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

From: Maxim Patlasov
Date: Wed Aug 27 2014 - 13:19:50 EST

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.

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