linux-kernel - Re: [PATCH 00/21] perf, c2c: Add new tool to analyze cacheline contention on NUMA systems

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Date:	Mon, 10 Feb 2014 22:18:25 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Don Zickus <dzickus@...hat.com>
Cc:	acme@...stprotocols.net, LKML <linux-kernel@...r.kernel.org>,
	jolsa@...hat.com, jmario@...hat.com, fowles@...each.com,
	eranian@...gle.com
Subject: Re: [PATCH 00/21] perf, c2c: Add new tool to analyze cacheline
 contention on NUMA systems

On Mon, Feb 10, 2014 at 12:28:55PM -0500, Don Zickus wrote:
> With the introduction of NUMA systems, came the possibility of remote memory accesses.
> Combine those remote memory accesses with contention on the remote node (ie a modified
> cacheline) and you have a possibility for very long latencies.  These latencies can
> bottleneck a program.
> 
> The program added by these patches, helps detect the situation where two nodes are
> 'tugging' on the same _data_ cacheline.  The term used through out this program and
> the various changelogs is called a HITM.  This means nodeX went to read a cacheline
> and it was discovered to be loaded in nodeY's LLC cache (hence the cacheHIT). The 
> remote cacheline was also in a 'M'odified state thus creating a 'HIT M' for hit in
> a modified state.  HITMs can happen locally and remotely.  This program's interest
> is mainly in remote HITMs as they cause the longest latencies.

All of that is true of the traditional SMP system too. Just use lower
level caches.
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