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

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Date:	Thu, 28 Aug 2014 10:10:02 +0800
From:	Ming Lei <ming.lei@...onical.com>
To:	Zach Brown <zab@...bo.net>
Cc:	Maxim Patlasov <mpatlasov@...allels.com>,
	Benjamin LaHaise <bcrl@...ck.org>,
	Jens Axboe <axboe@...nel.dk>,
	Christoph Hellwig <hch@...radead.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Kleikamp <dave.kleikamp@...cle.com>,
	Kent Overstreet <kmo@...erainc.com>, AIO <linux-aio@...ck.org>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	Dave Chinner <david@...morbit.com>
Subject: Re: [PATCH v1 5/9] block: loop: convert to blk-mq

On 8/28/14, Zach Brown <zab@...bo.net> 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
>> >> 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).
>
> 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.
>
> (And you might start to stumble into the terrifying territory of
> stacking fs write paths under fs write paths.. turn on lockdep! :))
>
>> 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.
>
> Yeah, there are a lot of rare potential blocking points throughout the
> fs aio submission paths.   In practice (aio+dio+block fs), I think
> submission tends to block waiting for congested block queues most often.

In case of null_blk, it shouldn't have blocked here since the defaul tag size
is enough for the single job test if Maxim didn't change the default parameter
of null_blk.


Thanks,
-- 
Ming Lei
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