On 12/28/18 12:41 AM, Michal Hocko wrote: >> >> It can be done in kernel page reclaim path, near the anonymous page >> swap out point. Instead of swapping out, we now have the option to >> migrate cold pages to PMEM NUMA nodes. > OK, this makes sense to me except I am not sure this is something that > should be pmem specific. Is there any reason why we shouldn't migrate > pages on memory pressure to other nodes in general? In other words > rather than paging out we whould migrate over to the next node that is > not under memory pressure. Swapout would be the next level when the > memory is (almost_) fully utilized. That wouldn't be pmem specific. Yeah, we don't want to make this specific to any particular kind of memory. For instance, with lots of pressure on expensive, small high-bandwidth memory (HBM), we might want to migrate some HBM contents to DRAM. We need to decide on whether we want to cause pressure on the destination nodes or not, though. I think you're suggesting that we try to look for things under some pressure and totally avoid them. That sounds sane, but I also like the idea of this being somewhat ordered. Think of if we have three nodes, A, B, C. A is fast, B is medium, C is slow. If A and B are "full" and we want to reclaim some of A, do we: 1. Migrate A->B, and put pressure on a later B->C migration, or 2. Migrate A->C directly ? Doing A->C is less resource intensive because there's only one migration involved. But, doing A->B/B->C probably makes the app behave better because the "A data" is presumably more valuable and is more appropriately placed in B rather than being demoted all the way to C.
