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

Mohamed Abuelfotoh, Hazem
2026-01-28T16:03:29Z

On 27/01/2026 08:50, Peter Zijlstra wrote:
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> 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.