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

[Maxim Patlasov <mpatlasov@parallels.com>]

On 08/27/2014 08:29 PM, Benjamin LaHaise wrote:
> On Wed, Aug 27, 2014 at 08:08:59PM +0400, Maxim Patlasov wrote:
> ...
>> 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.
> What filesystem is the backing file for loop0 on?  O_DIRECT access as
> Ming's patches use should be non-blocking, and if not, that's something
> to fix.

I used loop0 directly on top of null_blk driver (because my goal was to 
measure the overhead of processing requests in a separate thread).

In case of real-life filesystems, e.g. ext4, aio_kernel_submit() may 
easily block on something like bh_submit_read(), when fs reads file 
metadata to calculate the offset on block device by position in the file.

Thanks,
Maxim