added multiprocessing.set_start_method to safely determine method

vip_hci/fm/negfc_mcmc.py

@@ -50,7 +50,7 @@
 import numpy as np
 import os
 import emcee
-from multiprocessing import cpu_count, Pool, set_start_method
+import multiprocessing
 import inspect
 import datetime
 import corner
@@ -814,8 +814,8 @@ def mcmc_negfc_sampling(cube, angs, psfn, initial_state, algo=pca_annulus,
     else:
         raise TypeError("Interpolation not recognized.")
 
-    if nproc is None:
-        nproc = cpu_count() // 2      # Hyper-threading doubles the # of cores
+    if nproc is None:  # actually not used anymore, now that Pool has been implemented for EnsembleSampler
+        nproc = multiprocessing.cpu_count()
 
     # #########################################################################
     # Initialization of the variables
@@ -914,8 +914,13 @@ def mcmc_negfc_sampling(cube, angs, psfn, initial_state, algo=pca_annulus,
     os.environ["NUMEXPR_NUM_THREADS"] = "1"
     os.environ["OMP_NUM_THREADS"] = "1"
 
-    set_start_method("forkserver", force=True)
-    with Pool() as pool:
+    avail_methods = multiprocessing.get_all_start_methods()
+    if "forkserver" in avail_methods:
+        multiprocessing.set_start_method("forkserver", force=True)  # faster, better
+    else:
+        multiprocessing.set_start_method("spawn", force=True)  # slower, but on all platforms
+
+    with multiprocessing.Pool() as pool:
         sampler = emcee.EnsembleSampler(nwalkers, dim, lnprob,
                                         pool=pool, moves=emcee.moves.StretchMove(a=2),
                                         args=([bounds, cube, angs, psfn,
@@ -1078,7 +1083,7 @@ def mcmc_negfc_sampling(cube, angs, psfn, initial_state, algo=pca_annulus,
         timing(start_time)
 
     # reactivate multithreading
-    ncpus = cpu_count()
+    ncpus = multiprocessing.cpu_count()
     os.environ["MKL_NUM_THREADS"] = str(ncpus)
     os.environ["NUMEXPR_NUM_THREADS"] = str(ncpus)
     os.environ["OMP_NUM_THREADS"] = str(ncpus)