Re: [RFC][PATCH] x86, sched: allow topolgies where NUMA nodes share an LLC

From: Dave Hansen
Date: Thu Nov 09 2017 - 13:14:04 EST


On 11/09/2017 06:07 AM, Borislav Petkov wrote:
> On Wed, Nov 08, 2017 at 04:00:38PM -0800, Dave Hansen wrote:
>> I'd argue that those two end up looking pretty much the same to an app.
>> The only difference is that the slice-local and slice-remote cache hits
>> have slightly different access latencies. I don't think it's enough to
>> notice.
>
> So if it is not enough to notice, why do we even bother? I.e., is
> there any workload showing any advantages at all from the resources
> partitioning?

If you want the *absolutely* best latency available, you turn on SNC.
You get a small boost to slice-local access and a slight penalty to
remote-slice access compared to when Sub-NUMA-Clustering is off.

You can measure this for sure, but I'll still say that most folks will
never notice. In addition, if you have access interleaved everywhere,
the "slice-local boost" and "remote-slice penalty" roughly cancel
each-other out.