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Re: [Xen-users] slow dom0 write operations with 2.6.32 pv-ops and Xen4.0.0-rc4

On Mon, Mar 01, 2010 at 01:07:10PM +0100, benco@xxxxxxx wrote:
> > You're measuring buffered (cached) performance, so when you limit
> > the amount of cache to 2 GB the performance will be slower.
> > 
> > Try adding "oflag=direct" do your dd commands and test again.
> oflag=direct option produce almost the same results on xen and non-xen
> systems:
> #dd if=/dev/zero of=zerofile.tst bs=1k count=1000000 oflag=direct
> 1000000+0 records in
> 1000000+0 records out
> 1024000000 bytes (1.0 GB) copied, 616.219 s, 1.7 MB/s

Try bigger block size and smaller count, for example bs=64k. 
1k block are really small and won't give you good performance.

> what is interesting for me is the fact that with no cpu/memory limit and with
> caching the result is approx. 60% slowdown with xen kernel in comparison
> with non-xen kernel. Is it normal?

Yeah, it depends on a lot of things. Try with DIRECT io (and big enough block 
to get some real/meaningful numbers.

-- Pasi

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