Re: RFC: Memory Tiering Kernel Interfaces

[Dan Williams <dan.j.williams@intel.com>]

On Fri, Apr 29, 2022 at 8:59 PM Yang Shi <shy828301@gmail.com> wrote:
>
> Hi Wei,
>
> Thanks for the nice writing. Please see the below inline comments.
>
> On Fri, Apr 29, 2022 at 7:10 PM Wei Xu <weixugc@google.com> wrote:
> >
> > The current kernel has the basic memory tiering support: Inactive
> > pages on a higher tier NUMA node can be migrated (demoted) to a lower
> > tier NUMA node to make room for new allocations on the higher tier
> > NUMA node.  Frequently accessed pages on a lower tier NUMA node can be
> > migrated (promoted) to a higher tier NUMA node to improve the
> > performance.
> >
> > A tiering relationship between NUMA nodes in the form of demotion path
> > is created during the kernel initialization and updated when a NUMA
> > node is hot-added or hot-removed.  The current implementation puts all
> > nodes with CPU into the top tier, and then builds the tiering hierarchy
> > tier-by-tier by establishing the per-node demotion targets based on
> > the distances between nodes.
> >
> > The current memory tiering interface needs to be improved to address
> > several important use cases:
> >
> > * The current tiering initialization code always initializes
> >   each memory-only NUMA node into a lower tier.  But a memory-only
> >   NUMA node may have a high performance memory device (e.g. a DRAM
> >   device attached via CXL.mem or a DRAM-backed memory-only node on
> >   a virtual machine) and should be put into the top tier.
> >
> > * The current tiering hierarchy always puts CPU nodes into the top
> >   tier. But on a system with HBM (e.g. GPU memory) devices, these
> >   memory-only HBM NUMA nodes should be in the top tier, and DRAM nodes
> >   with CPUs are better to be placed into the next lower tier.
> >
> > * Also because the current tiering hierarchy always puts CPU nodes
> >   into the top tier, when a CPU is hot-added (or hot-removed) and
> >   triggers a memory node from CPU-less into a CPU node (or vice
> >   versa), the memory tiering hierarchy gets changed, even though no
> >   memory node is added or removed.  This can make the tiering
> >   hierarchy much less stable.
>
> I'd prefer the firmware builds up tiers topology then passes it to
> kernel so that kernel knows what nodes are in what tiers. No matter
> what nodes are hot-removed/hot-added they always stay in their tiers
> defined by the firmware. I think this is important information like
> numa distances. NUMA distance alone can't satisfy all the usecases
> IMHO.

Just want to note here that the platform firmware can only describe
the tiers of static memory present at boot. CXL hotplug breaks this
model and the kernel is left to dynamically determine the device's
performance characteristics and the performance of the topology to
reach that device. Now, the platform firmware does set expectations
for the perfomance class of different memory ranges, but there is no
way to know in advance the performance of devices that will be asked
to be physically or logically added to the memory configuration. That
said, it's probably still too early to define ABI for those
exceptional cases where the kernel needs to make a policy decision
about a device that does not fit into the firmware's performance
expectations, but just note that there are limits to the description
that platform firmware can provide.

I agree that NUMA distance alone is inadequate and the kernel needs to
make better use of data like ACPI HMAT to determine the default
tiering order.