Re: [PATCH v2 0/5] workqueue: Introduce a sharded cache affinity scope

[Chuck Lever]

On 3/23/26 12:26 PM, Breno Leitao wrote:
> Hello Chuck,
> 
> On Mon, Mar 23, 2026 at 11:28:49AM -0400, Chuck Lever wrote:
>> On 3/23/26 11:10 AM, Breno Leitao wrote:
>>>
>>> I am not convinced. The wq_cache_shard_size approach creates multiple
>>> pools on large systems while leaving small systems (<8 cores) unchanged.
>>
>> This is exactly my concern. Smaller systems /do/ experience measurable
>> contention in this area. I don't object to your series at all, it's
>> clean and well-motivated; but the cores-per-shard approach doesn't scale
>> down to very commonly deployed machine sizes.
> 
> I don't see why the cores-per-shard approach wouldn't scale down
> effectively.

Sharding the UNBOUND pool is fine. But with a fixed cores-per-shard
ratio of 8, it doesn't scale down to smaller systems.


> The sharding mechanism itself is independent of whether we use
> cores-per-shard or shards-per-LLC as the allocation strategy, correct?
> 
> Regardless of the approach, we retain full control over the granularity
> of the shards.
> 
>> We might also argue that the NFS client and other subsystems that make
>> significant use of UNBOUND workqueues in their I/O paths might be well
>> advised to modify their approach. (net/sunrpc/sched.c, hint hint)
>>
>>
>>> This eliminates the pathological lock contention we're observing on
>>> high-core-count machines without impacting smaller deployments.
>>
>>> In contrast, splitting pools per LLC would force fragmentation even on
>>> systems that aren't experiencing contention, increasing the need for
>>> manual tuning across a wider range of configurations.
>>
>> I claim that smaller deployments also need help. Further, I don't see
>> how UNBOUND pool fragmentation is a problem on such systems that needs
>> to be addressed (IMHO).
> 
> Are you suggesting we should reduce the default value to something like
> wq_cache_shard_size=2 instead of wq_cache_shard_size=8?

A shard size of 2 clearly won't scale properly to hundreds of cores. A
varying default cores-per-shard ratio would help scaling in both
directions, without having to manually tune.


-- 
Chuck Lever