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

From: Maxim Patlasov
Date: Fri Aug 29 2014 - 06:41:28 EST

On 8/28/14, Zach Brown<zab@xxxxxxxxx> wrote:

On Wed, Aug 27, 2014 at 09:19:36PM +0400, Maxim Patlasov wrote:
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
to implement pure non-blocking version of aio_kernel_submit() returning
error if blocking needed? Then loop driver (or any other in-kernel
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).
The relative overhead while doing nothing else. While zooming way down
in to micro benchmarks is fun and all, testing on an fs on brd might be
more representitive and so more compelling.

The measurements on an fs on brd are even more outrageous (the same fio script I posted a few messages above):

1) Baseline. no loopback device involved.

fio on /dev/ram0: 467K iops
fio on ext4 over /dev/ram0: 378K iops

2) Loopback device from 3.17.0-rc1 with Ming's patches (v1) applied:

fio on /dev/loop0 over /dev/ram0: 10K iops
fio on ext4 over /dev/loop0 over /dev/ram0: 9K iops

3) the same as above, but avoid extra context switch (call loop_queue_work() directly from loop_queue_rq()):

fio on /dev/loop0 over /dev/ram0: 267K iops
fio on ext4 over /dev/loop0 over /dev/ram0: 223K iops

The problem is not about huge relative overhead while doing nothing else. It's rather about introducing extra latency (~100 microseconds on commodity h/w I used) which might be noticeable on modern SSDs (and h/w RAIDs with caching).

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