Re: [Xen-users] Poor hard disk performance on xen-3/dom0

[Marcin Owsiany <porridge@xxxxxxxx>]

On Thu, Jan 19, 2006 at 03:14:50AM +1100, Woon Wai Keen wrote:
> In reply to myself, I just discovered a factor to the problem I'm facing.
> 
> # xm mem-set 0 120
> # hdparm -t /dev/sda
> 
>  /dev/sda:
>   Timing buffered disk reads:  180 MB in  3.02 seconds =  59.60 MB/sec
> 
> # xm mem-set 0 80
> # hdparm -t /dev/sda
> 
>  /dev/sda:
>   Timing buffered disk reads:   68 MB in  3.08 seconds =  22.08 MB/sec
> 
> I tried this in the non-xen kernel with a mem=64M boot flag, and found 
> that it does not happen (disk performs as expected). So I think that 
> something, somewhere, in the dom0 kernel underperforms with reduced RAM.
> 
> I'm fine with giving dom0 an extra 64MB just to avoid this problem, but 
> I'm also curious as to why this happens.

Seems like the benchmark is tainted because of buffer cache
interactions. Please try using a real benchmark program like bonnie++
and post your results.

Marcin
-- 
Marcin Owsiany
porridge@xxxxxxxx

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