[VLM] Replace torch.repeat_interleave with faster np.repeat for Qwen-VL series (#13736)

Co-authored-by: luoyuan.luo <luoyuan.luo@antgroup.com>

python/sglang/srt/models/qwen2_vl.py

@@ -44,6 +44,7 @@ from sglang.srt.managers.schedule_batch import MultimodalDataItem, MultimodalInp
 from sglang.srt.model_executor.forward_batch_info import ForwardBatch
 from sglang.srt.model_loader.weight_utils import default_weight_loader
 from sglang.srt.models.qwen2 import Qwen2Model
+from sglang.srt.models.utils import compute_cu_seqlens_from_grid_numpy
 from sglang.srt.utils import add_prefix
 from sglang.srt.utils.hf_transformers_utils import get_processor
 
@@ -387,10 +388,7 @@ class Qwen2VisionTransformer(nn.Module):
         emb = torch.cat((rotary_pos_emb, rotary_pos_emb), dim=-1)
         position_embeddings = (emb.cos(), emb.sin())
         # compute cu_seqlens
-        cu_seqlens = torch.repeat_interleave(
-            grid_thw[:, 1] * grid_thw[:, 2], grid_thw[:, 0]
-        ).cumsum(dim=0, dtype=torch.int32)
-        cu_seqlens = torch.cat([cu_seqlens.new_zeros(1), cu_seqlens])
+        cu_seqlens = compute_cu_seqlens_from_grid_numpy(grid_thw)
 
         # transformers
         x = x.unsqueeze(1)

python/sglang/srt/models/qwen3_vl.py

@@ -46,6 +46,7 @@ from sglang.srt.managers.schedule_batch import (
 from sglang.srt.model_executor.forward_batch_info import ForwardBatch, PPProxyTensors
 from sglang.srt.model_loader.weight_utils import default_weight_loader
 from sglang.srt.models.qwen3 import Qwen3Model
+from sglang.srt.models.utils import compute_cu_seqlens_from_grid_numpy
 from sglang.srt.utils import add_prefix
 from sglang.srt.utils.hf_transformers_utils import get_processor
 
@@ -434,15 +435,7 @@ class Qwen3VLMoeVisionModel(nn.Module):
         position_embeddings = (emb.cos(), emb.sin())
 
         # compute cu_seqlens
-        cu_seqlens = torch.repeat_interleave(
-            grid_thw[:, 1] * grid_thw[:, 2], grid_thw[:, 0]
-        ).cumsum(dim=0)
-        cu_seqlens = torch.cat(
-            [
-                torch.zeros(1, dtype=torch.int32, device=cu_seqlens.device),
-                cu_seqlens.to(torch.int32),
-            ]
-        )
+        cu_seqlens = compute_cu_seqlens_from_grid_numpy(grid_thw)
 
         x = x.unsqueeze(1)
 

python/sglang/srt/models/utils.py

@@ -12,6 +12,7 @@
 # limitations under the License.
 # ==============================================================================
 
+import numpy as np
 import torch
 
 from sglang.srt.layers.radix_attention import RadixAttention
@@ -59,3 +60,25 @@ def permute_inv(perm: torch.Tensor) -> torch.Tensor:
     inv_perm = torch.empty_like(perm)
     inv_perm[perm] = torch.arange(perm.numel(), device=perm.device, dtype=perm.dtype)
     return inv_perm
+
+
+def compute_cu_seqlens_from_grid_numpy(grid_thw: torch.Tensor) -> torch.Tensor:
+    """
+    Compute cu_seqlens from grid_thw using NumPy.
+
+    grid_thw: [T, 3] int tensor on CPU.
+              columns: [repeat_count, H, W]
+    Returns:
+        cu_seqlens: 1D int32 tensor on CPU, shape [N + 1]
+    """
+    assert (
+        grid_thw.device.type == "cpu"
+    ), "compute_cu_seqlens_from_grid_numpy expects a CPU tensor"
+    arr = grid_thw.numpy()
+
+    cu_seqlens = np.repeat(arr[:, 1] * arr[:, 2], arr[:, 0]).cumsum(
+        axis=0, dtype=np.int32
+    )
+    cu_seqlens = np.concatenate([np.zeros(1, dtype=np.int32), cu_seqlens])
+    cu_seqlens = torch.from_numpy(cu_seqlens)
+    return cu_seqlens

