[Hybrid]: Decouple Kernel Block Size from KV Page Size (#24486)

Signed-off-by: lizhiyuan <uniartisan2017@gmail.com>
Signed-off-by: Zhiyuan Li <uniartisan2017@gmail.com>

tests/v1/worker/test_gpu_input_batch.py

@@ -241,6 +241,7 @@ def test_sampling_metadata_in_input_batch(device: str, batch_size: int):
         pin_memory=is_pin_memory_available(),
         vocab_size=1024,
         block_sizes=[1],
+        kernel_block_sizes=[1],
     )
     reqs: list[CachedRequestState] = []
     req_id_reqs = {}
@@ -335,6 +336,7 @@ def test_swap_states_in_input_batch(device: str, batch_size: int, swap_list: lis
         pin_memory=is_pin_memory_available(),
         vocab_size=1024,
         block_sizes=[1],
+        kernel_block_sizes=[1],
     )
     ref_input_batch: InputBatch = InputBatch(
         max_num_reqs=batch_size,
@@ -344,6 +346,7 @@ def test_swap_states_in_input_batch(device: str, batch_size: int, swap_list: lis
         pin_memory=is_pin_memory_available(),
         vocab_size=1024,
         block_sizes=[1],
+        kernel_block_sizes=[1],
     )
 
     reqs: list[CachedRequestState] = []

tests/v1/worker/test_gpu_model_runner.py

@@ -68,6 +68,9 @@ def initialize_kv_cache(runner: GPUModelRunner):
         pin_memory=runner.pin_memory,
         vocab_size=runner.model_config.get_vocab_size(),
         block_sizes=[kv_cache_config.kv_cache_groups[0].kv_cache_spec.block_size],
+        kernel_block_sizes=[
+            kv_cache_config.kv_cache_groups[0].kv_cache_spec.block_size
+        ],
     )
     runner.initialize_attn_backend(kv_cache_config)
 
@@ -817,42 +820,231 @@ def test_hybrid_attention_mamba_tensor_shapes(monkeypatch):
         ssm_shape = vllm_ctx[layer_2].kv_cache[0][1].shape
 
         # assert we are using FlashInfer
-        assert attn_shape[0] == num_blocks
+        assert attn_shape[0] % num_blocks == 0
+        block_split_ratio = attn_shape[0] // num_blocks
+
+        # use small blocks for testing to avoid memory issues
+        test_block_size = min(2, len(blocks0), len(blocks1))
+
+        # use non-overlapping blocks to avoid data contamination
+        # Split kernel blocks: first half for attention, second half for mamba
+        mid_point = num_blocks // 2
+
+        # attention uses kernel blocks from first half (mapped to logical blocks)
+        kv_blocks_for_attention = np.array([0, 1])[:test_block_size]
+
+        # mamba uses kernel blocks from second half
+        kv_blocks_for_mamba = np.array([mid_point, mid_point + 1])[:test_block_size]
+
+        # create small constant tensors for testing with corrected shapes
+        # attention: [block_size, ...] starting from dimension 2
+        attn_constant_shape = attn_shape[2:]
+        conv_constant_shape = conv_shape[1:]
+        ssm_constant_shape = ssm_shape[1:]
 
         attn_blocks_constant = torch.full(
-            (len(blocks0), *attn_shape[1:]), device=DEVICE, fill_value=3.33
+            (test_block_size, *attn_constant_shape), device=DEVICE, fill_value=3.33
         )
         conv_blocks_constant = torch.full(
-            (len(blocks1), *conv_shape[1:]), device=DEVICE, fill_value=6.66
+            (test_block_size, *conv_constant_shape), device=DEVICE, fill_value=6.66
         )
         ssm_blocks_constant = torch.full(
-            (len(blocks1), *ssm_shape[1:]), device=DEVICE, fill_value=9.99
+            (test_block_size, *ssm_constant_shape), device=DEVICE, fill_value=9.99
         )
 
