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.
--
Jens Axboe
