On Tue, Jul 03, 2018 at 08:13:45AM -0600, Jens Axboe wrote:
> On 7/3/18 8:11 AM, Ming Lei wrote:
> > On Tue, Jul 03, 2018 at 08:03:23AM -0600, Jens Axboe wrote:
> >> On 7/3/18 2:34 AM, Ming Lei wrote:
> >>> It won't be efficient to dequeue request one by one from sw queue,
> >>> but we have to do that when queue is busy for better merge performance.
> >>>
> >>> This patch takes the Exponential Weighted Moving Average(EWMA) to figure
> >>> out if queue is busy, then only dequeue request one by one from sw queue
> >>> when queue is busy.
> >>
> >> I've started to come around to the approach, but can we add something
> >> that only triggers this busy tracking if we've even seen a BUSY
> >> condition? Basically, this:
> >>
> >> blk_mq_update_dispatch_busy(hctx, false);
> >>
> >> should be a no-op, if we've never called:
> >>
> >> blk_mq_update_dispatch_busy(hctx, true);
> >>
> >> Something ala the below, with the BLK_MQ_S_EWMA bit added, of course.
> >>
> >> static void __blk_mq_update_dispatch_busy(struct blk_mq_hw_ctx *hctx, bool busy)
> >> {
> >> unsigned int ewma = READ_ONCE(hctx->dispatch_busy);
> >>
> >> ewma *= BLK_MQ_DISPATCH_BUSY_EWMA_WEIGHT - 1;
> >> if (busy)
> >> ewma += 1 << BLK_MQ_DISPATCH_BUSY_EWMA_FACTOR;
> >> ewma /= BLK_MQ_DISPATCH_BUSY_EWMA_WEIGHT;
> >>
> >> WRITE_ONCE(hctx->dispatch_busy, ewma);
> >> }
> >
> > How about doing it in the following(simpler) way? By adding the check
> > at the entry of __blk_mq_update_dispatch_busy().
> >
> > if (!ewma && !busy)
> > return;
>
> That might be better indeed, though still would need the read once.
> The test_bit, for a constant bit, is basically free.
We can remove both READ_ONCE() and WRITE_ONCE(), I used it just for
document benefit since there is concurrent access on this shared variable,
but looks smp_read_barrier_depends() is added to READ_ONCE() recently.
Both the 32-bit read/write on hctx->dispatch_busy is atomic, meantime
not see any problem can be caused if compiler optimization is involved
on this read/write.
So I will remove READ_ONCE()/WRITE_ONCE() in V5, and add the above check
if you don't object.
Thanks,
Ming
