Re: [PATCH v3] x86,sched: allow topologies where NUMA nodes share an LLC

[Peter Zijlstra]

On Thu, Mar 29, 2018 at 07:37:29AM -0700, Dave Hansen wrote:
> On 03/29/2018 06:47 AM, Peter Zijlstra wrote:
> > Further I think Dave argued that we should not change the llc-size,
> > because while SNC presents a subset of the cache to local CPUs, for
> > remote data the whole cache is still available, again something some
> > applications might rely on.
> 
> BTW, I may have argued this in the past, but I don't think it's the best
> thing to do.
> 
> If anything, we should optimize for the _users_ of this information: the
> performance-sensitive ones who are digging up the cache topology.  They
> are also likely to be the most NUMA-affinitized and stay node-local with
> much of their memory traffic.  That would seem to point us in the
> direction of enumerating two separate, half-sized LLCs "shared" only by
> the slice when SNC mode is on.
> 
> That's what I've argued to the hardware folks lately, at least.

Fair enough; then we need change the reported cache-size and mention all
this in (preferably) both comment and Changelog.