throttling number of threads

[Brian_Budge]

Hi -

Is it possible to throttle the maximum number of threads used by the task scheduler or for a parallel launch? I searched the forums for this, but couldn't find anything. Probably just the wrong key words.

Thanks.

[RafSchietekat]

Provide a non-default argument to task_scheduler_init.

[turks]

Quoting - Raf Schietekat

Provide a non-default argument to task_scheduler_init.

Raf, could you explain further ? What would be a "non default argument" ?

Thanks
-T

[Vivek_Rajagopalan]

Quoting - turks

Raf, could you explain further ? What would be a "non default argument" ?

Thanks
-T

From the docs : http://www.threadingbuildingblocks.org/files/documentation/a00114.html

task_scheduler_init (int number_of_threads=automatic, stack_size_type thread_stack_size=0)
Shorthand for default constructor followed by call to intialize(number_of_threads).

-

task_scheduler_init tinit(4); // restrict tbb worker threads to 4, even if you have more or less hardware threads available

[Terry_W_Intel]

Quoting - turks

Raf, could you explain further ? What would be a "non default argument" ?

Thanks
-T

The task_scheduler_init() can take two optional arguments:

task_scheduler_init( int number_of_threads=automatic, stack_size_type thread_stack_size=0);

So, you can ask directly for p threads by doing this:

task_scheduler_init init(p);

Cheers,
Terry

[turks]

Quoting - Vivek Rajagopalan

From the docs : http://www.threadingbuildingblocks.org/files/documentation/a00114.html

task_scheduler_init (int number_of_threads=automatic, stack_size_type thread_stack_size=0)
Shorthand for default constructor followed by call to intialize(number_of_threads).

-

task_scheduler_init tinit(4); // restrict tbb worker threads to 4, even if you have more or less hardware threads available

Is there any way to know that if you ask for only 2 worker threads on an 8 core machine that you actually got 2 and
not 8 ? If I throttle back the worker threads at init time to 2 it uses as much CPU as when I init with 8.

[RafSchietekat]

Quoting - turks

Is there any way to know that if you ask for only 2 worker threads on an 8 core machine that you actually got 2 and
not 8 ? If I throttle back the worker threads at init time to 2 it uses as much CPU as when I init with 8.

The first task_scheduler_init instance sets the number of worker threads; any additional instances have no say in the matter. If you did provide the number to the first instance, what are the symptoms?

[turks]

Quoting - Raf Schietekat

The first task_scheduler_init instance sets the number of worker threads; any additional instances have no say in the matter. If you did provide the number to the first instance, what are the symptoms?

Symptoms of initing to 1 or 2 threads is that my pipeline is using a lot of CPU. I assume the default (if no argument is given) is 8 on a machine with 2 quad-core chips ?

[RafSchietekat]

"Symptoms of initing to 1 or 2 threads is that my pipeline is using a lot of CPU."
That's not very specific.

"I assume the default (if no argument is given) is 8 on a machine with 2 quad-core chips ?"
Yes, unless you also have hyperthreading.