Re: [PATCH 4/4] sched/fair: Proportional newidle balance
From: Mohamed Abuelfotoh, Hazem
Date: Wed Jan 28 2026 - 11:07:45 EST
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.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.
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?