On 7/18/19 1:08 AM, Paolo Valente wrote:
> Consider a sync bfq_queue Q that remains empty while in service, and
> suppose that, when this happens, there is a fair amount of already
> in-flight I/O not belonging to Q. In such a situation, I/O dispatching
> may need to be plugged (until new I/O arrives for Q), for the
> following reason.
>
> The drive may decide to serve in-flight non-Q's I/O requests before
> Q's ones, thereby delaying the arrival of new I/O requests for Q
> (recall that Q is sync). If I/O-dispatching is not plugged, then,
> while Q remains empty, a basically uncontrolled amount of I/O from
> other queues may be dispatched too, possibly causing the service of
> Q's I/O to be delayed even longer in the drive. This problem gets more
> and more serious as the speed and the queue depth of the drive grow,
> because, as these two quantities grow, the probability to find no
> queue busy but many requests in flight grows too.
>
> If Q has the same weight and priority as the other queues, then the
> above delay is unlikely to cause any issue, because all queues tend to
> undergo the same treatment. So, since not plugging I/O dispatching is
> convenient for throughput, it is better not to plug. Things change in
> case Q has a higher weight or priority than some other queue, because
> Q's service guarantees may simply be violated. For this reason,
> commit 1de0c4cd9ea6 ("block, bfq: reduce idling only in symmetric
> scenarios") does plug I/O in such an asymmetric scenario. Plugging
> minimizes the delay induced by already in-flight I/O, and enables Q to
> recover the bandwidth it may lose because of this delay.
>
> Yet the above commit does not cover the case of weight-raised queues,
> for efficiency concerns. For weight-raised queues, I/O-dispatch
> plugging is activated simply if not all bfq_queues are
> weight-raised. But this check does not handle the case of in-flight
> requests, because a bfq_queue may become non busy *before* all its
> in-flight requests are completed.
>
> This commit performs I/O-dispatch plugging for weight-raised queues if
> there are some in-flight requests.
>
> As a practical example of the resulting recover of control, under
> write load on a Samsung SSD 970 PRO, gnome-terminal starts in 1.5
> seconds after this fix, against 15 seconds before the fix (as a
> reference, gnome-terminal takes about 35 seconds to start with any of
> the other I/O schedulers).
>
> Fixes: commit 1de0c4cd9ea6 ("block, bfq: reduce idling only in symmetric scenarios")
Applied, but fixed up this line. The format is:
Fixes: 1de0c4cd9ea6 ("block, bfq: reduce idling only in symmetric scenarios")
--
Jens Axboe
