Hi,
On Thu, Aug 12, 2010 at 09:42:21AM -0400, Jeff Moyer wrote:
> Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx> 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.
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.
Thanks,
Naoya Horiguchi
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