Re: RFC: Memory Tiering Kernel Interfaces

[Wei Xu <weixugc@google.com>]

On Sun, May 1, 2022 at 11:35 AM Dan Williams <dan.j.williams@intel.com> wrote:
>
> 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.

Very useful clarification. It should be fine for the kernel to
dynamically determine the memory tier of each node.  I expect that it
can also be fine even if a node gets attached to a different memory
device and needs to be assigned into a different tier after another
round of hot-remove/hot-add.

What can be problematic is that a hot-added node not only changes its
own iter, but also causes other existing nodes to change their tiers.
This can mess up any tier-based memory accounting.

One approach to address this is to:

- have tiers being well-defined and stable, e.g. HBM is always in
tier-0, direct-attached DRAM and high-performance CXL.mem devices are
always in tier-1, slower CXL.mem devices are always in tier-2, and
PMEM is always in tier-3.  The tier definition is based on the device
performance, something similar to the class rating of storage devices
(e.g. SD cards).

- allow tiers being absent in the system, e.g. a machine may have only
tier-1 and tier-3, but have neither tier-0 nor tier-2.

- allow demotion to not only the immediate next lower tier, but all
lower tiers.  The actual selection of demotion order follows the
allocation fallback order.   This allows tier-1 to directly demote to
tier-3 without requiring the presence of tier-2.

This approach can ensure that the tiers of existing nodes are stable
and permit that the tier of a hot-plugged node is determined
dynamically.