> Il giorno 5 giu 2020, alle ore 16:16, Jan Kara <jack@xxxxxxx> ha scritto:
>
> Currently whenever bfq queue has a request queued we add now -
> last_completion_time to the think time statistics. This is however
> misleading in case the process is able to submit several requests in
> parallel because e.g. if the queue has request completed at time T0 and
> then queues new requests at times T1, T2, then we will add T1-T0 and
> T2-T0 to think time statistics which just doesn't make any sence (the
> queue's think time is penalized by the queue being able to submit more
> IO).
I've thought about this issue several times. My concern is that, by
updating the think time only when strictly meaningful, we reduce the
number of samples. In some cases, we may reduce it to a very low
value. For this reason, so far I have desisted from changing the
current scheme. IOW, I opted for dirtier statistics to avoid the risk
of too scarse statistics. Any feedback is very welcome.
Thanks,
Paolo
> So add to think time statistics only time intervals when the queue
> had no IO pending.
>
> Signed-off-by: Jan Kara <jack@xxxxxxx>
> ---
> block/bfq-iosched.c | 12 ++++++++++--
> 1 file changed, 10 insertions(+), 2 deletions(-)
>
> diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
> index c66c3eaa9e26..4b1c9c5f57b6 100644
> --- a/block/bfq-iosched.c
> +++ b/block/bfq-iosched.c
> @@ -5192,8 +5192,16 @@ static void bfq_update_io_thinktime(struct bfq_data *bfqd,
> struct bfq_queue *bfqq)
> {
> struct bfq_ttime *ttime = &bfqq->ttime;
> - u64 elapsed = ktime_get_ns() - bfqq->ttime.last_end_request;
> -
> + u64 elapsed;
> +
> + /*
> + * We are really interested in how long it takes for the queue to
> + * become busy when there is no outstanding IO for this queue. So
> + * ignore cases when the bfq queue has already IO queued.
> + */
> + if (bfqq->dispatched || bfq_bfqq_busy(bfqq))
> + return;
> + elapsed = ktime_get_ns() - bfqq->ttime.last_end_request;
> elapsed = min_t(u64, elapsed, 2ULL * bfqd->bfq_slice_idle);
>
> ttime->ttime_samples = (7*ttime->ttime_samples + 256) / 8;
> --
> 2.16.4
>
