On 7/3/18 8:34 AM, Ming Lei wrote:
> 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.
Yep, that sounds great to me.
--
Jens Axboe
