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