[libvirt] [PATCH resend V10 03/12] Resctrl: Add new xml element to support cache tune

[Martin Kletzander]

On Wed, Mar 15, 2017 at 02:23:26PM +0000, Daniel P. Berrange wrote:
>On Wed, Mar 15, 2017 at 03:11:26PM +0100, Martin Kletzander wrote:
>> On Mon, Mar 06, 2017 at 06:06:32PM +0800, Eli Qiao wrote:
>> > This patch adds new xml element to support cache tune as:
>> >
>> > <cputune>
>> >  ...
>> >  <cachetune id='1' host_id='0' type='l3' size='2816' unit='KiB'
>>
>> Again, this was already discussed, probably, I just can't find the
>> source of it.  But host_id actually already selects what cache is
>> supposed to be used, so instead of type='l3' we only need scope='both'
>> (or data/instruction) there.  Or am I missing something?  What I'm
>> concerned about is that the host_id is mostly stable on one host (when
>> the previously mentioned problems are fixed), but it will make no sense
>> when the VM is migrated to another one.  Unfortunately, the only
>> solution I can think of is using multiple keys to precisely describe the
>> bank we want (e.g. host's cpu id, cache level and scope), but that seems
>> very unclean.
>
>I tend to view use of this cachetune setting as being similar to
>using host CPU passthrough - you're intentionally trading off
>migratability of your  guest to get a perf boost.
>
>Even without the host_id bit, this is still non-portable, as you
>might be requesting separate regions for code + data, but the
>target host of migration may only support shared regions.
>

Sure, but those are things we can check during migration.  I'd be OK
with disabling migration (or making it --unsafe) when migrating with
cachetune.

>Regards,
>Daniel
>--
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