Re: [PATCH v1 0/2] mm/vmscan: optimize preferred target demotion node selection

[Andrew Morton <akpm@linux-foundation.org>]

On Wed,  7 Jan 2026 07:28:12 +0000 Bing Jiao <bingjiao@google.com> wrote:

> In tiered memory systems, the demotion aims to move cold folios to the
> far-tier nodes. To maintain system performance, the demotion target
> should ideally be the node with the shortest NUMA distance from the
> source node.
> 
> However, the current implementation has two suboptimal behaviors:
> 
> 1. Unbalanced Fallback: When the primary preferred demotion node is full,
>    the allocator falls back to other nodes in a way that often skews
>    toward zones that closer to the primary preferred node rather than
>    distributing the load evenly across fallback nodes.
> 
> 2. Suboptimal Target Selection: demote_folio_list() randomly select
>    a preferred node from the allowed mask, potentially selecting
>    a very distant node.
> 
> This series optimizes the selection logic while ensuring balanced
> allocation across fallback nodes.
> 
> Patch 1/2 introduces a randomized fallback mechanism in
> alloc_demote_folio() to prevent allocation hotspots when the preferred
> node is under memory pressure.
> 
> Patch 2/2 updates demote_folio_list() to traverse the demotion targets
> hierarchically, ensuring the perferred target is always the closest
> available node.

Yes, those things sound rather suboptimal.  Do you have any data which
will help us understand the performance benefit of these changes?

(2/2 has a typo in the subject and in the comment.  perferred->preferred)