Force OMP_NUM_THREADS=1 if unset (#6998)

* force omp

* update

* set

* workers

* link

python/ray/__init__.py

@@ -1,7 +1,11 @@
 import os
+import logging
+import multiprocessing
 from os.path import dirname
 import sys
 
+logger = logging.getLogger(__name__)
+
 # MUST add pickle5 to the import path because it will be imported by some
 # raylet modules.
 
@@ -10,6 +14,12 @@ if "pickle5" in sys.modules:
                       "requires a specific version of pickle5 (which is "
                       "packaged along with Ray).")
 
+if "OMP_NUM_THREADS" not in os.environ and multiprocessing.cpu_count() > 8:
+    logger.warning("[ray] Forcing OMP_NUM_THREADS=1 to avoid performance "
+                   "degradation with many workers (issue #6998). You can "
+                   "override this by explicitly setting OMP_NUM_THREADS.")
+    os.environ["OMP_NUM_THREADS"] = "1"
+
 # Add the directory containing pickle5 to the Python path so that we find the
 # pickle5 version packaged with ray and not a pre-existing pickle5.
 pickle5_path = os.path.join(