linux-kernel - Re: [PATCH 4/4] sched/fair: Proportional newidle balance

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Message-ID: <20260129091937.GY166857@noisy.programming.kicks-ass.net>
Date: Thu, 29 Jan 2026 10:19:37 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: "Mohamed Abuelfotoh, Hazem" <abuehaze@...zon.com>
Cc: Mario Roy <marioeroy@...il.com>, Chris Mason <clm@...a.com>,
	Joseph Salisbury <joseph.salisbury@...cle.com>,
	Adam Li <adamli@...amperecomputing.com>,
	Josh Don <joshdon@...gle.com>, mingo@...hat.com,
	juri.lelli@...hat.com, vincent.guittot@...aro.org,
	dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
	mgorman@...e.de, vschneid@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/4] sched/fair: Proportional newidle balance

On Wed, Jan 28, 2026 at 03:48:13PM +0000, Mohamed Abuelfotoh, Hazem wrote:

> Below are the hardware specs for both c7i.4xlarge & c7a.4xlarge.
> 
> c7i.4xlarge
> 
> CPU Model: Intel(R) Xeon(R) Platinum 8488C
> Number of CPUs: 16
> Memory: 32 GB
> Number of sockets: 1

But the 8488C is a 56 core part, with 112 threads. So you're handing out
8 core partitions of that thing, for 7 such instances on one machine?

(Also, calling anything 16 core with 32GB 'large' is laughable, that's
laptop territory.)

> -------------------------------------------------------------------------
> 
> c7a.4xlarge
> 
> CPU Model: AMD EPYC 9R14
> Number of CPUs: 16
> Memory: 32 GB
> Number of sockets: 1

And that 9r14 is a 96 core part, 12 CCDs, 8 cores each. So you're again
handing out partitions of that.

For both cases, are these partitions fixed? Specifically in the AMD case,
are you handing out exactly 1 CCDs per partition?

Because if so, I'm mighty confused by the results. 8 cores, 16 threads
is exactly one CCD worth of Zen4 and should therefore be a single L3 and
behave exactly like the Intel thing.

Something is missing here.