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Fri, 19 Dec 2025 12:56:01 +0000 Message-ID: <94c8c1af-d9a5-411b-bc54-a7b28d6cff29@intel.com> Date: Fri, 19 Dec 2025 20:55:46 +0800 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 19/23] sched/cache: Avoid cache-aware scheduling for memory-heavy processes To: K Prateek Nayak , Vern Hao CC: Juri Lelli , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Valentin Schneider , Madadi Vineeth Reddy , "Hillf Danton" , Shrikanth Hegde , "Jianyong Wu" , Yangyu Chen , Tingyin Duan , Vern Hao , Len Brown , Aubrey Li , Zhao Liu , Chen Yu , Adam Li , Aaron Lu , Tim Chen , , Tim Chen , Peter Zijlstra , "Vincent Guittot" , "Gautham R . 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If RSS >>>>> exceeds the LLC size, skip cache-aware scheduling. >>>> Restricting RSS prevents many applications from benefiting from this optimization. I believe this restriction should be lifted. For memory- intensive workloads, the optimization may simply yield no gains, but it certainly shouldn't make performance worse. We need to further refine this logic. >>> >>> Memory-intensive workloads may trigger performance regressions when >>> memory bandwidth(from L3 cache to memory controller) is saturated due >> RSS size and bandwidth saturation are not necessarily linked, In my view, the optimization should be robust enough that it doesn't cause a noticeable drop in performance, no matter how large the RSS is. > > Easier said than done. I agree RSS size is not a clear indication of > bandwidth saturation. With NUMA Balancing enabled, we can use the > hinting faults to estimate the working set and make decisions but for > systems that do not have NUMA, short of programming some performance > counters, there is no real way to estimate the working set. > > Hinting faults are known to cause overheads so enabling them without > NUMA can cause noticeable overheads with no real benefits. > >> We need to have a more profound discussion on this. > > What do you have in mind? > > From where I stand, having the RSS based bailout for now won't make > things worse for these tasks with huge memory reserves and when we can > all agree on some generic method to estimate the working set of a task, > we can always add it into exceed_llc_capacity(). > Prateek, thanks very much for the practical callouts - using RSS seems to be the best trade-off we can go with for now. Vern, I get your point about the concern between RSS and actual memory footprint. However, detecting the working set doesn’t seem to be accurate or generic in kernel space - even with NUMA fault statistics sampling. One reliable way I can think of to detect the working set is in user space, via resctrl (Intel RDT, AMD QoS, Arm MPAM). So maybe we can leverage that information to implement fine-grained control on a per-process or per-task basis later. thanks, Chenyu