Fedora xen - Jun 2006 - slow disk performance in Xen0 FC5

Hi all,

My dom0 and domU machines are showing terrible disk performance degradation.

For instance my avg data read rate ( tested using hdparm -t /dev/hda)
is 55MB/s using the regular smp kernel but it falls to 2.6MB/s when I
boot into dom0. No other domains are runnign at this point. I have
looked for incrementing interrupts in ide0 , there are none. hdparm
shows that dma mode is enabled. dmesg shows that the same driver is
being used for both smp and dom0 cases ( Uniform Multi-Platform E-IDE
driver Revision: 7.00alpha2 ).

I have total of 2Gig RAM in the box and dom0 is restricted to 512M.

Any help appreciated !

Thanks,
-ashe

[root@medusa1 ~]# rpm -qa | grep -i xen
kernel-xenU-2.6.16-1.2080_FC5
kernel-xen0-2.6.16-1.2080_FC5
xen-3.0.1-4

It helped me a lot (but did not really fix the problem) when I set my  
disk queueing mode to deadline, from whatever it had been before. For  
instance, on my domU:
echo deadline > /sys/block/xvda/queue/scheduler

The command is similar on the dom0, just use the appropriate block  
device.

BUT, what *really* helped me out was to turn on write caching on my  
SATA raid card. It helped such an absurd amount that I have to  
suspect some corner case performance problem was being hit before,  
but I don''t know what it would have been.

On May 31, 2006, at 11:07 PM, Ashe Canvar wrote:
> Hi all,
>
> My dom0 and domU machines are showing terrible disk performance  
> degradation.
>
> For instance my avg data read rate ( tested using hdparm -t /dev/hda)
> is 55MB/s using the regular smp kernel but it falls to 2.6MB/s when I
> boot into dom0. No other domains are runnign at this point. I have
> looked for incrementing interrupts in ide0 , there are none. hdparm
> shows that dma mode is enabled. dmesg shows that the same driver is
> being used for both smp and dom0 cases ( Uniform Multi-Platform E-IDE
> driver Revision: 7.00alpha2 ).
>
> I have total of 2Gig RAM in the box and dom0 is restricted to 512M.
>
> Any help appreciated !
>
> Thanks,
> -ashe
>
> [root@medusa1 ~]# rpm -qa | grep -i xen
> kernel-xenU-2.6.16-1.2080_FC5
> kernel-xen0-2.6.16-1.2080_FC5
> xen-3.0.1-4
>
> --
> Fedora-xen mailing list
> Fedora-xen@redhat.com
> https://www.redhat.com/mailman/listinfo/fedora-xen

Hi,

On Wed, 2006-05-31 at 23:07 -0700, Ashe Canvar wrote:
> For instance my avg data read rate ( tested using hdparm -t /dev/hda)
> is 55MB/s using the regular smp kernel but it falls to 2.6MB/s when I
> boot into dom0. No other domains are runnign at this point. I have
> looked for incrementing interrupts in ide0 , there are none.
If interrupt delivery is not happening, then that may certainly explain
poor performance!

There were some important fixes to Xen''s irq delivery mechanism that
have been added to FC5 since the 1.2080 kernel you are running.  Could
you please try updating to a newer kernel and seeing if you can
reproduce the problem?

Thanks,
 Stephen