Re: [PATCH 3/3] readahead: introduce context readahead algorithm

Jeff Moyer
2009-04-15T17:57:23Z

Hi, Fengguang,

Wu Fengguang <fengguang.wu@intel.com> writes:

>> I tested out your patches. Below are some basic iozone numbers for a
>> single NFS client reading a file. The iozone command line is:
>>
>> iozone -s 2000000 -r 64 -f /mnt/test/testfile -i 1 -w
>
> Jeff, thank you very much for the testing out!
>
>> The file system is unmounted after each run to flush the cache. The
>> numbers below reflect only a single run each. The file system was also
>> unmounted on the NFS client after each run.
>>
>> KEY
>> ---
>> vanilla: 2.6.30-rc1
>> readahead: 2.6.30-rc1 + your 10 readahead patches
>> context readahead: 2.6.30-rc1 + your 10 readahead patches + the 3
>> context readahead patches.
>> nfsd's: number of NFSD threads on the server
>
> I guess you are applying the readahead patches to the server side?

That's right.

> What's the NFS mount options and client/server side readahead size?
> The context readahead is pretty sensible to these parameters.

Default options everywhere.

>> I'll note that the cfq in 2.6.30-rc1 is crippled, and that Jens has a
>> patch posted that makes the numbers look at least a little better, but
>> that's immaterial to this discussion, I think.
>>
>> vanilla
>>
>> nfsd's | 1 | 2 | 4 | 8
>> --------+---------------+-------+------
>> cfq | 43127 | 22354 | 20858 | 21179
>> deadline| 43732 | 68059 | 76659 | 83231
>>
>> readahead
>>
>> nfsd's | 1 | 2 | 4 | 8
>> --------+---------------+-------+------
>> cfq | 42471 | 21913 | 21252 | 20979
>> deadline| 42801 | 70158 | 82068 | 82406
>>
>> context readahead
>>
>> nfsd's | 1 | 2 | 4 | 8
>> --------+---------------+-------+------
>> cfq | 42827 | 21882 | 20678 | 21508
>> deadline| 43040 | 71173 | 82407 | 86583
>
> Let me transform them into relative numbers:
>
> A B C A..B A..C
> cfq-1 43127 42471 42827 -1.5% -0.7%
> cfq-2 22354 21913 21882 -2.0% -2.1%
> cfq-4 20858 21252 20678 +1.9% -0.9%
> cfq-8 21179 20979 21508 -0.9% +1.6%
>
> deadline-1 43732 42801 43040 -2.1% -1.6%
> deadline-2 68059 70158 71173 +3.1% +4.6%
> deadline-4 76659 82068 82407 +7.1% +7.5%
> deadline-8 83231 82406 86583 -1.0% +4.0%
>
> Summaries:
> 1) the overall numbers are slightly negative for CFQ and looks better
> with deadline.

The variance is probably 1-2%. I'll try to quantify that for you.

> Anyway we have the io context problem for CFQ. And I'm planning to
> dive into the CFQ code and your patch on that :-)

Jens already reworked the patch and included it in his for-linus branch
of the block tree. So, you can start there. ;-)

> 2) the single thread case performance consistently dropped by 1-2%.

> It seems not related to the behavior changes introduced by the mmap
> readahead patches and context readahead patches. And looks more like
> some overheads created by the code reorganization and the patch
> "readahead: apply max_sane_readahead() limit in ondemand_readahead()"
> which adds a bit overhead with the call max_sane_readahead().
>
> I'll try to root cause it.
>
> Thanks again for the numbers!

No problem.

Cheers,
Jeff