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

[Ming Lei]

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.

V3 need to rewrite against recent vfs's change(in -next already)
about splitting kiocb patches, and thanks to Christoph's patches,
aio_kernel_() can be implemented just inside loop and it may
become a bit simple.

>
> Jens, if you have some fast storage at hand, could you please measure IOPS
> for Ming's patches vs. raw block device -- to ensure that the patches do not
> impose too low limit on performance.
>
> Thanks,
> Maxim
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