Re: [RFC PATCH 0/2] zswap: fix placement inversion in memory tiering systems

[Yosry Ahmed]

March 29, 2025 at 1:02 PM, "Nhat Pham" <nphamcs@xxxxxxxxx> wrote:

> Currently, systems with CXL-based memory tiering can encounter the
> following inversion with zswap: the coldest pages demoted to the CXL
> tier can return to the high tier when they are zswapped out,
> creating memory pressure on the high tier.
> This happens because zsmalloc, zswap's backend memory allocator, does
> not enforce any memory policy. If the task reclaiming memory follows
> the local-first policy for example, the memory requested for zswap can
> be served by the upper tier, leading to the aformentioned inversion.
> This RFC fixes this inversion by adding a new memory allocation mode
> for zswap (exposed through a zswap sysfs knob), intended for
> hosts with CXL, where the memory for the compressed object is requested
> preferentially from the same node that the original page resides on.

I didn't look too closely, but why not just prefer the same node by default? Why is a knob needed?

Or maybe if there's a way to tell the "tier" of the node we can prefer to allocate from the same "tier"?