Re: [RFC] [PATCH 2/4] dio: add page locking for direct I/O

[Jeff Moyer <jmoyer@redhat.com>]

Naoya Horiguchi <n-horiguchi@ah.jp.nec.com> writes:

> Hi,
>
> On Thu, Aug 12, 2010 at 09:42:21AM -0400, Jeff Moyer wrote:
>> Naoya Horiguchi <n-horiguchi@ah.jp.nec.com> writes:
>> 
>> > Basically it is user's responsibility to take care of race condition
>> > related to direct I/O, but some events which are out of user's control
>> > (such as memory failure) can happen at any time. So we need to lock and
>> > set/clear PG_writeback flags in dierct I/O code to protect from data loss.
>> 
>> Did you do any performance testing of this?  If not, please do and
>> report back.  I'm betting users won't be pleased with the results.
>
> Here is the result of my direct I/O benchmarck, which mesures the time
> it takes to do direct I/O for 20000 pages on 2MB buffer for four types
> of I/O. Each I/O is issued for one page unit and each number below is
> the average of 25 runs.
>
>                                   with patchset          2.6.35-rc3
>    Buffer      I/O type        average(s)  STD(s)   average(s)  STD(s)   diff(s)
>   hugepage   Sequential Read      3.87      0.16       3.88      0.20    -0.01
>              Sequential Write     7.69      0.43       7.69      0.43     0.00
>              Random Read          5.93      1.58       6.49      1.45    -0.55
>              Random Write        13.50      0.28      13.41      0.30     0.09
>   anonymous  Sequential Read      3.88      0.21       3.89      0.23    -0.01
>              Sequential Write     7.86      0.39       7.80      0.34     0.05
>              Random Read          7.67      1.60       6.86      1.27     0.80
>              Random Write        13.50      0.25      13.52      0.31    -0.01
>
> From this result, although fluctuation is relatively large for random read,
> differences between vanilla kernel and patched one are within the deviations and
> it seems that adding direct I/O lock makes little or no impact on performance.

First, thanks for doing the testing!

> And I know the workload of this benchmark can be too simple, so please
> let me know if you think we have another workload to be looked into.

Well, as distasteful as this sounds, I think a benchmark that does I/O
to partial pages would show the problem best.  And yes, this does happen
in the real world.  ;-)  So, sequential 512 byte or 1k or 2k I/Os, or
just misalign larger I/Os so that two sequential I/Os will hit the same
page.

I believe you can use fio to generate such a workload;  see iomem_align
in the man page.  Something like the below *might* work.  If not, then
simply changing the bs=4k to bs=2k and getting rid of iomem_align should
show the problem.

Cheers,
Jeff

[global]
ioengine=libaio
iodepth=32
bs=4k
direct=1
size=2g
overwrite=1

[test1]
rw=write
iomem_align=2k

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