-        # fill all attention blocks with constant
+        # Fill attention blocks with constants using kv block indices
+        kernel_blocks_for_attention = kv_blocks_for_attention * block_split_ratio
+
         for layer in [layer_0, layer_1]:
-            vllm_ctx[layer].kv_cache[0][blocks0, :] = (
-                attn_blocks_constant.detach().clone()
-            )
+            # attention: kv_cache[0][kernel_block_idx, kv_idx, ...]
+            for i, kernel_block in enumerate(kernel_blocks_for_attention):
+                vllm_ctx[layer].kv_cache[0][kernel_block, :] = attn_blocks_constant[i]
 
-        # fill all mamba blocks with constant
+        # fill mamba blocks with constants using kernel block indices
         for layer in [layer_2, layer_3, layer_4, layer_5]:
-            vllm_ctx[layer].kv_cache[0][0][blocks1, :] = (
-                conv_blocks_constant.detach().clone()
-            )
-            vllm_ctx[layer].kv_cache[0][1][blocks1, :] = (
-                ssm_blocks_constant.detach().clone()
-            )
+            # mamba: kv_cache[0][component][kernel_block_idx, ...]
+            for i, kv_block in enumerate(kv_blocks_for_mamba):
+                vllm_ctx[layer].kv_cache[0][0][kv_block, :] = conv_blocks_constant[i]
+                vllm_ctx[layer].kv_cache[0][1][kv_block, :] = ssm_blocks_constant[i]
 
