Re: [RFC][PATCH v2 00/21] PMEM NUMA node and hotness accounting/migration

[Dave Hansen]

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.