On Wed, Mar 15, 2017 at 02:11:23PM -0300, Marcelo Tosatti wrote:
On Wed, Mar 15, 2017 at 03:59:31PM +0100, Martin Kletzander wrote: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.This is the same conditions as pinning a vcpu to a pcpu. So yes, it might be that you want to migrate to a host where a different "host ID" number is used (which might or might not be a different socket).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.Then migration should fail.
Yes, it will automatically fail when you can't do the same allocations, that's easy. The difference with host_id is that you cannot foresee whether keeping the host_id makes sense on the destination as well.
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.Migration support is required (for NFV usecase they want to migrate VMs around).
So we at least need a check for the fact that either the caches are the somehow same or destination XML is supplied. The other option would be what I hinted above, that is to change host_id= to something like level, scope, and pcpu(s)/socket(s). That is fairly easy change from the code point of view, but I'm trying to think more about the user experience. Martin
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