From 165331a2004fbda1531091341ed081d6a39d2162 Mon Sep 17 00:00:00 2001
From: Lianmin Zheng <lianminzheng@gmail.com>
Date: Tue, 9 Jun 2026 20:33:46 -0700
Subject: [PATCH] Share BCG output buffers across capture sizes (#27659)

---
 .../breakable_cuda_graph_runner.py            | 74 ++++++++++++++++++-
 1 file changed, 71 insertions(+), 3 deletions(-)

diff --git a/python/sglang/srt/model_executor/breakable_cuda_graph_runner.py b/python/sglang/srt/model_executor/breakable_cuda_graph_runner.py
index d5efcd421..d7718518f 100644
--- a/python/sglang/srt/model_executor/breakable_cuda_graph_runner.py
+++ b/python/sglang/srt/model_executor/breakable_cuda_graph_runner.py
@@ -25,7 +25,7 @@ from __future__ import annotations
 import bisect
 import inspect
 import logging
-from typing import TYPE_CHECKING, Union
+from typing import TYPE_CHECKING, Any, Union
 
 import torch
 import tqdm
@@ -386,6 +386,7 @@ class BreakableCudaGraphRunner:
                 if get_tensor_model_parallel_rank() == 0
                 else reversed(self.capture_num_tokens)
             )
+            shared_output_buffer = None
             for num_tokens in capture_range:
                 if get_tensor_model_parallel_rank() == 0:
                     avail_mem = get_available_gpu_memory(
@@ -397,7 +398,11 @@ class BreakableCudaGraphRunner:
                         f"[BCG] Capturing ({num_tokens=} {avail_mem=:.2f} GB)"
                     )
 
-                graph, output = self._capture_one(num_tokens, pool, stream)
+                graph, output = self._capture_one(
+                    num_tokens, pool, stream, shared_output_buffer
+                )
+                if shared_output_buffer is None:
+                    shared_output_buffer = output
                 self.graphs[num_tokens] = graph
                 self.output_buffers[num_tokens] = output
 
@@ -429,7 +434,62 @@ class BreakableCudaGraphRunner:
                     return False
         return num_tokens <= self.max_num_tokens
 
-    def _capture_one(self, num_tokens, pool, stream):
+    def _slice_output(self, output: Any, num_tokens: int) -> Any:
+        if output is None:
+            return None
+        if torch.is_tensor(output):
+            return output[:num_tokens]
+        if isinstance(output, PPProxyTensors):
+            return output[:num_tokens]
+        if isinstance(output, tuple):
+            return tuple(self._slice_output(item, num_tokens) for item in output)
+        if isinstance(output, list):
+            return [self._slice_output(item, num_tokens) for item in output]
+        raise TypeError(f"Unsupported BCG output type: {type(output)}")
+
+    def _copy_output_to_buffer(
+        self, output: Any, output_buffer: Any, num_tokens: int
+    ) -> None:
+        if output is None or output_buffer is None:
+            if output is None and output_buffer is None:
+                return
+            raise ValueError(
+                "BCG output structure changed between capture sizes: "
+                f"{type(output)} vs {type(output_buffer)}"
+            )
+        if torch.is_tensor(output) and torch.is_tensor(output_buffer):
+            output_buffer[:num_tokens].copy_(output[:num_tokens])
+            return
+        if isinstance(output, PPProxyTensors) and isinstance(
+            output_buffer, PPProxyTensors
+        ):
+            if output.tensors.keys() != output_buffer.tensors.keys():
+                raise ValueError(
+                    "BCG output proxy structure changed between capture sizes: "
+                    f"{output.tensors.keys()} != {output_buffer.tensors.keys()}"
+                )
+            for key, tensor in output.tensors.items():
+                self._copy_output_to_buffer(
+                    tensor, output_buffer.tensors[key], num_tokens
+                )
+            return
+        if isinstance(output, (list, tuple)) and isinstance(
+            output_buffer, type(output)
+        ):
+            if len(output) != len(output_buffer):
+                raise ValueError(
+                    "BCG output sequence structure changed between capture sizes: "
+                    f"{len(output)} != {len(output_buffer)}"
+                )
+            for item, buffer in zip(output, output_buffer):
+                self._copy_output_to_buffer(item, buffer, num_tokens)
+            return
+        raise TypeError(
+            "Unsupported BCG output buffer pair: "
+            f"{type(output)} vs {type(output_buffer)}"
+        )
+
+    def _capture_one(self, num_tokens, pool, stream, shared_output_buffer=None):
         """Capture a breakable CUDA graph for one token size."""
         forward_batch = self._build_capture_forward_batch(num_tokens)
         self._init_forward_metadata_for_capture(forward_batch, num_tokens)
@@ -448,6 +508,14 @@ class BreakableCudaGraphRunner:
             graph = BreakableCUDAGraph()
             with BreakableCUDAGraphCapture(cuda_graph=graph, pool=pool, stream=stream):
                 output = run_once()
+                if shared_output_buffer is not None:
+                    self._copy_output_to_buffer(
+                        output, shared_output_buffer, num_tokens
+                    )
+            if shared_output_buffer is None:
+                output = self._slice_output(output, num_tokens)
+            else:
+                output = self._slice_output(shared_output_buffer, num_tokens)
 
         return graph, output