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

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Message-ID: <20260127085025.GW171111@noisy.programming.kicks-ass.net>
Date: Tue, 27 Jan 2026 09:50:25 +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 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). The regression is caused by the commit 33cf66d88306
> ("sched/fair: Proportional newidle balance"). We were able to reclaim the
> performance back after reverting this commit. We also noticed that the
> impact is higher on AMD vs Intel.
> 
> Benchmark Name |  Description				    | Unit
> postgresql     |  HammerDB workload (TPC-C-like benchmark)  | NOPM
> nginx_lb       |  Testing NGINX as a load balancer	    | RPS
> memcached      |  Testing using Lancet load generator       | QPS
> 
> **Results on v6.12.66**
> 
> Benchmark name | SUT EC2 Instance | Regression percentage
> postgresql     | c7a.4xlarge      | -4.0%
> postgresql     | c7i.4xlarge      | -4.0%
> nginx_lb       | c7a.4xlarge      | -5.0%
> memcached      | c7a.4xlarge      | -11.0%

So only postgres has a regression on Intel? Memcached doesn't show
anything?