Does -openmp turn off Turbo Boost?

[Greynolds__Alan]

I'm using 11.1.067 on a 12-core Mac Pro booted into 64-bit Windows 7 and compile my application with -fast and with/without -openmp. Using the Intel Turbo Boost Monitor gadget, I notice that compiling with -openmp and setting OMP_NUM_THREADS=1 the processor consistently stays at 2.67 Ghz when executing a computationally intensive test case. However compiling without -openmp and running the same test case. the processor consistently jumps to2.93 Ghz. During all this no other processis consuming cycles (e.g. virus checking is disabled). Is -openmp turning off Turbo Boost?

Al

[jimdempseyatthecove]

Al,

Can you determine which core(s) are used with each version?
There may be an issue relating to placement since the Turbo Boost can be better thought of as a over heat protection (detune). The temperature can be sensitive to which core is selected when running each app.

Try using either an API to set affinity to the same core for each app or using the Task Manager to set the affinity for the app. Same with the Turbo Boost Monitor. IOW keep runtime environment the same.

Jim Dempsey

[Greynolds__Alan]

Jim,

Thanks for the advice. I'll try it this evening (the machine is located at home). I'm not complaining at this point since I am running benchmarks for an OpenMP-related paper and would really like to have Turbo Boost disabled. Does anyone know how to do this on a Mac Pro booted to Windows 7 since the Mac does not have a normal "bios" where these things are usually done?

Al

[Ron_Green]

Al,

OMP_NUM_THREADS sets the number of worker threads. The OMP runtimes have changed over time, so I'll have to check what we're doing currently - but versions of the OMP runtime may have a control thread running in addition to the worker thread pool ( 1 thread in your case ). Thus, there may be a control thread and a worker thread running for OMP_NUM_THREADS=1, thus touching 2 cores and preventing turbo boost from activation.

I'll also research how to disable turbo boost w/o bios. I've been asked this question recently and I thought I had heard of a way to do this via a software utility.

UPDATE: updating the BIOS is the only known method to disable turbo boost. And since the Mac doesn't use BIOS ... out of luck. Here is an interesting discussion on an Apple forum on the matter: http://discussions.apple.com/thread.jspa?threadID=2221702

ron

[Greynolds__Alan]

Ron.

Tmonitor from CPUID.com can disable Turbo Boost directly from Windows even on my non-BIOS Mac.

Al