Re: [Xen-devel] PV performance degraded after live migration

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 15/03/17 15:43, Alan Robinson wrote:
> Hi Olaf,
>
> On Wed, Mar 15, 2017 at 12:20:44PM +0100, Olaf Hering wrote:
>> After reports about degraded performance after a PV domU was migrated
>> from one dom0 to another it turned out that this issue happens with
>> every version of Xen and every version of domU kernel.
>>
>> The used benchmark is 'sysbench memory'. I hacked it up to show how long
>> the actual work takes, and that loop takes longer to execute after the
>> domU is migrated. In my testing the loop (memory_execute_event) takes
>> about 1200ns, after migration it takes about 1500ns. It just writes 0 to
>> an array of memory. In total sysbench reports 6500 MiB/sec, after
>> migration its just 3350 MiB/sec.
> We have seen something similar being caused by the superpage
> attribute getting dropped when the domU was migrated. The new
> copy of the domU only has 4K pages instead of 2MB pages eventually
> this would seem to fit your memory access sysbench results.

PV guests with superpages can't be migrated at all.  There is no way to
guarantee that there is an available superpage on the destination, and
the absence of one can't be handled on behalf of the guest.

Before my migration v2 work, this was pot luck and you might resume with
the 2M mapping simply missing.  After my migration v2 work, this
situation is detected and causes an abort on the send side.

~Andrew

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