> > more smooth, and has way less impact on other system activity.
> > Background writeback should be, by definition, background
> > activity. The fact that we flush huge bundles of it at the time
> > means that it potentially has heavy impacts on foreground workloads,
> > which isn't ideal. We can't easily limit the sizes of writes that
> > we do, since that would impact file system layout in the presence
> > of delayed allocation. So just throttle back buffered writeback,
> > unless someone is waiting for it.
> >
> > The algorithm for when to throttle takes its inspiration in the
> > CoDel networking scheduling algorithm. Like CoDel, blk-wb monitors
> > the minimum latencies of requests over a window of time. In that
> > window of time, if the minimum latency of any request exceeds a
> > given target, then a scale count is incremented and the queue depth
> > is shrunk. The next monitoring window is shrunk accordingly. Unlike
> > CoDel, if we hit a window that exhibits good behavior, then we
> > simply increment the scale count and re-calculate the limits for that
> > scale value. This prevents us from oscillating between a
> > close-to-ideal value and max all the time, instead remaining in the
> > windows where we get good behavior.
> >
> > Unlike CoDel, blk-wb allows the scale count to to negative. This
> > happens if we primarily have writes going on. Unlike positive
> > scale counts, this doesn't change the size of the monitoring window.
> > When the heavy writers finish, blk-bw quickly snaps back to it's
> > stable state of a zero scale count.
> >
> > The patch registers two sysfs entries. The first one, 'wb_window_usec',
> > defines the window of monitoring. The second one, 'wb_lat_usec',
> > sets the latency target for the window. It defaults to 2 msec for
> > non-rotational storage, and 75 msec for rotational storage. Setting
> > this value to '0' disables blk-wb. Generally, a user would not have
> > to touch these settings.
> >
> > We don't enable WBT on devices that are managed with CFQ, and have
> > a non-root block cgroup attached. If we have a proportional share setup
> > on this particular disk, then the wbt throttling will interfere with
> > that. We don't have a strong need for wbt for that case, since we will
> > rely on CFQ doing that for us.
>
> Just one nit: Don't you miss wbt_exit() call for legacy block layer? I
> don't see where that happens.

Huh yes, good point, that must have been lost along the way. I'll readd
it.

--
Jens Axboe