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RE: [Xen-devel] RE: freezing when using GPLPV drivers (including Dom0)

>From: James Harper [mailto:james.harper@xxxxxxxxxxxxxxxx] 
>Sent: Wednesday, December 31, 2008 11:16 AM
>> >From: James Harper
>> >Sent: Wednesday, December 31, 2008 10:46 AM
>> >
>> >I am suspecting that maybe the problem is disk starvation 
>but I don't
>> >quite understand why the lockup happens for so long. I'm also not
>> >why I'm only seeing the problem when using my GPLPV drivers - one
>> >possibility is that the increased performance puts more load on the
>> >storage system.
>> >
>> Maybe you can check cycles spent on kernel thread/event handler
>> in backend driver side. I'm not sure whether heavy communication
>> between be/fe could disturb dom0 scheduler if care is not taken in
>> current design. E.g. back kernel thread may eat too many cycles
>> before giving up, or your GPLPV fe driver may issue too many events
>> to break be side...
>I am running the restore again and monitoring using:
>. xentop running in dom0
>. arping to the DomU running from an external machine
>. ping to Dom0 running from an external machine
>With arping and ping running I have noticed that the freeze is not
>always long enough to cause the TCP connections to time out - 
>I was only
>noticing the ones that were long enough.
>During the freeze, xentop shows very low Dom0 and DomU CPU, 
>arping stops
>receiving replies to the arp requests, but the ping to Dom0 
>keeps going.
>The freeze that just occurred was not long enough for me to tell if the
>DomU xentop counters for network and disk were increasing or not.
>(xentop keeps running, lending weight to the freeze only concerning
>tasks that want to access the disk).
>Is there a way under Linux of monitoring disk queue length? I am using
>LVM on top of a low end HP 'Smart Array' (E200) running two RAID1
>volumes using SATA disks.

'sar' could provide such info, IMO.

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