fix issues with set_workers

Author: Vahid Tavanashad

CHANGELOG.md

@@ -15,6 +15,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 * NumPy interface `mkl_fft.interfaces.numpy_fft` is aligned with numpy-2.x.x [gh-139](https://github.com/IntelPython/mkl_fft/pull/139), [gh-157](https://github.com/IntelPython/mkl_fft/pull/157)
 * To set `mkl_fft` as the backend for SciPy is only possible through `mkl_fft.interfaces.scipy_fft` [gh-179](https://github.com/IntelPython/mkl_fft/pull/179)
 
+### Fixed
+* Fixed issues with `set_workers` function in SciPy interface `mkl_fft.interfaces.scipy_fft` [gh-183](https://github.com/IntelPython/mkl_fft/pull/183)
+
 ## [1.3.14] (04/10/2025)
 
 resolves gh-152 by adding an explicit `mkl-service` dependency to `mkl-fft` when building the wheel

mkl_fft/interfaces/README.md

@@ -42,6 +42,8 @@ This interface is a drop-in replacement for the [`scipy.fft`](https://scipy.gith
 
 * Helper functions: `fftshift`, `ifftshift`, `fftfreq`, `rfftfreq`, `set_workers`, `get_workers`. All of these functions, except for `set_workers` and `get_workers`, serve as a fallback to the SciPy implementation and are included for completeness.
 
+Note that in computing FFTs, the default value of `workers` parameter is the maximum number of threads available unlike the default behavior of SciPy where only one thread is used.
+
 The following example shows how to use this interface for calculating a 1D FFT.
 
 ```python
@@ -102,3 +104,24 @@ with scipy.fft.set_backend(mkl_backend, only=True):
 print(f"Time with OneMKL FFT backend installed: {t2:.1f} seconds")
 # Time with MKL FFT backend installed: 9.1 seconds
 ```
+
+In the following example, we use `set_worker` to control the number of threads when `mkl_fft` is being used as a backend for SciPy.
+
+```python
+import numpy, mkl, scipy
+import mkl_fft.interfaces.scipy_fft as mkl_fft
+import scipy
+a = numpy.random.randn(128, 64) + 1j*numpy.random.randn(128, 64)
+scipy.fft.set_global_backend(mkl_fft)  # set mkl_fft as global backend
+
+mkl.verbose(1)
+# True
+mkl.get_max_threads()
+# 112
+y = scipy.signal.fftconvolve(a, a)  # Note that Nthr:112
+# MKL_VERBOSE FFT(dcbo256x128,input_strides:{0,128,1},output_strides:{0,128,1},bScale:3.05176e-05,tLim:56,unaligned_input,unaligned_output,desc:0x563aefe86180) 165.02us CNR:OFF Dyn:1 FastMM:1 TID:0  NThr:112
+
+with mkl_fft.set_workers(4):
+    y = scipy.signal.fftconvolve(a, a)  # Note that Nthr:4
+# MKL_VERBOSE FFT(dcbo256x128,input_strides:{0,128,1},output_strides:{0,128,1},bScale:3.05176e-05,tLim:4,unaligned_output,desc:0x563aefe86180) 187.37us CNR:OFF Dyn:1 FastMM:1 TID:0  NThr:4
+```

mkl_fft/interfaces/_scipy_fft.py

@@ -66,31 +66,13 @@
 ]
 
 
-class _cpu_max_threads_count:
-    def __init__(self):
-        self.cpu_count = None
-        self.max_threads_count = None
-
-    def get_cpu_count(self):
-        if self.cpu_count is None:
-            max_threads = self.get_max_threads_count()
-            self.cpu_count = max_threads
-        return self.cpu_count
-
-    def get_max_threads_count(self):
-        if self.max_threads_count is None:
-            # pylint: disable=no-member
-            self.max_threads_count = mkl.get_max_threads()
-
-        return self.max_threads_count
-
-
 class _workers_data:
     def __init__(self, workers=None):
-        if workers:
-            self.workers_ = workers
+        if workers is not None:  # workers = 0 should be handled
+            self.workers_ = _workers_to_num_threads(workers)
         else:
-            self.workers_ = _cpu_max_threads_count().get_cpu_count()
+            # Unlike SciPy, the default value is maximum number of threads
+            self.workers_ = mkl.get_max_threads()  # pylint: disable=no-member
         self.workers_ = operator.index(self.workers_)
 
