linux-kernel - Re: Odd performance results

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Date:	Tue, 12 Jul 2016 10:49:58 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	paulmck@...ux.vnet.ibm.com, Peter Zijlstra <peterz@...radead.org>
Cc:	tglx@...utronix.de, mingo@...e.hu, ak@...ux.intel.com,
	linux-kernel@...r.kernel.org
Subject: Re: Odd performance results

On 07/12/16 08:05, Paul E. McKenney wrote:
> On Tue, Jul 12, 2016 at 04:55:51PM +0200, Peter Zijlstra wrote:
>> On Sun, Jul 10, 2016 at 07:43:27AM -0700, Paul E. McKenney wrote:
>>> On Sun, Jul 10, 2016 at 07:17:19AM +0200, Peter Zijlstra wrote:
>>>>
>>>>
>>>> On 10 July 2016 06:26:39 CEST, "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:
>>>>> Hello!
>>>>>
>>>>> So I ran a quick benchmark which showed stair-step results.  I
>>>>> immediately
>>>>> thought "Ah, this is due to CPU 0 and 1, 2 and 3, 4 and 5, and 6 and 7
>>>>> being threads in a core."  Then I thought "Wait, this is an x86!"
>>>>> Then I dumped out cpu*/topology/thread_siblings_list, getting the
>>>>> following:
>>>>>
>>>>> 	cpu0/topology/thread_siblings_list: 0-1
>>>>> 	cpu1/topology/thread_siblings_list: 0-1
>>>>> 	cpu2/topology/thread_siblings_list: 2-3
>>>>> 	cpu3/topology/thread_siblings_list: 2-3
>>>>> 	cpu4/topology/thread_siblings_list: 4-5
>>>>> 	cpu5/topology/thread_siblings_list: 4-5
>>>>> 	cpu6/topology/thread_siblings_list: 6-7
>>>>> 	cpu7/topology/thread_siblings_list: 6-7
>>>>
>>>>
>>>> I'm guessing this is an AMD bulldozer like machine?
>>>
>>> /proc/cpuinfo thinks otherwise:
>>>
>>> processor	: 0
>>> vendor_id	: GenuineIntel
>>> cpu family	: 6
>>> model		: 60
>>> model name	: Intel(R) Core(TM) i7-4710MQ CPU @ 2.50GHz
>>
>> Weird, I've never seen an Intel box do that before... hpa, any idea? or
>> is this just one weird BIOS.
> 
> ;-)
> 
> It is a Lenovo W541 laptop, for whatever that might be worth.  Roughly
> on year old.
> 

Well, the obvious thing here is that CPUs 0-1, 2-3, 4-5, and 6-7 *are*
indeed threads in a core... Intel x86 products have supported
multithreading since the Pentium 4.  So the "wait, this is an x86!" bit
is strange to me.

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?

Also, you mentioned absolutely nothing about what kind of benchmark it
was, or what the "stairstepping" results imply, so it doesn't really
make it any easier...

	-hpa