misc: add offload warning

colossalai/zero/low_level/low_level_optim.py

@@ -1,5 +1,6 @@
 # this code is inspired by the DeepSpeed library and implemented with our own design from scratch
 import copy
+import warnings
 from contextlib import contextmanager
 from functools import partial
 from typing import Dict, Iterator, List, Optional, Tuple
@@ -87,6 +88,11 @@ class LowLevelZeroOptimizer(OptimizerWrapper):
         self._partition_grads = partition_grad
 
         self._cpu_offload = cpu_offload
+        if cpu_offload:
+            warnings.warn(
+                "Please enlarge OMP_NUM_THREADS to speed up the CPU computation. "
+                "Use default OMP_NUM_THREADS=1 would significantly slow down the training."
+            )
 
         # grad accumulation
         self.require_grad_sync = True