Multicore performance drop under Windows x64

[lajszlo]

I'm using MKL for matrix multiplication (cblas_sgemm function) under 64-bit Windows. This far I'been using 32-bit code, and -- according to the Windows Task Manager -- my routine exploited both my cores to 100%. Now I compiled a 64-bit version from the code, and now Windows Task Manager shows only 50-60% processor use. interestingly, one of the cores is used at about 80%, and the other one about 20% (compared to the earlier 100%-100%). Is there anything I can do to have the same performance with the 64-bit version than with the 32-bit version?
Thanks a lot,

[TimP]

It looks like you're getting just one thread. This would be expected MKL default behavior if you run on a single core HyperThreaded CPU. In case the OS is confusing MKL into thinking you have a single core when you have dual, does MKL_DYNAMIC help?
Windows 7 is said to have scheduling improvements over earlier versions of Windows X64.

[lajszlo]

You are right, the 32-bit executable has 3 threads (according to Task Manager), while the 64-bit version has only 1 thread. I forgot to mention that I have Windows XP 64-bit version.
I'm absolutely not familiar with threading in MKL. I added the following two lines to my code:
mkl_set_dynamic(0);
mkl_set_num_threads(2, MKL_BLAS);
but nothing has changed, still there is only one thread. Do you mean this or something else by "does MKL_DYNAMIC help"?
Thanks

[Gennady_F_Intel]

Which libraries do you use for linking application?

[lajszlo]

I'm using MKL 10.1.1.022. For the 32-bit version (which works fine) I link mkl_c.lib, mkl_sequential.lib, libiomp5md.lib from the ia32\lib directory. For the x64 version there is no mkl_c.lib in the em64t\lib directory, so I link mkl_core.lib, mkl_intel_lp64.lib, mkl_sequential.lib, libiomp5md.lib. Is that OK? Thanks.

[Gennady_F_Intel · Accepted solution]

No, that not correct. You are using non-threaded libraries. Please try to use,

mkl_intel_lp64.lib mkl_intel_thread.lib mkl_core.liblibiomp5md.lib

as an examplefor static linking with supporting LP64 interface.

For more info, please refer to the Users Guide or see here the Linker Adviser.

[lajszlo]

This solved the problem, thanks a lot!!