# Controlling Thread Allocation

The KMP_HW_SUBSET and KMP_AFFINITY environment variables allow you to control how the OpenMP* runtime uses the hardware threads on the processors. These environment variables allow you to try different thread distributions on the cores of the processors and determine how these threads are bound to the cores. You can use the environment variables to work out what is optimal for your application.

The KMP_HW_SUBSET variable controls the allocation of hardware resources and the KMP_AFFINITY variable controls how the OpenMP threads are bound to those resources.

## Controlling Thread Distribution

The KMP_HW_SUBSET variable controls the hardware resource that will be used by the program. This variable specifies the number of sockets to use, how many cores to use per socket and how many threads to assign per core. While specifying two threads per core often yields better performance than one thread per core, specifying three or four threads per core may or may not improve the performance. This variable enables you to conveniently measure the performance of up to four threads per core.

For example, you can determine the effects of assigning 24, 48, 72, or the maximum 96 OpenMP threads in a system with 24 cores by specifying the following variable settings:

To Assign This Number of Threads ... | ... Use This Setting
--- | ---
24 | `KMP_HW_SUBSET=24c,1t`
48 | `KMP_HW_SUBSET=24c,2t`
72 | `KMP_HW_SUBSET=24c,3t`
96 | `KMP_HW_SUBSET=24c,4t`

### Note

Take care when using the OMP_NUM_THREADS variable along with this variable. Using the OMP_NUM_THREADS variable can result in over or under subscription.

## Controlling Thread Bindings

The KMP_AFFINITY variable controls how the OpenMP threads are bound to the hardware resources allocated by the KMP_HW_SUBSET variable. While this variable can be set to several binding or affinity types, the following are the recommended affinity types to use to run your OpenMP threads on the processor:

- compact: sequentially distribute the threads among the cores that share the same cache.

- scatter: distribute the threads among the cores without regard to the cache.

The following table shows how the threads are bound to the cores when you want to use three threads per core on two cores by specifying `KMP_HW_SUBSET=2c,3t`:

Affinity | OpenMP Threads on Core 0 | OpenMP Threads on Core 1
--- | --- | ---
`KMP_AFFINITY=compact` | 0, 1, 2 | 3, 4, 5
`KMP_AFFINITY=scatter` | 0, 2, 4 | 1, 3, 5

## Determining the Best Setting

To determine the best thread distribution and bindings using these variables, use the following:

1. Ensure that your OpenMP code is working properly before using these environment variables.

2. Establish a baseline with your current OpenMP code to compare to the performance when you allocate the threads to a processor.

3. Measure the performance of distributing one, two, three, or four threads per core by use the KMP_HW_SUBSET variable.

4. Measure the performance of binding the threads to the cores by using the KMP_AFFINITY variable.

Parent topic: OpenMP* Support

## See Also

Thread Affinity Interface
Supported Environment Variables