Adds `json_count_leaves` utility function (#23899)

Signed-off-by: aditchawdhary <aditxy@hotmail.com>

tests/utils_/test_utils.py

@@ -948,6 +948,36 @@ def test_join_host_port():
     assert join_host_port("::1", 5555) == "[::1]:5555"
 
 
+def test_json_count_leaves():
+    """Test json_count_leaves function from jsontree utility."""
+    from vllm.utils.jsontree import json_count_leaves
+
+    # Single leaf values
+    assert json_count_leaves(42) == 1
+    assert json_count_leaves("hello") == 1
+    assert json_count_leaves(None) == 1
+
+    # Empty containers
+    assert json_count_leaves([]) == 0
+    assert json_count_leaves({}) == 0
+    assert json_count_leaves(()) == 0
+
+    # Flat structures
+    assert json_count_leaves([1, 2, 3]) == 3
+    assert json_count_leaves({"a": 1, "b": 2}) == 2
+    assert json_count_leaves((1, 2, 3)) == 3
+
+    # Nested structures
+    nested_dict = {"a": 1, "b": {"c": 2, "d": 3}}
+    assert json_count_leaves(nested_dict) == 3
+
+    nested_list = [1, [2, 3], 4]
+    assert json_count_leaves(nested_list) == 4
+
+    mixed_nested = {"list": [1, 2], "dict": {"x": 3}, "value": 4}
+    assert json_count_leaves(mixed_nested) == 4
+
+
 def test_convert_ids_list_to_tokens():
     tokenizer = AutoTokenizer.from_pretrained("Qwen/Qwen2.5-1.5B-Instruct")
     token_ids = tokenizer.encode("Hello, world!")

vllm/multimodal/cache.py

@@ -10,7 +10,8 @@ from typing_extensions import TypeAlias, override
 
 from vllm.logger import init_logger
 from vllm.utils import GiB_bytes, LRUCache
-from vllm.utils.jsontree import json_map_leaves, json_reduce_leaves
+from vllm.utils.jsontree import (json_count_leaves, json_map_leaves,
+                                 json_reduce_leaves)
 
 from .inputs import (MultiModalFeatureSpec, MultiModalFieldElem,
                      MultiModalKwargs, MultiModalKwargsItem,
@@ -127,11 +128,32 @@ class MultiModalCache:
         )
 
         if debug:
-            logger.debug("Calculated size of %s to be %.2f GiB", type(value),
-                         size / GiB_bytes)
+            leaf_count = json_count_leaves(value)
+            logger.debug(
+                "Calculated size of %s to be %.2f GiB (%d leaves)",
+                type(value),
+                size / GiB_bytes,
+                leaf_count,
+            )
 
         return size
 
+    @classmethod
+    def get_item_complexity(cls, value: MultiModalCacheValue) -> int:
+        """
+        Get the number of leaf elements in a multi-modal cache value.
+
+        This provides a measure of structural complexity that can be useful
+        for debugging cache performance and understanding data patterns.
+
+        Args:
+            value: The multi-modal cache value to analyze.
+
+        Returns:
+            The number of leaf elements in the nested structure.
+        """
+        return json_count_leaves(value)
+
     @classmethod
     def get_lru_cache(
         cls,

vllm/utils/jsontree.py

 # SPDX-License-Identifier: Apache-2.0
 # SPDX-FileCopyrightText: Copyright contributors to the vLLM project
 """Helper functions to work with nested JSON structures."""
+
 from collections.abc import Iterable
 from functools import reduce
 from typing import Callable, TypeVar, Union, overload
@@ -8,8 +9,12 @@ from typing import Callable, TypeVar, Union, overload
 _T = TypeVar("_T")
 _U = TypeVar("_U")
 
-JSONTree = Union[dict[str, "JSONTree[_T]"], list["JSONTree[_T]"],
-                 tuple["JSONTree[_T]", ...], _T]
+JSONTree = Union[
+    dict[str, "JSONTree[_T]"],
+    list["JSONTree[_T]"],
+    tuple["JSONTree[_T]", ...],
+    _T,
+]
 """A nested JSON structure where the leaves need not be JSON-serializable."""
 
 
@@ -78,3 +83,8 @@ def json_reduce_leaves(
         json_iter_leaves(value),
         initial,
     )
+
+
+def json_count_leaves(value: JSONTree[_T]) -> int:
+    """Count the number of leaves in a nested JSON structure."""
+    return sum(1 for _ in json_iter_leaves(value))