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# Global Configuration Options

NumPy has a few import-time, compile-time, or runtime configuration options which change the global behaviour. Most of these are related to performance or for debugging purposes and will not be interesting to the vast majority of users.

## Performance-related options

### Number of threads used for linear algebra

NumPy itself is normally intentionally limited to a single thread during function calls, however it does support multiple Python threads running at the same time. Note that for performant linear algebra NumPy uses a BLAS backend such as OpenBLAS or MKL, which may use multiple threads that may be controlled by environment variables such as `OMP_NUM_THREADS` depending on what is used. One way to control the number of threads is the package threadpoolctl

### madvise hugepage on Linux

When working with very large arrays on modern Linux kernels, you can experience a significant speedup when transparent hugepage is used. The current system policy for transparent hugepages can be seen by:

    cat /sys/kernel/mm/transparent_hugepage/enabled

When set to `madvise` NumPy will typically use hugepages for a performance boost. This behaviour can be modified by setting the environment variable:

    NUMPY_MADVISE_HUGEPAGE=0

or setting it to `1` to always enable it. When not set, the default is to use madvise on Kernels 4.6 and newer. These kernels presumably experience a large speedup with hugepage support. This flag is checked at import time.

### SIMD feature selection

Setting `NPY_DISABLE_CPU_FEATURES` will exclude simd features at runtime. See Runtime dispatch.

## Debugging-related options

### Warn if no memory allocation policy when deallocating data

Some users might pass ownership of the data pointer to the `ndarray` by setting the `OWNDATA` flag. If they do this without setting (manually) a memory allocation policy, the default will be to call `free`. If `NUMPY_WARN_IF_NO_MEM_POLICY` is set to `"1"`, a `RuntimeWarning` will be emitted. A better alternative is to use a `PyCapsule` with a deallocator and set the `ndarray.base`.

Changed in version 1.25.2: This variable is only checked on the first import.