Re: [PATCH 0/4] Page based O_DIRECT v2

[Jens Axboe]

On Wed, Aug 19 2009, Alan D. Brunelle wrote:
> Hi Jens - 
> 
> I'm not using loop, but it appears that there may be a regression in
> regular asynchronous direct I/O sequential write performance when these
> patches are applied. Using my "small" machine (16-way x86_64, 256GB, two
> dual-port 4GB FC HBAs connected through switches to 4 HP MSA1000s - one
> MSA per port), I'm seeing a small but noticeable drop in performance for
> sequential writes on the order of 2 to 6%. Random asynchronous direct
> I/O and sequential reads appear to unaffected.
> 
> http://free.linux.hp.com/~adb/2009-08-19/nc.png
> 
> has a set of graphs showing the data obtained when utilizing LUNs
> exported by the MSAs (increasing the number of MSAs being used along the
> X-axis). The critical sequential write graph has numbers like (numbers
> expressed in GB/second):
> 
> Kernel                    1MSA  2MSAs 3MSAs 4MSAs
> ------------------------  ----- ----- ----- -----
> 2.6.31-rc6              :  0.17  0.33  0.50  0.65 
> 2.6.31-rc6 + loop-direct:  0.15  0.31  0.46  0.61
> 
> Using all 4 devices we're seeing a drop of slightly over 6%. 
> 
> I also typically do runs utilizing just the caches on the MSAs (getting
> rid of physical disk interactions (seeks &c).). Even here we see a small
> drop off in sequential write performance (on the order of about 2.5%
> when using all 4 MSAs)- but noticeable gains for both random reads and
> (especially) random writes. That graph can be seen at:
> 
> http://free.linux.hp.com/~adb/2009-08-19/ca.png
> 
> BTW: The grace/xmgrace files that generated these can be found at - 
> 
> http://free.linux.hp.com/~adb/2009-08-19/nc.agr
> http://free.linux.hp.com/~adb/2009-08-19/ca.agr
> 
> - as the specifics can be seen better whilst running xmgrace on those
> files.

Thanks a lot for the test run, Alan. I wonder why writes are down while
reads are up. One possibility could be a WRITE vs WRITE_ODIRECT
difference, though I think they should be the same. The patches I posted
have not been benchmarked at all, it's still very much a work in
progress. I just wanted to show the general direction that I thought
would be interesting. So I have done absolutely zero performance
testing, it's only been tested for whether it still worked or not (to
some degree :-)...

I'll poke a bit at it here, too. I want to finish the unplug/wait
problem first. Is your test case using read/write or readv/writev?

-- 
Jens Axboe

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