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

Peter Zijlstra
2026-01-27T08:50:34Z

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?