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

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Message-ID: <5314a23f-6873-41b7-a3de-cb6f54e982db@amazon.com>
Date: Wed, 28 Jan 2026 16:03:09 +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 08:50, 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). 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?
True, memcached performance on Intel is exactly the same with and 
without commit 33cf66d88306 ("sched/fair: Proportional newidle 
balance"). The memcached regression is only visible on AMD. I also 
tested on arm64 VMs using Graviton3(based on Arm Neoverse V1) and 
Graviton4(based on Arm Neoverse V2) and I don't see any memcached 
regression there.