         # verify attention and mamba contents are correct
         for layer in [layer_0, layer_1]:
-            assert torch.equal(
-                vllm_ctx[layer].kv_cache[0][blocks0, :], attn_blocks_constant
-            )
+            for i, kernel_block in enumerate(kernel_blocks_for_attention):
+                actual_kv = vllm_ctx[layer].kv_cache[0][kernel_block, :]
+                expected = attn_blocks_constant[i]
+
+                # Check K and V separately
+                assert torch.equal(actual_kv[0], expected)
+                assert torch.equal(actual_kv[1], expected)
+
         for layer in [layer_2, layer_3, layer_4, layer_5]:
-            assert torch.equal(
-                vllm_ctx[layer].kv_cache[0][0][blocks1, :], conv_blocks_constant
+            for i, kv_block in enumerate(kv_blocks_for_mamba):
+                actual_conv = vllm_ctx[layer].kv_cache[0][0][kv_block, :]
+                actual_ssm = vllm_ctx[layer].kv_cache[0][1][kv_block, :]
+                expected_conv = conv_blocks_constant[i]
+                expected_ssm = ssm_blocks_constant[i]
+
+                assert torch.equal(actual_conv, expected_conv)
+                assert torch.equal(actual_ssm, expected_ssm)
+
+        for layer in [layer_2, layer_3, layer_4, layer_5]:
+            for i, kv_block in enumerate(kv_blocks_for_mamba):
+                actual_conv = vllm_ctx[layer].kv_cache[0][0][kv_block, :]
+                actual_ssm = vllm_ctx[layer].kv_cache[0][1][kv_block, :]
+                expected_conv = conv_blocks_constant[i]
+                expected_ssm = ssm_blocks_constant[i]
+                assert torch.equal(actual_conv, expected_conv)
+                assert torch.equal(actual_ssm, expected_ssm)
+
+
+def test_hybrid_block_table_initialization():
+    """Test hybrid block table with different kernel and kvcache_manager block
+    sizes."""
+    from vllm.v1.worker.block_table import BlockTable
+
+    # Test configuration: kvcache_manager block size = 32,
+    # kernel block size = 16
+    block_size = 32
+    kernel_block_sizes = [16]
+    max_num_reqs = 10
+    max_num_blocks_per_req = 20
+    max_num_batched_tokens = 512
+
+    block_table = BlockTable(
+        block_size=block_size,
+        max_num_reqs=max_num_reqs,
+        max_num_blocks_per_req=max_num_blocks_per_req,
+        max_num_batched_tokens=max_num_batched_tokens,
+        pin_memory=False,
+        device=torch.device(DEVICE),
+        kernel_block_size=kernel_block_sizes[0],
+    )
+
+    # Verify hybrid block configuration
+    assert block_table.use_hybrid_blocks is True
+    assert block_table.block_size == kernel_block_sizes[0]
+    assert block_table.blocks_per_kv_block == (
+        block_size // kernel_block_sizes[0]
+    )  # Changed to use first element
+
+    # Test block table conversion logic
+    # One kvcache_manager block should map to multiple kernel blocks
+    kvcache_manager_blocks = [0, 1, 2]
+
+    # Verify that kvcache_manager blocks can be converted to kernel blocks
+    # and that block table operations work correctly.
+    req_index = 0
+    block_table.append_row(kvcache_manager_blocks, req_index)
+    # Get expected kernel blocks from the implementation for verification.
+    expected_kernel_blocks = block_table._map_to_kernel_blocks(
+        np.array(kvcache_manager_blocks)
+    )
+    # Verify block table state
+    assert block_table.num_blocks_per_row[req_index] == len(expected_kernel_blocks)
+    assert np.array_equal(
+        block_table.block_table.np[req_index, : len(expected_kernel_blocks)],
+        expected_kernel_blocks,
+    )
+
+
+def test_input_batch_with_kernel_block_sizes():
+    """Test InputBatch initialization with kernel_block_sizes parameter."""
+    max_num_reqs = 10
+    max_model_len = 512
+    max_num_batched_tokens = 512
+    device = torch.device(DEVICE)
+    pin_memory = False
+    vocab_size = 50272
+
+    # Test with different kernel block sizes
+    block_sizes = [32, 64]
+    kernel_block_sizes = [16, 32]
+
+    input_batch = InputBatch(
+        max_num_reqs=max_num_reqs,
+        max_model_len=max_model_len,
+        max_num_batched_tokens=max_num_batched_tokens,
+        device=device,
+        pin_memory=pin_memory,
+        vocab_size=vocab_size,
+        block_sizes=block_sizes,
+        kernel_block_sizes=kernel_block_sizes,
+    )
+
+    # Verify that block tables were created with kernel block sizes
+    assert len(input_batch.block_table.block_tables) == len(block_sizes)
+
+    for i, (kv_size, kernel_size) in enumerate(zip(block_sizes, kernel_block_sizes)):
+        block_table = input_batch.block_table.block_tables[i]
+        if kv_size != kernel_size:
+            assert block_table.use_hybrid_blocks is True
+            assert block_table.block_size == kernel_size
+        else:
+            assert block_table.use_hybrid_blocks is False
+            assert block_table.block_size == kernel_size
+
+
+def test_hybrid_cache_integration(model_runner, dist_init):
+    """Test hybrid cache architecture integration with GPUModelRunner."""
+    # Create a new model runner with hybrid cache configuration
+    vllm_config = get_vllm_config()
+
+    # Configure hybrid cache with different kvcache_manager block size
+    vllm_config.cache_config.block_size = 32
+
+    model_config = vllm_config.model_config
+    num_heads = model_config.get_num_kv_heads(vllm_config.parallel_config)
+    head_size = model_config.get_head_size()
+    vllm_config.compilation_config.static_forward_context["layer.0"] = Attention(
+        num_heads, head_size, 0.1
+    )
+
+    runner = GPUModelRunner(vllm_config, DEVICE)
+
+    # Initialize KV cache with configuration
+    attn_spec = FullAttentionSpec(
+        block_size=16,  # Use kernel block size directly
+        num_kv_heads=runner.model_config.get_num_kv_heads(runner.parallel_config),
+        head_size=runner.model_config.get_head_size(),
+        dtype=runner.kv_cache_dtype,
     )
-            assert torch.equal(
-                vllm_ctx[layer].kv_cache[0][1][blocks1, :], ssm_blocks_constant
+    tensor_size = attn_spec.page_size_bytes * NUM_BLOCKS
+    kv_cache_config = KVCacheConfig(
+        num_blocks=NUM_BLOCKS,
+        kv_cache_tensors=[
+            KVCacheTensor(size=tensor_size, shared_by=["layer.0"]),
+        ],
+        kv_cache_groups=[
+            KVCacheGroupSpec(layer_names=["layer.0"], kv_cache_spec=attn_spec)
+        ],
     )
+    runner.kv_cache_config = kv_cache_config
+
+    # Initialize input batch with kernel block sizes
+    runner.input_batch = InputBatch(
+        max_num_reqs=runner.max_num_reqs,
+        max_model_len=runner.max_model_len,
+        max_num_batched_tokens=runner.max_num_tokens,
+        device=runner.device,
+        pin_memory=runner.pin_memory,
+        vocab_size=runner.model_config.get_vocab_size(),
+        block_sizes=[kv_cache_config.kv_cache_groups[0].kv_cache_spec.block_size],
+        kernel_block_sizes=[16],
+    )  # Use kernel block size
+
+    runner.initialize_attn_backend(kv_cache_config)
+
+    # Verify hybrid block table configuration
+    block_table = runner.input_batch.block_table.block_tables[0]
+    assert block_table.block_size == (
+        kv_cache_config.kv_cache_groups[0].kv_cache_spec.block_size
+    )
+
+    # Test request processing with hybrid blocks
+    req_id = "hybrid_req_0"
+    scheduler_output = _schedule_new_request(req_id)
+
+    # Update states should work with hybrid blocks
+    runner._update_states(scheduler_output)
+    assert _is_req_scheduled(runner, req_id)
+    assert _is_req_state_block_table_match(runner, req_id)

