Re: [EXT] Re: [RFC PATCH v2 0/2] Node migration between memory tiers

[Gregory Price]

On Tue, Jan 09, 2024 at 11:41:11AM +0800, Huang, Ying wrote:
> Gregory Price <gregory.price@xxxxxxxxxxxx> writes:
> 
> > On Thu, Jan 04, 2024 at 02:05:01PM +0800, Huang, Ying wrote:
> >> >
> >> > From  https://lpc.events/event/16/contributions/1209/attachments/1042/1995/Live%20In%20a%20World%20With%20Multiple%20Memory%20Types.pdf
> >> > abstract_distance_offset: override by users to deal with firmware issue.
> >> >
> >> > say firmware can configure the cxl node into wrong tiers, similar to
> >> > that it may also configure all cxl nodes into single memtype, hence
> >> > all these nodes can fall into a single wrong tier.
> >> > In this case, per node adistance_offset would be good to have ?
> >> 
> >> I think that it's better to fix the error firmware if possible.  And
> >> these are only theoretical, not practical issues.  Do you have some
> >> practical issues?
> >> 
> >> I understand that users may want to move nodes between memory tiers for
> >> different policy choices.  For that, memory_type based adistance_offset
> >> should be good.
> >> 
> >
> > There's actually an affirmative case to change memory tiering to allow
> > either movement of nodes between tiers, or at least base placement on
> > HMAT information. Preferably, membership would be changable to allow
> > hotplug/DCD to be managed (there's no guarantee that the memory passed
> > through will always be what HMAT says on initial boot).
> 
> IIUC, from Jonathan Cameron as below, the performance of memory
> shouldn't change even for DCD devices.
> 
> https://lore.kernel.org/linux-mm/20231103141636.000007e4@xxxxxxxxxx/
> 
> It's possible to change the performance of a NUMA node changed, if we
> hot-remove a memory device, then hot-add another different memory
> device.  It's hoped that the CDAT changes too.
> 
> So, all in all, HMAT + CDAT can help us to put the memory device in
> appropriate memory tiers.  Now, we have HMAT support in upstream.  We
> will working on CDAT support.

That should be sufficient assuming the `-numa hmat-lb` setting in QEMU
does the right thing.  I suppose we also need to figure out a way to set
CDAT information for a memory device that isn't related to CXL (from the
perspective of the guest).  I'll take a look if I get cycles.

~Gregory