Re: [FIX PATCH] mm: pcp: fix pcp->free_count reduction on page allocation

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

On Tue, 7 Jan 2025 14:47:24 +0530 Nikhil Dhama <nikhil.dhama@amd.com> wrote:

> In current PCP auto-tuning desgin, free_count was introduced to track
> the consecutive page freeing with a counter, This counter is incremented
> by the exact amount of pages that are freed, but reduced by half on
> allocation. This is causing a 2-node iperf3 client to server's network
> bandwidth to drop by 30% if we scale number of client-server pairs from 32
> (where we achieved peak network bandwidth) to 64.
> 
> To fix this issue, on allocation, reduce free_count by the exact number
> of pages that are allocated instead of halving it.

The present division by two appears to be somewhat randomly chosen. 
And as far as I can tell, this patch proposes replacing that with
another somewhat random adjustment.

What's the actual design here?  What are we attempting to do and why,
and why is the proposed design superior to the present one?

> On a 2-node AMD server, one running iperf3 clients and other iperf3
> sever, This patch restores the performance drop.

Nice, but might other workloads on other machines get slower?