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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 15.03.17 at 16:43, <Alan.Robinson@xxxxxxxxxxxxxx> wrote:
> 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. 

That would be an explanation for HVM, but here we consider PV only.
And whether a HVM guest can be re-created using large pages
mainly depends on how fragmented the destination host memory is.

Jan


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