Re: [PATCH v3 15/21] sched/cache: Disable cache aware scheduling for processes with high thread counts

[Madadi Vineeth Reddy <vineethr@linux.ibm.com>]

Hi Peter,

Sorry for the delayed response. Wanted to be sure before responding.

On 20/02/26 15:23, Peter Zijlstra wrote:
> On Fri, Feb 20, 2026 at 12:10:21PM +0530, Madadi Vineeth Reddy wrote:
>> Hi Peter,
>>
>> On 19/02/26 22:25, Peter Zijlstra wrote:
>>> On Wed, Feb 18, 2026 at 11:24:05PM +0530, Madadi Vineeth Reddy wrote:
>>>> Is there a way to make this useful for architectures with small LLC
>>>> sizes? One possible approach we were exploring is to have LLC at a
>>>> hemisphere level that comprise multiple SMT4 cores.
>>>
>>> Is this hemisphere an actual physical cache level, or would that be
>>> artificial?
>>
>> It's artificial. There is no cache being shared at this level but this is
>> still the level where some amount of cache-snooping takes place and it is
>> relatively faster to access the data from the caches of the cores
>> within this domain.
>>
>> We verified with this producer consumer workload where the producer
>> and consumer threads placed in the same hemisphere showed measurably
>> better latency compared to cross-hemisphere placement.
> 
> So I just read the Power10 Wikipedia entry; that seems to suggest there
> actually is a significant L3 at the hemisphere level.
> 
> That thing states that Power10 has:
> 
>  - 16 cores in two hemispheres of 8 cores each.
>  - each core has 2M L2 cache
>  - each hemi has 64M of L3 cache

The Wikipedia entry is incorrect. On Power10, L3 is at the SMT4
small core level (4M per core), not at the hemisphere level. This
is documented in the Power10 user manual [1] (Page 175). L3 is
also a victim cache on Power10.

> 
> Then there appears to be a 'funny' in that there's always one 'dead'
> core, so you end up with 8+7, and the small hemi looses an 8M L3 slice
> due to that.
> 
> Now, I'm just reading a Wiki pages written by a random person on the
> interweb, so perhaps this is wrong (in which case I would suggest you

Yes, the Wikipedia page is wrong on this. We will get it corrected
with proper references.

[1] https://files.openpower.foundation/s/EgCy7C43p2NSRfR

Thanks,
Vineeth

> get someone from IBM to go and edit that page and provide references),
> or there has been a miscommunication somewhere else, and perhaps there
> really is L3 at the hemi level, and arch/powerpc/ 'forgot' to expose
> that :-)