linux-kernel - Re: [CRM114spam]: Re: Odd performance results

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Message-Id: <20160712195926.GY7094@linux.vnet.ibm.com>
Date:	Tue, 12 Jul 2016 12:59:26 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	"Dr. David Alan Gilbert" <dave@...blig.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	"H. Peter Anvin" <hpa@...or.com>, tglx@...utronix.de,
	mingo@...e.hu, ak@...ux.intel.com, linux-kernel@...r.kernel.org
Subject: Re: [CRM114spam]: Re: Odd performance results

On Tue, Jul 12, 2016 at 08:10:49PM +0100, Dr. David Alan Gilbert wrote:
> * Peter Zijlstra (peterz@...radead.org) wrote:
> > 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.
> 
> Paul's isn't unique:
> cat /sys/devices/system/cpu/cpu*/topology/thread_siblings_list
> 0-1
> 0-1
> 2-3
> 2-3
> 
> i7-3520M CPU @ 2.90GHz   (Dual core with hyperthread, Thinkpad t530, fedora 24)

Glad that it is not just me!  ;-)

								Thanx, Paul