Re: [PATCH v2 0/5] blk-mq-sched: support request batch dispatching for sq elevator

[Yu Kuai <yukuai1@huaweicloud.com>]

Hi,

在 2025/07/31 17:25, Ming Lei 写道:
> On Thu, Jul 31, 2025 at 04:42:10PM +0800, Yu Kuai wrote:
>> Hi,
>>
>> 在 2025/07/31 16:18, Ming Lei 写道:
>>> batch dispatch may hurt io merge performance which is important for
>>> elevator, so please provide test data on real HDD. & SSD., instead of
>>> null_blk only, and it can be perfect if merge sensitive workload
>>> is evaluated.
>>
>> Ok, I'll provide test data on HDD and SSD that I have for now.
>>
>> For the elevator IO merge case, what I have in mind is that we issue
>> small sequential IO one by one with multiple contexts, so that bios
>> won't be merged in plug, and this will require IO issue > IO done, is
>> this case enough?
> 
> Long time ago, I investigated one such issue which is triggered in qemu
> workload, but not sure if I can find it now.
> 
> Also many scsi devices may easily run into queue busy, then scheduler merge
> starts to work, and it may perform worse if you dispatch more in this
> situation.

I think we won't dispatch more in this case, on the one hand we will get
budgets first, to make sure never dispatch more than queue_depth; on the
onther hand, in the case hctx->dispatch_busy is set, we still fallback
to the old case to dispatch one at a time;

So if the IO merge start because many rqs are accumulated inside
elevator and the driver if full, I will expect IO merge will behave the
same with this set. I'll have a test to check.

Thanks,
Kuai

> 
> Thanks,
> Ming
> 
> 
> .
>