vllm/attention/backends/abstract.py

@@ -2,7 +2,7 @@
 # SPDX-FileCopyrightText: Copyright contributors to the vLLM project
 
 from abc import ABC, abstractmethod
-from typing import Generic, Optional, Protocol, TypeVar
+from typing import Generic, Optional, Protocol, TypeVar, Union
 
 import torch
 
@@ -26,6 +26,13 @@ class AttentionType:
     """Attention between dec. Q and enc. K/V for encoder-decoder."""
 
 
+class MultipleOf:
+    base: int
+
+    def __init__(self, base: int):
+        self.base = base
+
+
 class AttentionBackend(ABC):
     """Abstract class for attention backends."""
 
@@ -57,6 +64,10 @@ class AttentionBackend(ABC):
     def get_metadata_cls() -> type["AttentionMetadata"]:
         raise NotImplementedError
 
+    @classmethod
+    def get_supported_kernel_block_size(cls) -> list[Union[int, MultipleOf]]:
+        return cls.get_impl_cls().get_supported_kernel_block_size()
+
     @classmethod
     def make_metadata(cls, *args, **kwargs) -> "AttentionMetadata":
         return cls.get_metadata_cls()(*args, **kwargs)
@@ -157,6 +168,11 @@ class AttentionImpl(ABC, Generic[T]):
     ) -> None:
         raise NotImplementedError
 
+    @staticmethod
+    def get_supported_kernel_block_size() -> list[Union[int, MultipleOf]]:
+        # TODO: implement this function for all backends.
+        return [MultipleOf(1)]
+
     @abstractmethod
     def forward(
         self,

vllm/model_executor/models/config.py

@@ -365,6 +365,23 @@ class HybridAttentionMambaModelConfig(VerifyAndUpdateConfig):
             block_size=model_config.max_model_len,
         ).page_size_bytes
 
