Quote Originally Posted by sniper_sung View Post
Many of you must have noticed that disabling HT could result in more Gflops in LinX. Some of you have already known the reason. I myself have done some small research into this.

I have wrote some simple MATLAB code to measure the Gflops of my 980X by timing large matrix multiplications. I have noticed that MATLAB only utilizes 6 logical cores out of the 12, delivering 40 Gflops from my code. The CPU load shown in Windows task manager is capped at 50%.

Then I ran two instances of my MATLAB code concurrently. By doing so, all 12 logical cores are utilized and the CPU load shown in Windows task manager is 100%. However, each instance of my MATLAB code is only delivering 15 Gflops, which means all 12 logical cores are doing only 30 Gflops in total, which is actually slower than the 40 Gflops from 6 logical cores.

Note that MATLAB is based on Intel Math Kernel Library. Intel claims that:



Source: http://software.intel.com/en-us/arti...abled-systems/

In other words, to get more Gflops from LinX, it is better to disable HT.
IMHO cache trashing is the major cause of your performance drop, because you are running two instances. Probably each instance is unware of another and tries to make use of the full cache.
(Correct me if that is not the case, because I have never used MATLAB)

It would be another story for a single instance linpack.