Re: [PATCH v2 4/4] block: loop: support to submit I/O via kernel aio based

[Ming Lei]

On Fri, Mar 20, 2015 at 12:37 AM, Maxim Patlasov
<mpatlasov@xxxxxxxxxxxxx> wrote:
> On 03/18/2015 07:57 PM, Ming Lei wrote:
>>
>> On Thu, Mar 19, 2015 at 2:28 AM, Maxim Patlasov <mpatlasov@xxxxxxxxxxxxx>
>> wrote:
>>>
>>> On 01/13/2015 07:44 AM, Ming Lei wrote:
>>>>
>>>> Part of the patch is based on Dave's previous post.
>>>>
>>>> This patch submits I/O to fs via kernel aio, and we
>>>> can obtain following benefits:
>>>>
>>>>          - double cache in both loop file system and backend file
>>>>          gets avoided
>>>>          - context switch decreased a lot, and finally CPU utilization
>>>>          is decreased
>>>>          - cached memory got decreased a lot
>>>>
>>>> One main side effect is that throughput is decreased when
>>>> accessing raw loop block(not by filesystem) with kernel aio.
>>>>
>>>> This patch has passed xfstests test(./check -g auto), and
>>>> both test and scratch devices are loop block, file system is ext4.
>>>>
>>>> Follows two fio tests' result:
>>>>
>>>> 1. fio test inside ext4 file system over loop block
>>>> 1) How to run
>>>>          - linux kernel base: 3.19.0-rc3-next-20150108(loop-mq merged)
>>>>          - loop over SSD image 1 in ext4
>>>>          - linux psync, 16 jobs, size 200M, ext4 over loop block
>>>>          - test result: IOPS from fio output
>>>>
>>>> 2) Throughput result:
>>>>          -------------------------------------------------------------
>>>>          test cases          |randread   |read   |randwrite  |write  |
>>>>          -------------------------------------------------------------
>>>>          base                |16799      |59508  |31059      |58829
>>>>          -------------------------------------------------------------
>>>>          base+kernel aio     |15480      |64453  |30187      |57222
>>>>          -------------------------------------------------------------
>>>
>>>
>>> Ming, it's important to understand the overhead of aio_kernel_()
>>> implementation. So could you please add test results for raw SSD device
>>> to
>>> the table above next time (in v3 of your patches).
>>
>> what aio_kernel_() does is to just call ->read_iter()/->write_iter(),
>> so it should not have introduced extra overload.
>>
>>  From performance view, the effect is only from switching to
>> O_DIRECT. With O_DIRECT, double cache can be avoided,
>> meantime both page caches and CPU utilization can be decreased.
>
>
> The way how you reused loop_queue_rq() --> queue_work() functionality (added
> early, by commit b5dd2f604) may affect performance of O_DIRECT operations.
> It can be easily demonstrated on ram-drive, but measurements on real storage
> h/w would be more convincing.

The test data in the commit log is on real storage h/w, which is attached
to one sata 3.0Gbps drive.

blk-mq may affect performance a bit on ram-drive too, which can be
demonstrated from null_blk test(blk_mq vs. bio), but looks not
a big deal since it isn't a real use case.

>
> Btw, when you wrote "linux psync, 16 jobs, size 200M, ext4 over loop block"
> -- does it mean that there were 16 threads in userspace submitting I/O
> concurrently? If yes, throughput comparison for a single job test would be
> also useful to look at.

Yes, it is the 'numjobs' in fio config file because performance can only
be got higher for 'sync' I/O by increasing number of I/O threads.

No problem, throughput comparison for single job will be provided in V3.

Thanks,
Ming Lei
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