Messages in this thread | | | Subject | Re: [PATCH 8/8] block: hook up writeback throttling | From | Jens Axboe <> | Date | Tue, 8 Nov 2016 08:16:35 -0700 |
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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
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