     @property
@@ -108,21 +90,22 @@ def workers(self, workers_val):
 
 
 def _workers_to_num_threads(w):
-    """Handle conversion of workers to a positive number of threads in the
-    same way as scipy.fft.helpers._workers.
+    """
+    Handle conversion of workers to a positive number of threads in the
+    same way as scipy.fft._pocketfft.helpers._workers.
     """
     if w is None:
         return _workers_global_settings.get().workers
     _w = operator.index(w)
     if _w == 0:
         raise ValueError("Number of workers must not be zero")
     if _w < 0:
-        ub = os.cpu_count()
-        _w += ub + 1
+        _cpu_count = os.cpu_count()
+        _w += _cpu_count + 1
         if _w <= 0:
             raise ValueError(
-                "workers value out of range; got {}, must not be"
-                " less than {}".format(w, -ub)
+                f"workers value out of range; got {w}, must not be less "
+                f"than {-_cpu_count}"
             )
     return _w
 
@@ -134,14 +117,16 @@ def __init__(self, workers):
 
     def __enter__(self):
         try:
+            # mkl.set_num_threads_local sets the number of threads to the
+            # given input number, and returns the previous number of threads
             # pylint: disable=no-member
             self.prev_num_threads = mkl.set_num_threads_local(self.n_threads)
         except Exception as e:
             raise ValueError(
-                "Class argument {} result in invalid number of threads {}".format(
-                    self.workers, self.n_threads
-                )
+                f"Class argument {self.workers} results in invalid number of "
+                f"threads {self.n_threads}"
             ) from e
+        return self
 
     def __exit__(self, *args):
         # restore old value
@@ -696,21 +681,19 @@ def get_workers():
 
 
 @contextlib.contextmanager
-def set_workers(n_workers):
+def set_workers(workers):
     """
     Set the value of workers used by default, returns the previous value.
 
     For full documentation refer to `scipy.fft.set_workers`.
 
     """
-    nw = operator.index(n_workers)
+    nw = operator.index(workers)
     token = None
     try:
         new_wd = _workers_data(nw)
         token = _workers_global_settings.set(new_wd)
         yield
     finally:
-        if token:
+        if token is not None:
             _workers_global_settings.reset(token)
-        else:
-            raise ValueError

mkl_fft/tests/third_party/scipy/test_multithreading.py

@@ -52,7 +52,7 @@ def _mt_fft(x):
     return fft.fft(x, workers=2)
 
 
-@pytest.mark.slow
+# @pytest.mark.slow
 def test_mixed_threads_processes(x):
     # Test that the fft threadpool is safe to use before & after fork
 
@@ -79,10 +79,11 @@ def test_invalid_workers(x):
         fft.ifft(x, workers=-cpus - 1)
 
 
-@pytest.mark.skip()
 def test_set_get_workers():
     cpus = os.cpu_count()
-    assert fft.get_workers() == 1
+
+    # default value is max number of threads unlike stock SciPy
+    assert fft.get_workers() == cpus
     with fft.set_workers(4):
         assert fft.get_workers() == 4
 
@@ -91,13 +92,13 @@ def test_set_get_workers():
 
         assert fft.get_workers() == 4
 
-    assert fft.get_workers() == 1
+    # default value is max number of threads unlike stock SciPy
+    assert fft.get_workers() == cpus
 
     with fft.set_workers(-cpus):
         assert fft.get_workers() == 1
 
 
-@pytest.mark.skip("mkl_fft does not validate workers")
 def test_set_workers_invalid():
 
     with pytest.raises(ValueError, match="workers must not be zero"):