Re: [PATCH v1 5/9] block: loop: convert to blk-mq

[Ming Lei]

On Thu, Aug 21, 2014 at 10:58 AM, Jens Axboe <axboe@xxxxxxxxx> wrote:
> On 2014-08-20 21:54, Ming Lei wrote:
>>>>
>>>>   From my investigation, context switch increases almost 50% with
>>>> workqueue compared with kthread in loop in a quad-core VM. With
>>>> kthread, requests may be handled as batch in cases which won't be
>>>> blocked in read()/write()(like null_blk, tmpfs, ...), but it is
>>>> impossible
>>>> with
>>>> workqueue any more.  Also block plug&unplug should have been used
>>>> with kthread to optimize the case, especially when kernel AIO is
>>>> applied,
>>>> still impossible with work queue too.
>>>
>>>
>>>
>>> OK, that one is actually a good point, since one need not do per-item
>>> queueing. We could handle different units, though. And we should have
>>> proper
>>> marking of the last item in a chain of stuff, so we might even be able to
>>> offload based on that instead of doing single items. It wont help the
>>> sync
>>> case, but for that, workqueue and kthread would be identical.
>>
>>
>> We may do that by introducing callback of queue_rq_list in blk_mq_ops,
>> and I will figure out one patch today to see if it can help the case.
>
>
> I don't think we should add to the interface, I prefer keeping it clean like
> it is right now. At least not if we can get around it. My point is that the
> driver already knows when the chain is complete, when REQ_LAST is set. So
> before that event triggers, it need not kick off IO, or at least i could do
> it in batches before that. That may not be fully reliable in case of
> queueing errors, but if REQ_LAST or 'error return' is used as the way to
> kick off pending IO, then that should be good enough. Haven't audited this
> in a while, but at least that is the intent of REQ_LAST.

Another point is that running N queue_work(rq) may cost more
than running one time queue_work(N rqs) since context still may
switch back and forth when executing queue_work().

Anyway I need to run test first to see if it can bring back throughout
on sequential read by handling them as batch.


Thanks,
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