+        # Model may be marked as is_hybrid
+        #  but mamba is skipped via config,
+        #  return directly
+        if mamba_page_size == 0:
+            return
+
+        # Attention backend constraints:
+        # - FlashAttention (FA) requires block size to be multiple of 16
+        # - MLA (Multi-head Latent Attention) requires larger alignment:
+        #   * CUTLASS_MLA backend: 128-byte alignment
+        #   * Other MLA backends: 64-byte alignment
+        if model_config.use_mla:
+            use_cutlass_mla = envs.VLLM_ATTENTION_BACKEND == "CUTLASS_MLA"
+            kernel_block_alignment_size = 128 if use_cutlass_mla else 64
+        else:
+            kernel_block_alignment_size = 16
+
         if cache_config.enable_prefix_caching:
             # With prefix caching, select attention block size to
             # optimize for mamba kernel performance
@@ -381,19 +398,28 @@ class HybridAttentionMambaModelConfig(VerifyAndUpdateConfig):
             # TODO(tdoublep): this constraint can be relaxed fairly
             # easily by changing the way we layout chunks in the
             # mamba2 kernels.
-            chunk_size = model_config.get_mamba_chunk_size()
+
+            from math import gcd
+
+            def lcm(a, b):
+                return a * b // gcd(a, b)
+
+            base_chunk_size = model_config.get_mamba_chunk_size()
             attn_tokens_per_mamba_state = cdiv(mamba_page_size, attn_page_size_1_token)
+
+            chunk_size = lcm(base_chunk_size, kernel_block_alignment_size)
             attn_block_size = chunk_size * cdiv(attn_tokens_per_mamba_state, chunk_size)
             cache_config.mamba_block_size = attn_block_size
         else:
             # Without prefix caching, select minimum valid attention block size
             # to minimize mamba state padding
 
-            # some attention backends (e.g. FA) only support setting
-            # block size to multiple of 16, so let's suggest a value
-            # that would work (note: FA is currently not compatible
-            # with mamba layers, use FlashInfer instead).
-            attn_block_size = 16 * cdiv(mamba_page_size, 16 * attn_page_size_1_token)
+            # Calculate minimum attention block size that satisfies both:
+            # 1. Backend alignment requirements (kernel_block_alignment_size)
+            # 2. Mamba page size compatibility (attn_page_size >= mamba_page_size)
+            attn_block_size = kernel_block_alignment_size * cdiv(
+                mamba_page_size, kernel_block_alignment_size * attn_page_size_1_token
+            )
 
         # override attention block size if either (a) the
         # user has not set it or (b) the user has set it

vllm/platforms/cuda.py

@@ -118,7 +118,15 @@ class CudaPlatformBase(Platform):
 
         # TODO(lucas): handle this more gracefully
         # Note: model_config may be None during testing
-        if model_config is not None and model_config.use_mla:
+        # Note: block_size is initialized in
+        # HybridAttentionMambaModelConfig.verify_and_update_config
+        # for models with both attention and mamba,
+        # and doesn't need to be reinitialized here
+        if (
+            model_config is not None
+            and model_config.use_mla
+            and cache_config.block_size is not None
+        ):
             use_sparse = hasattr(vllm_config.model_config.hf_config, "index_topk")
             # If `VLLM_ATTENTION_BACKEND` is not set and we are using MLA,
             # then we default to FlashMLA backend for non-blackwell GPUs,
@@ -151,18 +159,22 @@ class CudaPlatformBase(Platform):
             if (
                 use_flashmla
                 and is_flashmla_dense_supported()[0]
-                and cache_config.block_size != 64
+                and cache_config.block_size % 64 != 0
             ):
                 cache_config.block_size = 64
                 logger.info("Forcing kv cache block size to 64 for FlashMLA backend.")
 
