linux-kernel - Re: Odd performance results

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Date:	Tue, 12 Jul 2016 20:51:20 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	paulmck@...ux.vnet.ibm.com, tglx@...utronix.de, mingo@...e.hu,
	ak@...ux.intel.com, linux-kernel@...r.kernel.org
Subject: Re: Odd performance results

On Tue, Jul 12, 2016 at 10:49:58AM -0700, H. Peter Anvin wrote:
> On 07/12/16 08:05, Paul E. McKenney wrote:
> The CPU in question (and /proc/cpuinfo should show this) has four cores
> with a total of eight threads.  The "siblings" and "cpu cores" fields in
> /proc/cpuinfo should show the same thing.  So I am utterly confused
> about what is unexpected here?

Typically threads are enumerated differently on Intel parts. Namely:

	cpu_id = code_id + nr_cores * smt_id

which gives, for a 4 core, 2 thread part:

	0-3: core 0-3, smt0
	4-7: core 0-3, smt1

My Core i7-2600k for example has:

$ cat /sys/devices/system/cpu/cpu*/topology/thread_siblings_list
0,4
1,5
2,6
3,7
0,4
1,5
2,6
3,7

The ordering Paul has, namely 0,1 for core0,smt{0,1} is not something
I've ever seen on an Intel part. AMD otoh does enumerate their CMT stuff
like what Paul has.