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

[Maxim Patlasov]

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.

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.

Thanks,
Maxim
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