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

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Message-ID: <c6ae2cf8-18e9-44ec-a9ed-287bb1d90c23@amazon.com>
Date: Wed, 28 Jan 2026 15:48:13 +0000
From: "Mohamed Abuelfotoh, Hazem" <abuehaze@...zon.com>
To: Peter Zijlstra <peterz@...radead.org>
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 27/01/2026 08:44, Peter Zijlstra wrote:
> CAUTION: This email originated from outside of the organization. Do not click links or open attachments unless you can confirm the sender and know the content is safe.
> 
> 
> 
> On Sun, Jan 25, 2026 at 12:22:21PM +0000, Mohamed Abuelfotoh, Hazem wrote:
>> I can confirm that we are seeing a 4-11% performance regression in v6.12.66
>> on multiple benchmarks running on c7a.4xlarge AWS EC2 instances that are
>> powered by AMD EPYC 9R14-series CPU (code-named Genoa) and c7i.4xlarge which
>> is powered by 4th-Generation Intel Xeon Scalable processor (code-named
>> Sapphire Rapids).
> 
> For those not speaking Amazon; what actual system setup is that Xeon? Is
> that single socket or multi-socket?
> 
> Going by the name, the 4x would suggest a quad-socket Xeon, which are
> somewhat beastly, but if I google this 'c7i.4xlarge' identifier, I get a
> puny single socket 16cpu thing.
> 
> What is it?

Hi Peter,

Apologize for the confusion, the "4x" is just an Amazon naming for EC2 
instance sizing,  basically number of CPUs & Memory and Network 
Bandwidth. The naming has nothing to do with exact number of sockets 
within the VM.

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

-------------------------------------------------------------------------

c7a.4xlarge

CPU Model: AMD EPYC 9R14
Number of CPUs: 16
Memory: 32 GB
Number of sockets: 1