[RFC -V2 0/8] autonuma: Optimize memory placement in memory tiering system

["Huang, Ying" <ying.huang@intel.com>]

From: Huang Ying <ying.huang@intel.com>

With the advent of various new memory types, there may be multiple
memory types in one system, e.g. DRAM and PMEM (persistent memory).
Because the performance and cost of the different types of memory may
be different, the memory subsystem of the machine could be called
the memory tiering system.

After commit c221c0b0308f ("device-dax: "Hotplug" persistent memory
for use like normal RAM"), the PMEM could be used as the
cost-effective volatile memory in separate NUMA nodes.  In a typical
memory tiering system, there are CPUs, DRAM and PMEM in each physical
NUMA node.  The CPUs and the DRAM will be put in one logical node,
while the PMEM will be put in another (faked) logical node.

To optimize the system overall performance, the hot pages should be
placed in DRAM node.  To do that, we need to identify the hot pages in
the PMEM node and migrate them to DRAM node via NUMA migration.

While in autonuma, there are a set of existing mechanisms to identify
the pages recently accessed by the CPUs in a node and migrate the
pages to the node.  So we can reuse these mechanisms to build
mechanisms to optimize page placement in the memory tiering system.
This has been implemented in this patchset.

At the other hand, the cold pages should be placed in PMEM node.  So,
we also need to identify the cold pages in the DRAM node and migrate
them to PMEM node.

In the following patchset,

[PATCH 0/4] [RFC] Migrate Pages in lieu of discard
https://lore.kernel.org/linux-mm/20191016221148.F9CCD155@viggo.jf.intel.com/

A mechanism to demote the cold DRAM pages to PMEM node under memory
pressure is implemented.  Based on that, the cold DRAM pages can be
demoted to PMEM node proactively to free some memory space on DRAM
node.  And this frees space on DRAM node for the hot PMEM pages to be
migrated to.  This has been implemented in this patchset too.

The patchset is based on the following not-yet-merged patchset
(after being rebased to v5.5):

[PATCH 0/4] [RFC] Migrate Pages in lieu of discard
https://lore.kernel.org/linux-mm/20191016221148.F9CCD155@viggo.jf.intel.com/

This is part of a larger patch set.  If you want to apply these or
play with them, I'd suggest using the tree from below,

https://github.com/hying-caritas/linux/commits/autonuma-r2

With all above optimization, the score of pmbench memory accessing
benchmark with 80:20 read/write ratio and normal access address
distribution improves 116% on a 2 socket Intel server with Optane DC
Persistent Memory.

Changelog:

v2:

- Addressed comments for V1.

- Rebased on v5.5.

Best Regards,
Huang, Ying