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

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Message-ID: <0b42d417-9c68-3e48-7736-db33310ba332@linux.intel.com>
Date:   Thu, 9 Nov 2017 10:13:53 -0800
From:   Dave Hansen <dave.hansen@...ux.intel.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org, tony.luck@...el.com,
        tim.c.chen@...ux.intel.com, hpa@...ux.intel.com,
        rientjes@...gle.com, imammedo@...hat.com, prarit@...hat.com,
        toshi.kani@...com, brice.goglin@...il.com, mingo@...nel.org
Subject: Re: [RFC][PATCH] x86, sched: allow topolgies where NUMA nodes share
 an LLC

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.