Re: [PATCH v2 19/23] sched/cache: Avoid cache-aware scheduling for memory-heavy processes

["Chen, Yu C" <yu.c.chen@intel.com>]

On 12/19/2025 11:14 AM, K Prateek Nayak wrote:
> Hello Vern,
> 
> On 12/18/2025 3:12 PM, Vern Hao wrote:
>>
>> On 2025/12/18 16:32, Chen, Yu C wrote:
>>> On 12/18/2025 11:59 AM, Vern Hao wrote:
>>>>
>>>> On 2025/12/4 07:07, Tim Chen wrote:
>>>>> From: Chen Yu <yu.c.chen@intel.com>
>>>>>
>>>>> Prateek and Tingyin reported that memory-intensive workloads (such as
>>>>> stream) can saturate memory bandwidth and caches on the preferred LLC
>>>>> when sched_cache aggregates too many threads.
>>>>>
>>>>> To mitigate this, estimate a process's memory footprint by comparing
>>>>> its RSS (anonymous and shared pages) to the size of the LLC. 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