Warn if OMP_NUM_THREADS is set (#6729)

doc/source/iter.rst

@@ -109,8 +109,8 @@ element ``x_{i+1}`` is fetched from the source actor. This is useful if you need
 update the source actor between iterator steps. Note that for async gather, this
 ordering only applies per shard.
 
-**Operator state**: Operator state is preserved for each shard. This means that you can pass a stateful callable to ``.foreach()``:
-For example, this means you can pass a stateful callable to ``.foreach()``:
+**Operator state**: Operator state is preserved for each shard.
+This means that you can pass a stateful callable to ``.foreach()``:
 
 .. code-block:: python
 

python/ray/worker.py

@@ -675,6 +675,11 @@ def init(address=None,
     else:
         driver_mode = SCRIPT_MODE
 
+    if "OMP_NUM_THREADS" in os.environ:
+        logger.warning("OMP_NUM_THREADS={} is set, this may impact "
+                       "object transfer performance.".format(
+                           os.environ["OMP_NUM_THREADS"]))
+
     if setproctitle is None:
         logger.warning(
             "WARNING: Not updating worker name since `setproctitle` is not "