-            if use_cutlass_mla and cache_config.block_size != 128:
+            if use_cutlass_mla and cache_config.block_size % 128 != 0:
                 cache_config.block_size = 128
                 logger.info(
                     "Forcing kv cache block size to 128 for CUTLASS_MLA backend."
                 )
 
-            if use_flashinfer_mla and cache_config.block_size not in [32, 64]:
+            if (
+                use_flashinfer_mla
+                and cache_config.block_size != 32
+                and cache_config.block_size % 64 != 0
+            ):
                 cache_config.block_size = 64
                 logger.info(
                     "Forcing kv cache block size to 64 for FlashInferMLA backend."
@@ -269,12 +281,12 @@ class CudaPlatformBase(Platform):
             use_cutlassmla = selected_backend == _Backend.CUTLASS_MLA or (
                 selected_backend is None
                 and cls.is_device_capability(100)
-                and block_size == 128
+                and block_size % 128 == 0
             )
             use_flashinfermla = selected_backend == _Backend.FLASHINFER_MLA or (
                 selected_backend is None
                 and cls.is_device_capability(100)
-                and block_size in [32, 64]
+                and (block_size == 32 or block_size % 64 == 0)
             )
             use_flashmla = selected_backend == _Backend.FLASHMLA or (
                 selected_backend is None and is_flashmla_dense_supported()[0]
@@ -298,7 +310,7 @@ class CudaPlatformBase(Platform):
                     "vllm.v1.attention.backends.mla.flashinfer_mla.FlashInferMLABackend"
                 )
             if use_flashmla:
-                if block_size != 64:
+                if block_size % 64 != 0:
                     logger.warning(
                         "FlashMLA backend is not supported for block size %d"
                         " (currently only supports block size 64).",

vllm/v1/attention/backends/flash_attn.py

@@ -3,7 +3,7 @@
 """Attention layer with FlashAttention."""
 
 from dataclasses import dataclass
-from typing import Optional
+from typing import Optional, Union
 
 import numpy as np
 import torch
@@ -14,6 +14,7 @@ from vllm.attention.backends.abstract import (
     AttentionImpl,
     AttentionMetadata,
     AttentionType,
+    MultipleOf,
     is_quantized_kv_cache,
 )
 from vllm.attention.layer import Attention
@@ -57,6 +58,10 @@ class FlashAttentionBackend(AttentionBackend):
     def get_supported_head_sizes(cls) -> list[int]:
         return [32, 64, 96, 128, 160, 192, 224, 256]
 
+    @staticmethod
+    def get_supported_kernel_block_size() -> list[Union[int, MultipleOf]]:
+        return [MultipleOf(16)]
+
     @classmethod
     def validate_head_size(cls, head_size: int) -> None:
         supported_head_sizes = cls.get_supported_head_sizes()

vllm/v1/attention/backends/flashinfer.py

@@ -23,6 +23,7 @@ from vllm.attention.backends.abstract import (
     AttentionBackend,
     AttentionImpl,
     AttentionType,
+    MultipleOf,
 )
 from vllm.config import CUDAGraphMode, VllmConfig
 from vllm.logger import init_logger
@@ -165,6 +166,13 @@ class FlashInferBackend(AttentionBackend):
         # https://github.com/flashinfer-ai/flashinfer/blob/3d55c71a62052c590c130897d3a3db49b14fcc34/include/flashinfer/utils.cuh#L157
         return [64, 128, 256]
 
+    @staticmethod
+    def get_supported_kernel_block_size() -> list[Union[int, MultipleOf]]:
+        # Note: Not sure for all platforms,
+        # but on Blackwell, only support a page size of
+        # 16, 32, 64
+        return [16, 32, 64]
+
     @classmethod
     def validate_head_size(cls, head_size: int) -> None:
         supported_head_sizes = cls.get_supported_head_sizes()

vllm/v1/attention/backends/mla/cutlass_mla.py

@@ -10,6 +10,7 @@ import vllm._custom_ops as ops
 from vllm.attention.backends.abstract import (
     AttentionLayer,
     AttentionType,
+    MultipleOf,
     is_quantized_kv_cache,
 )
 from vllm.logger import init_logger
@@ -44,6 +45,10 @@ class CutlassMLABackend(MLACommonBackend):
     def get_builder_cls() -> type["CutlassMLAMetadataBuilder"]:
         return CutlassMLAMetadataBuilder
 
