Re: [PATCH RFC 0/5] workqueue: add WQ_AFFN_CACHE_SHARD affinity scope

[Chuck Lever]

On 3/17/26 7:32 AM, Breno Leitao wrote:
> Hello Tejun,
> 
> On Fri, Mar 13, 2026 at 07:57:20AM -1000, Tejun Heo wrote:
>> Hello,
>>
>> Applied 1/5. Some comments on the rest:
>>
>> - The sharding currently splits on CPU boundary, which can split SMT
>>   siblings across different pods. The worse performance on Intel compared
>>   to SMT scope may be indicating exactly this - HT siblings ending up in
>>   different pods. It'd be better to shard on core boundary so that SMT
>>   siblings always stay together.
> 
> Thank you for the insight. I'll modify the sharding to operate at the
> core boundary rather than at the SMT/thread level to ensure sibling CPUs
> remain in the same pod.
> 
>> - How was the default shard size of 8 picked? There's a tradeoff
>> between the number of kworkers created and locality. Can you also
>> report the number of kworkers for each configuration? And is there
>> data on different shard sizes? It'd be useful to see how the numbers
>> change across e.g. 4, 8, 16, 32.
> 
> The choice of 8 as the default shard size was somewhat arbitrary – it was
> selected primarily to generate initial data points.

Perhaps instead of basing the sharding on a particular number of CPUs
per shard, why not cap the total number of shards? IIUC that is the main
concern about ballooning the number of kworker threads.


> I'll run tests with different shard sizes and report the results.
> 
> I'm currently working on finding a suitable workload with minimal noise.
> Testing on real NVMe devices shows significant jitter that makes analysis
> difficult. I've also been experimenting with nullblk, but haven't had much
> success yet.
> 
> If you have any suggestions for a reliable workload or benchmark, I'd
> appreciate your input.
> 
>> - Can you also test on AMD machines? Their CCD topology (16 or 32
>> threads per LLC) would be a good data point.
> 
> Absolutely, I'll test on AMD machines as well.
> 
> Thanks,
> --breno


-- 
Chuck Lever