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
