[zero] add chunk size search for chunk manager (#1052)

colossalai/tensor/chunk.py

@@ -268,3 +268,41 @@ class ChunkManager:
             for i, chunk in enumerate(group):
                 msg += f'[{i}] {chunk}\n'
         return msg
+
+    @staticmethod
+    def get_chunk_util(chunk_size: int, params_numel: List[int]) -> float:
+        assert len(params_numel) > 0
+        total_size = 0
+        total_utilized_size = 0
+        cur_chunk_utilized_size = 0
+        for size in params_numel:
+            assert chunk_size >= size
+            total_utilized_size += size
+            if total_size == 0 or cur_chunk_utilized_size + size > chunk_size:
+                total_size += chunk_size
+                cur_chunk_utilized_size = 0
+            cur_chunk_utilized_size += size
+        return total_utilized_size / total_size
+
+    @staticmethod
+    def search_chunk_size(module: torch.nn.Module,
+                          search_range: int,
+                          n_grids: int,
+                          min_chunk_size: Optional[int] = None) -> int:
+        assert search_range % n_grids == 0
+        # TODO(ver217): sort params and filter unused ones
+        params_numel = [p.numel() for p in module.parameters()]
+        max_param_numel = max(params_numel)
+        if min_chunk_size is not None:
+            assert min_chunk_size >= max_param_numel
+        else:
+            min_chunk_size = max_param_numel
+        step_size = search_range // n_grids
+        max_chunk_util = -1
+        best_chunk_size = -1
+        for chunk_size in range(min_chunk_size, min_chunk_size + search_range + 1, step_size):
+            chunk_util = ChunkManager.get_chunk_util(chunk_size, params_numel)
+            if chunk_util > max_chunk_util:
+                max_chunk_util = chunk_util
+                best_chunk_size = chunk_size
+        return best_chunk_size