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SubjectRe: [PATCH 00/11] [RFC] 512K readahead size with thrashing safe readahead
On Wed, Feb 03, 2010 at 02:27:56PM +0800, Wu Fengguang wrote:
> Vivek,
>
> On Wed, Feb 03, 2010 at 06:38:03AM +0800, Vivek Goyal wrote:
> > On Tue, Feb 02, 2010 at 11:28:35PM +0800, Wu Fengguang wrote:
> > > Andrew,
> > >
> > > This is to lift default readahead size to 512KB, which I believe yields
> > > more I/O throughput without noticeably increasing I/O latency for today's HDD.
> > >
> >
> > Hi Fengguang,
> >
> > I was doing a quick test with the patches. I was using fio to run some
> > sequential reader threads. I have got one access to one Lun from an HP
> > EVA. In my case it looks like with the patches throughput has come down.
>
> Thank you for the quick testing!
>
> This patchset does 3 things:
>
> 1) 512K readahead size
> 2) new readahead algorithms
> 3) new readahead tracing/stats interfaces
>
> (1) will impact performance, while (2) _might_ impact performance in
> case of bugs.
>
> Would you kindly retest the patchset with readahead size manually set
> to 128KB? That would help identify the root cause of the performance
> drop:
>
> DEV=sda
> echo 128 > /sys/block/$DEV/queue/read_ahead_kb
>

I have got two paths to the HP EVA and got multipath device setup(dm-3). I
noticed with vanilla kernel read_ahead_kb=128 after boot but with your patches
applied it is set to 4. So looks like something went wrong with device
size/capacity detection hence wrong defaults. Manually setting
read_ahead_kb=512, got me better performance as compare to vanilla kernel.

AVERAGE[bsr]
-------
job Set NR ReadBW(KB/s) MaxClat(us) WriteBW(KB/s) MaxClat(us)
--- --- -- ------------ ----------- ------------- -----------
bsr 3 1 190302 97937.3 0 0
bsr 3 2 185636 223286 0 0
bsr 3 4 185986 363658 0 0
bsr 3 8 184352 428478 0 0
bsr 3 16 185646 594311 0 0
Thanks
Vivek

> The readahead stats provided by the patchset are very useful for
> analyzing the problem:
>
> mount -t debugfs none /debug
>
> # for each benchmark:
> echo > /debug/readahead/stats # reset counters
> # do benchmark
> cat /debug/readahead/stats # check counters
>
> Thanks,
> Fengguang


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