Re: Odd performance results

[Ingo Molnar <mingo@kernel.org>]

* Peter Zijlstra <peterz@infradead.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 = core_id + nr_cores * smt_id

Yeah, they are 'interleaved' at the thread/core level - I suppose to 'mix' them on 
OS schedulers that don't know about SMT.

(Fortunately this interleaving is not done across NUMA domains.)

> $ cat /sys/devices/system/cpu/cpu*/topology/thread_siblings_list

Btw., this command will print out the mappings in order even on larger systems and 
shows the CPU # as well:

 $ grep -i . /sys/devices/system/cpu/cpu*/topology/thread_siblings_list | sort -t u -k +3 -n

/sys/devices/system/cpu/cpu0/topology/thread_siblings_list:0,60
/sys/devices/system/cpu/cpu1/topology/thread_siblings_list:1,61
/sys/devices/system/cpu/cpu2/topology/thread_siblings_list:2,62
/sys/devices/system/cpu/cpu3/topology/thread_siblings_list:3,63
/sys/devices/system/cpu/cpu4/topology/thread_siblings_list:4,64
/sys/devices/system/cpu/cpu5/topology/thread_siblings_list:5,65
/sys/devices/system/cpu/cpu6/topology/thread_siblings_list:6,66
/sys/devices/system/cpu/cpu7/topology/thread_siblings_list:7,67
/sys/devices/system/cpu/cpu8/topology/thread_siblings_list:8,68
/sys/devices/system/cpu/cpu9/topology/thread_siblings_list:9,69
/sys/devices/system/cpu/cpu10/topology/thread_siblings_list:10,70
/sys/devices/system/cpu/cpu11/topology/thread_siblings_list:11,71
...
/sys/devices/system/cpu/cpu116/topology/thread_siblings_list:56,116
/sys/devices/system/cpu/cpu117/topology/thread_siblings_list:57,117
/sys/devices/system/cpu/cpu118/topology/thread_siblings_list:58,118
/sys/devices/system/cpu/cpu119/topology/thread_siblings_list:59,119

> 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.

That's more the natural 'direct' mapping from CPU internal topology to CPU id: 
what's close to each other physically is close to each other in the CPU id space 
as well.

Thanks,

	Ingo