[Doc] Clarify cudagraph capture size logic and default behavior in scheduler (#18698)

Signed-off-by: Zazzle516 <2405677060@qq.com>
Signed-off-by: Harry Mellor <19981378+hmellor@users.noreply.github.com>
Co-authored-by: Harry Mellor <19981378+hmellor@users.noreply.github.com>

vllm/config/__init__.py

@@ -3579,30 +3579,40 @@ class VllmConfig:
 
     def _set_cudagraph_sizes(self):
         """
-        cudagraph batchsize padding logic:
+        vLLM defines the default candidate list of batch sizes for CUDA graph
+        capture as:
 
-        `[1, 2, 4] + [8 * i for i in range(1, 1025)]` is a list of all possible
-        batch sizes that cudagraph will capture.
-
-        Depending on the engine's configuration of `max_num_seqs`, the
-        candidate batch sizes to capture cudagraph will shrink to the subset
-        which just cover the range of `[1, max_num_seqs]`. In the common case,
-        `max_num_seqs` is 256, and the cudagraph batch sizes will be
-        `[1, 2, 4, 8, 16, 24, 32, 40, ..., 256]`.
-
-        However, if users specify the cudagraph capture sizes through
-        compilation config, we will use the specified sizes instead.
+        ```python
+        max_graph_size = min(max_num_seqs * 2, 512)
+        # 1, 2, 4, then multiples of 8 up to max_graph_size
+        cuda_graph_sizes = [1, 2, 4, 8, 16, 24, 32, 40, ..., max_graph_size]
 
         In the end, `vllm_config.compilation_config.cudagraph_capture_sizes`
         will be the final sizes to capture cudagraph (in descending order).
 
-        During runtime, if batchsize is larger than
-        `vllm_config.compilation_config.cudagraph_capture_sizes`,
-        no cudagraph will be used.
-        If the batch size is no larger than
-        `vllm_config.compilation_config.cudagraph_capture_sizes`,
-        we can quickly find the padded graph size for a given batch size by
-        looking up `vllm_config.compilation_config.bs_to_padded_graph_size`.
+        These sizes are used to capture and reuse CUDA graphs for
+        performance-critical paths (e.g., decoding). Capturing enables
+        significantly faster kernel dispatch by avoiding Python overhead. The
+        list is then filtered based on `max_num_batched_tokens` (e.g., 8192 on
+        most GPUs), which controls the total allowed number of tokens in a
+        batch. Since each sequence may have a variable number of tokens, the
+        maximum usable batch size will depend on actual sequence lengths.
+
+        Example:
+            With `max_num_batched_tokens = 8192`, and typical sequences
+            averaging ~32 tokens, most practical batch sizes fall below 256.
+            However, the system will still allow capture sizes up to 512 if
+            shape and memory permit.
+
+        Note:
+            If users explicitly specify cudagraph capture sizes in the
+            compilation config, those will override this default logic.
+            At runtime:
+
+            - If batch size <= one of the `cudagraph_capture_sizes`, the closest
+            padded CUDA graph will be used.
+            - If batch size > largest `cudagraph_capture_sizes`, cudagraph will
+            not be used.
         """
 
         # calculate the default `batch_size_capture_list`