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

[Woon Wai Keen <list+xen@xxxxxxxxxxxxxx>]

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.
Ian Pratt wrote:
> Compare the boot messages of vanilla vs. xen -- you're likely using a
> different driver.
>
> I know everyone always ignores me whenever I say this, but users really
> should be using the -xen kernel rather than -xen0/U. The latter two are
> just to make build times quicker for developers. The -xen kernel has way
> more drivers, though built as modules, so you'll need to make an initrd.
>
> Ian
>
> > On a vanilla 2.6.12 kernel, I get:
> >
> >   60MB/s for hdparm -t /dev/sd[a|b] (on an Intel ICH7 SATA 
> > controller).
> >   30MB/s for hdparm -t /dev/hda (Intel ICH7)
> >
> > While on the xen dom0 kernel, I get 20MB/s for all three 
> > devices. I confirm that DMA is enabled for the PATA hard 
> > disk. I don't know how to check that for the SATA hard disks, 
> > but I assume that they're on DMA mode too.
> > Xen is compiled with the following:
> >
> > xen_changeset          : Fri Dec  9 11:05:06 2005 +0000 
> > 8311:53cff3f88e45
> > cc_compiler            : gcc version 3.3.5 (Debian 1:3.3.5-13)
> >
> > The dom0 kernel is compiled with almost the same options as 
> > the vanilla kernel, barring the xen-exclusive and 
> > nonxen-exclusive options.
> > Does anyone else face similar problems?
--
Regards,
wK (www.doubleukay.com)

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