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

[Maxim Patlasov]

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
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

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) 
might firstly try that non-blocking submit as fast-path, and, only if 
it's failed, fall back to queueing.

Thanks,
Maxim
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