Why does Linux's scheduler put two threads onto the same physical core on processors with HyperThreading?

Question

I\'ve read in multiple places that Linux\'s default scheduler is hyperthreading aware on multi-core machines, meaning that if you have a machine with 2 real cor

Answer 1

I'm unable to reproduce this on 3.13.0-48 with my Intel(R) Xeon(R) CPU E5-1650 0 @ 3.20GHz.

I have 6 cores with hyperthreading, where logical core N maps to physical core N mod 6.

Here's a typical output of top with stress -c 4 in two columns, so that each row is one physical core (I left out a few cores because my system is not idle):

%Cpu0  :100.0 us,   %Cpu6  :  0.0 us, 
%Cpu1  :100.0 us,   %Cpu7  :  0.0 us, 
%Cpu2  :  5.9 us,   %Cpu8  :  2.0 us, 
%Cpu3  :100.0 us,   %Cpu9  :  5.7 us, 
%Cpu4  :  3.9 us,   %Cpu10 :  3.8 us, 
%Cpu5  :  0.0 us,   %Cpu11 :100.0 us, 

Here it is after killing and restarting stress:

%Cpu0  :100.0 us,   %Cpu6  :  2.6 us, 
%Cpu1  :100.0 us,   %Cpu7  :  0.0 us, 
%Cpu2  :  0.0 us,   %Cpu8  :  0.0 us, 
%Cpu3  :  2.6 us,   %Cpu9  :  0.0 us, 
%Cpu4  :  0.0 us,   %Cpu10 :100.0 us, 
%Cpu5  :  2.6 us,   %Cpu11 :100.0 us, 

I did this several times, and did not see any instances where 4 threads across 12 logical cores would schedule on the same physical core.

With -c 6 I tend to get results like this, where Linux appears to be helpfully scheduling other processes on their own physical cores. Even so, they're distributed way better than chance:

%Cpu0  : 18.2 us,   %Cpu6  :  4.5 us, 
%Cpu1  :  0.0 us,   %Cpu7  :100.0 us, 
%Cpu2  :100.0 us,   %Cpu8  :100.0 us, 
%Cpu3  :100.0 us,   %Cpu9  :  0.0 us, 
%Cpu4  :100.0 us,   %Cpu10 :  0.0 us, 
%Cpu5  :100.0 us,   %Cpu11 :  0.0 us,