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

From: Ming Lei
Date: Wed Aug 27 2014 - 22:06:32 EST

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:
>> 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.

Ming Lei
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