linux-kernel - Re: [RFC patch v3 00/20] Cache aware scheduling

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Message-ID: <ddb9d558-d114-41db-9d4b-296fc2ecdbb4@linux.ibm.com>
Date: Fri, 4 Jul 2025 01:30:09 +0530
From: Shrikanth Hegde <sshegde@...ux.ibm.com>
To: K Prateek Nayak <kprateek.nayak@....com>, Tim Chen
 <tim.c.chen@...el.com>,
        Chen Yu <yu.c.chen@...el.com>
Cc: Juri Lelli <juri.lelli@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>,
        Mel Gorman <mgorman@...e.de>, Valentin Schneider <vschneid@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Libo Chen <libo.chen@...cle.com>, Abel Wu <wuyun.abel@...edance.com>,
        Madadi Vineeth Reddy <vineethr@...ux.ibm.com>,
        Hillf Danton <hdanton@...a.com>, Len Brown <len.brown@...el.com>,
        linux-kernel@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        "Gautham R . Shenoy" <gautham.shenoy@....com>
Subject: Re: [RFC patch v3 00/20] Cache aware scheduling


> 
> tl;dr
> 
> o Benchmark that prefer co-location and run in threaded mode see
>    a benefit including hackbench at high utilization and schbench
>    at low utilization.
> 
> o schbench (both new and old but particularly the old) regresses
>    quite a bit on the tial latency metric when #workers cross the
>    LLC size.
> 
> o client-server benchmarks where client and servers are threads
>    from different processes (netserver-netperf, tbench_srv-tbench,
>    services of DeathStarBench) seem to noticeably regress due to
>    lack of co-location between the communicating client and server.
> 
>    Not sure if WF_SYNC can be an indicator to temporarily ignore
>    the preferred LLC hint.
> 
> o stream regresses in some runs where the occupancy metrics trip
>    and assign a preferred LLC for all the stream threads bringing
>    down performance in !50% of the runs.
> 

- When you have SMT systems, threads will go faster if they run in ST mode.
If aggregation happens in a LLC, they might end up with lower IPC.

> Full data from my testing is as follows:
> 
> o Machine details
> 
> - 3rd Generation EPYC System
> - 2 sockets each with 64C/128T
> - NPS1 (Each socket is a NUMA node)
> - C2 Disabled (POLL and C1(MWAIT) remained enabled)
> 
> o Kernel details
> 
> tip:      tip:sched/core at commit 914873bc7df9 ("Merge tag
>             'x86-build-2025-05-25' of
>             git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip")
> 
> llc-aware-lb-v3: tip + this series as is
> 
>