test/srt/ops/test_repeat_interleave.py

@@ -0,0 +1,141 @@
+import time
+from typing import Tuple
+
+import numpy as np
+import pytest
+import torch
+
+from sglang.srt.models.utils import compute_cu_seqlens_from_grid_numpy as cpu_numpy_impl
+
+
+def torch_ref_impl(grid_thw: torch.Tensor) -> torch.Tensor:
+    """
+    Pure PyTorch implementation of cu_seqlens computation.
+    Assumes grid_thw is already on the correct device (CPU here).
+    Shape: [T, 3], columns: [repeat_count, H, W]
+    """
+    cu_seqlens = torch.repeat_interleave(
+        grid_thw[:, 1] * grid_thw[:, 2], grid_thw[:, 0]
+    ).cumsum(dim=0)
+    cu_seqlens = torch.cat(
+        [
+            torch.zeros(1, dtype=torch.int32, device=cu_seqlens.device),
+            cu_seqlens.to(torch.int32),
+        ]
+    )
+    return cu_seqlens
+
+
+def benchmark_once(fn, grid_thw, iters: int = 1000):
+    """
+    Run a function `fn` on the same input `grid_thw` for `iters` times
+    and measure total elapsed time.
+    """
+    start = time.perf_counter()
+    for _ in range(iters):
+        out = fn(grid_thw)
+    end = time.perf_counter()
+    return (end - start), out
+
+
+# (T, repeat_min, repeat_max)
+GRID_TEST_CONFIGS: list[Tuple[int, int, int]] = [
+    (16, 1, 4),  # small T, small repeat counts
+    (128, 0, 4),  # allow repeat=0 to test edge cases
+    (512, 1, 8),
+    (1024, 1, 16),
+]
+
+NUM_CASES_PER_CONFIG = 10
+
+
+def _generate_random_grid(T: int, repeat_min: int, repeat_max: int) -> torch.Tensor:
+    """
+    grid_thw: [T, 3]
+    col0: repeat count
+    col1, col2: arbitrary positive integers (here 1..16)
+    """
+    repeats = torch.randint(repeat_min, repeat_max + 1, (T, 1), dtype=torch.int32)
+    th = torch.randint(1, 17, (T, 1), dtype=torch.int32)
+    tw = torch.randint(1, 17, (T, 1), dtype=torch.int32)
+    grid_thw = torch.cat([repeats, th, tw], dim=1)
+    return grid_thw
+
+
+class TestRepeatInterleave:
+    @classmethod
+    def setup_class(cls):
+        torch.set_num_threads(1)
+
+    def setup_method(self, method):
+        torch.manual_seed(0)
+        np.random.seed(0)
+
+    @pytest.mark.parametrize(
+        "T,repeat_min,repeat_max",
+        GRID_TEST_CONFIGS,
+    )
+    @pytest.mark.parametrize("case_idx", range(NUM_CASES_PER_CONFIG))
+    def test_cpu_correctness_random_cases(
+        self,
+        T: int,
+        repeat_min: int,
+        repeat_max: int,
+        case_idx: int,
+    ):
+        torch.manual_seed(case_idx)
+        np.random.seed(case_idx)
+
+        grid_thw = _generate_random_grid(T, repeat_min, repeat_max)
+
+        grid_clone = grid_thw.clone()
+
+        out_torch = torch_ref_impl(grid_thw)
+        out_numpy = cpu_numpy_impl(grid_thw)
+
+        assert torch.equal(grid_thw, grid_clone), "Function modified input grid_thw!"
+
+        assert (
+            out_torch.shape == out_numpy.shape
+        ), f"Shape mismatch: torch={out_torch.shape}, numpy={out_numpy.shape}"
+
+        assert (
+            out_torch.dtype == torch.int32
+        ), f"Unexpected torch dtype: {out_torch.dtype}"
+        assert (
+            out_numpy.dtype == torch.int32
+        ), f"Unexpected numpy impl dtype: {out_numpy.dtype}"
+
+        if not torch.equal(out_torch.cpu(), out_numpy.cpu()):
+            diff_idx = (out_torch.cpu() != out_numpy.cpu()).nonzero(as_tuple=False)
+            idx0 = diff_idx[0].item()
+            pytest.fail(
+                f"Value mismatch, T={T}, case_idx={case_idx}, first differing index={idx0}, "
+                f"torch={out_torch[idx0].item()}, "
+                f"numpy={out_numpy[idx0].item()}"
+            )
+
+    def test_zero_repeat_edge_case(self):
+        T = 4
+        grid_thw = torch.tensor(
+            [
+                [0, 4, 4],
+                [1, 2, 3],  # 6
+                [2, 1, 5],  # 5, 5
+                [0, 7, 7],  # 0
+            ],
+            dtype=torch.int32,
+        )
+
+        grid_clone = grid_thw.clone()
+
+        out_torch = torch_ref_impl(grid_thw)
+        out_numpy = cpu_numpy_impl(grid_thw)
+
+        assert torch.equal(
+            grid_thw, grid_clone
+        ), "Function modified input grid_thw with zero repeats!"
+
+        assert torch.equal(
+            out_torch.cpu(), out_numpy.cpu()
+        ), f"Zero-repeat case mismatch: torch={out_torch}, numpy={out_numpy}"

test/srt/run_suite.py

@@ -46,6 +46,7 @@ suites = {
         TestFile("openai_server/validation/test_matched_stop.py", 60),
         TestFile("openai_server/validation/test_openai_server_ignore_eos.py", 85),
         TestFile("openai_server/validation/test_request_length_validation.py", 31),
+        TestFile("ops/test_repeat_interleave.py", 60),
         TestFile("quant/test_block_int8.py", 22),
         TestFile("quant/test_fp8_kernel.py", 8),
         TestFile("quant/test_int8_kernel.py", 8),