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

[Ming Lei]

On 8/28/14, Maxim Patlasov <mpatlasov@xxxxxxxxxxxxx> wrote:
> On 08/21/2014 09:44 AM, Ming Lei wrote:
>> On Wed, Aug 20, 2014 at 4:50 AM, Jens Axboe <axboe@xxxxxxxxx> wrote:
>>
>>>
>>> Reworked a bit more:
>>>
>>> http://git.kernel.dk/?p=linux-block.git;a=commit;h=a323185a761b9a54dc340d383695b4205ea258b6
>> One big problem of the commit is that it is basically a serialized
>> workqueue
>> because of single &hctx->run_work, and per-req work_struct has to be
>> used for concurrent implementation.  So looks the approach isn't flexible
>> enough compared with doing that in driver, or any idea about how to fix
>> that?
>>
>
> I'm interested what's the price of handling requests in a separate
> thread at large. I used the following fio script:
>
>      [global]
>      direct=1
>      bsrange=512-512
>      timeout=10
>      numjobs=1
>      ioengine=sync
>
>      filename=/dev/loop0 # or /dev/nullb0
>
>      [f1]
>      rw=randwrite
>
> to compare the performance of:
>
> 1) /dev/loop0 of 3.17.0-rc1 with Ming's patches applied -- 11K iops

If you enable BLK_MQ_F_WQ_CONTEXT, it isn't strange to see this
result since blk-mq implements a serialized workqueue.

> 2) the same as above, but call loop_queue_work() directly from
> loop_queue_rq() -- 270K iops
> 3) /dev/nullb0 of 3.17.0-rc1 -- 380K iops

In my recent investigation and discussion with Jens, using workqueue
may introduce some regression for cases like loop over null_blk, tmpfs.

And 270K vs. 380K is a bit similar with my result, and it was observed that
context switch is increased by more than 50% with introducing workqueue.

I will post V3 which will use previous kthread, with blk-mq & kernel aio, which
should make full use of blk-mq and kernel aio, and won't introduce regression
for cases like above.

> Taking into account so big difference (11K vs. 270K), would it be worthy
> to implement pure non-blocking version of aio_kernel_submit() returning
> error if blocking needed? Then loop driver (or any other in-kernel user)

The kernel aio submit is very similar with user space's implementation,
except for block plug&unplug usage in user space aio submit path.

If it is blocked in aio_kernel_submit(), you should observe similar thing
with io_submit() too.

Thanks,
-- 
Ming Lei
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