Re: RFC: Memory Tiering Kernel Interfaces

[Davidlohr Bueso]

Nice summary, thanks. I don't know who of the interested parties will be
at lsfmm, but fyi we have a couple of sessions on memory tiering Tuesday
at 14:00 and 15:00.

On Fri, 29 Apr 2022, Wei Xu 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.

Regardless of the promotion algorithm, at some point I see the NUMA hinting
fault mechanism being in the way of performance. It would be nice if hardware
began giving us page "heatmaps" instead of having to rely on faulting or
sampling based ways to identify hot memory.

> 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.

At least the CXL memory (volatile or not) will still be slower than
regular DRAM, so I think that we'd not want this to be top-tier. But
in general, yes I agree that defining top tier as whether or not the
node has a CPU a bit limiting, as you've detailed here.

> Tiering Hierarchy Initialization
> ================================
>
> By default, all memory nodes are in the top tier (N_TOPTIER_MEMORY).
>
> A device driver can remove its memory nodes from the top tier, e.g.
> a dax driver can remove PMEM nodes from the top tier.
>
> The kernel builds the memory tiering hierarchy and per-node demotion
> order tier-by-tier starting from N_TOPTIER_MEMORY.  For a node N, the
> best distance nodes in the next lower tier are assigned to
> node_demotion[N].preferred and all the nodes in the next lower tier
> are assigned to node_demotion[N].allowed.
>
> node_demotion[N].preferred can be empty if no preferred demotion node
> is available for node N.

Upon cases where there more than one possible demotion node (with equal
cost), I'm wondering if we want to do something better than choosing
randomly, like we do now - perhaps round robin? Of course anything
like this will require actual performance data, something I have seen
very little of.

> Memory tiering hierarchy is rebuilt upon hot-add or hot-remove of a
> memory node, but is NOT rebuilt upon hot-add or hot-remove of a CPU
> node.

I think this makes sense.

Thanks,
Davidlohr