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;
Thanks,
Ming
