Multithreading with Intel and AMD

[Rainer]

D:\Temp>crcbench
1 thread(s) 1701523 cpu cycles (100 % of 1 thread). MT scaling factor: 1.00
2 thread(s) 1750000 cpu cycles (102 % of 1 thread). MT scaling factor: 1.94
3 thread(s) 1858069 cpu cycles (109 % of 1 thread). MT scaling factor: 2.75
4 thread(s) 1915659 cpu cycles (112 % of 1 thread). MT scaling factor: 3.55
5 thread(s) 2278678 cpu cycles (133 % of 1 thread). MT scaling factor: 3.73
6 thread(s) 2275967 cpu cycles (133 % of 1 thread). MT scaling factor: 4.49
7 thread(s) 2331998 cpu cycles (137 % of 1 thread). MT scaling factor: 5.11
8 thread(s) 2340575 cpu cycles (137 % of 1 thread). MT scaling factor: 5.82

D:\Temp>crcbench
1 thread(s) 1700522 cpu cycles (100 % of 1 thread). MT scaling factor: 1.00
2 thread(s) 1750055 cpu cycles (102 % of 1 thread). MT scaling factor: 1.94
3 thread(s) 1857493 cpu cycles (109 % of 1 thread). MT scaling factor: 2.75
4 thread(s) 1915634 cpu cycles (112 % of 1 thread). MT scaling factor: 3.55
5 thread(s) 2248357 cpu cycles (132 % of 1 thread). MT scaling factor: 3.78
6 thread(s) 2307894 cpu cycles (135 % of 1 thread). MT scaling factor: 4.42
7 thread(s) 2329516 cpu cycles (136 % of 1 thread). MT scaling factor: 5.11
8 thread(s) 2340757 cpu cycles (137 % of 1 thread). MT scaling factor: 5.81

D:\Temp>

This is a Xeon E3-1265LV2, Performance Setting "CPU always 100%", and Turbo Mode disabled in BIOS.

 

[mrle]

This is a Xeon E3-1265LV2, Performance Setting "CPU always 100%", and Turbo Mode disabled in BIOS.

Sure about the turbo? From the results it looks like it is definitely on (from 1 to 4 threads there should be no difference on a 4-core cpu).

 

[Rainer]

Sure about the turbo?

Well, there's a BIOS menu titled "Turbo Boost Technology" in which you can set "Turbo Mode" to [Disabled], which I did.

It's very well possible that it doesn't have any effect, but there's no way for me to tell. If you improved your little program so that it reports clock speed as well, we would know more

Edit: Indeed it didn't have any effect. Apparently the "Turbo Boost Technology" setting gets ignored unless another BIOS entry named "Power Technology" is set from "Energy Efficient" to "Custom". If we do that, the results looks closer to what we would expect:

1 thread(s) 2375559 cpu cycles (100 % of 1 thread). MT scaling factor: 1.00
2 thread(s) 2375657 cpu cycles (100 % of 1 thread). MT scaling factor: 2.00
3 thread(s) 2375894 cpu cycles (100 % of 1 thread). MT scaling factor: 3.00
4 thread(s) 2376540 cpu cycles (100 % of 1 thread). MT scaling factor: 4.00
5 thread(s) 2743742 cpu cycles (115 % of 1 thread). MT scaling factor: 4.33
6 thread(s) 2832707 cpu cycles (119 % of 1 thread). MT scaling factor: 5.03
7 thread(s) 2875679 cpu cycles (121 % of 1 thread). MT scaling factor: 5.78
8 thread(s) 2900571 cpu cycles (122 % of 1 thread). MT scaling factor: 6.55

Nonetheless I still think that reporting clock speed dependent on thread count would make a nice additional feature in your program

 

[mrle]

I just found out that it might be possible to temporarily disable turbo in Windows without rebooting and changing BIOS settings. Apparently, all it takes is to go to Control Panel > Power Options > Advanced settings > Processor power management and set Maximum processor state to 99%. Can anyone try it out? Right now I don't have a Windows machine with turbo cpu to test, but I can confirm that a similar procedure disables turbo on linux:

cpupower frequency-set --governor userspace
cpupower -c all frequency-set -f 2899999

In the example above, 2899999 is 1 less than base clock of 2900000.

This will bring it back on:

cpupower frequency-set --governor ondemand