Re: [v2 RFC PATCH 0/9] Another Approach to Use PMEM as NUMA Node

From: Dave Hansen
Date: Tue Apr 16 2019 - 17:22:38 EST

On 4/16/19 12:19 PM, Yang Shi wrote:
> would we prefer to try all the nodes in the fallback order to find the
> first less contended one (i.e. DRAM0 -> PMEM0 -> DRAM1 -> PMEM1 -> Swap)?

Once a page went to DRAM1, how would we tell that it originated in DRAM0
and is following the DRAM0 path rather than the DRAM1 path?

Memory on DRAM0's path would be:

DRAM0 -> PMEM0 -> DRAM1 -> PMEM1 -> Swap

Memory on DRAM1's path would be:

DRAM1 -> PMEM1 -> DRAM0 -> PMEM0 -> Swap

Keith Busch had a set of patches to let you specify the demotion order
via sysfs for fun. The rules we came up with were:
1. Pages keep no history of where they have been
2. Each node can only demote to one other node
3. The demotion path can not have cycles

That ensures that we *can't* follow the paths you described above, if we
follow those rules...