+    @staticmethod
+    def get_supported_kernel_block_size() -> list[Union[int, MultipleOf]]:
+        return [128]
+
 
 class SM100Workspace:
     def __init__(self, initial_workspace_size):

vllm/v1/attention/backends/mla/flashmla.py

@@ -6,7 +6,7 @@ from typing import ClassVar, Optional, Union
 
 import torch
 
-from vllm.attention.backends.abstract import AttentionLayer, AttentionType
+from vllm.attention.backends.abstract import AttentionLayer, AttentionType, MultipleOf
 from vllm.attention.ops.flashmla import (
     flash_mla_with_kvcache,
     get_mla_metadata,
@@ -44,6 +44,10 @@ class FlashMLABackend(MLACommonBackend):
     def get_impl_cls() -> type["FlashMLAImpl"]:
         return FlashMLAImpl
 
+    @staticmethod
+    def get_supported_kernel_block_size() -> list[Union[int, MultipleOf]]:
+        return [64]
+
 
 @dataclass
 class FlashMLADecodeMetadata(MLACommonDecodeMetadata):

vllm/v1/attention/backends/rocm_aiter_fa.py

@@ -3,7 +3,7 @@
 """Attention layer with AiterFlashAttention."""
 
 from dataclasses import dataclass
-from typing import Optional
+from typing import Optional, Union
 
 import torch
 
@@ -12,6 +12,7 @@ from vllm.attention.backends.abstract import (
     AttentionImpl,
     AttentionMetadata,
     AttentionType,
+    MultipleOf,
 )
 from vllm.config import VllmConfig
 from vllm.logger import init_logger
@@ -359,6 +360,10 @@ class AiterFlashAttentionBackend(AttentionBackend):
     def get_supported_head_sizes(cls) -> list[int]:
         return [64, 128, 256]
 
+    @staticmethod
+    def get_supported_kernel_block_size() -> list[Union[int, MultipleOf]]:
+        return [MultipleOf(16)]
+
     @classmethod
     def validate_head_size(cls, head_size: int) -> None:
         supported_head_sizes = cls.get_supported_head_sizes()

vllm/v1/attention/backends/tree_attn.py

@@ -4,7 +4,7 @@
 
 import ast
 from dataclasses import dataclass
-from typing import Optional
+from typing import Optional, Union
 
 import torch
 
@@ -14,6 +14,7 @@ from vllm.attention.backends.abstract import (
     AttentionImpl,
     AttentionMetadata,
     AttentionType,
+    MultipleOf,
 )
 from vllm.attention.ops.triton_unified_attention import unified_attention
 from vllm.config import VllmConfig
@@ -39,6 +40,10 @@ class TreeAttentionBackend(AttentionBackend):
     def get_supported_head_sizes(cls) -> list[int]:
         return [32, 64, 96, 128, 160, 192, 224, 256]
 
+    @staticmethod
+    def get_supported_kernel_block_size() -> list[Union[int, MultipleOf]]:
+        return [MultipleOf(16)]
+
     @classmethod
     def validate_head_size(cls, head_size: int) -> None:
         supported_head_sizes = cls.get_supported_head_sizes()

vllm/v1/attention/backends/triton_attn.py

@@ -3,7 +3,7 @@
 """High-Performance Triton-only Attention layer."""
 
 from dataclasses import dataclass
-from typing import ClassVar, Optional
+from typing import ClassVar, Optional, Union
 
 import torch
 
@@ -12,6 +12,7 @@ from vllm.attention.backends.abstract import (
     AttentionImpl,
     AttentionMetadata,
     AttentionType,
+    MultipleOf,
 )
 from vllm.attention.ops.triton_reshape_and_cache_flash import (
     triton_reshape_and_cache_flash,
@@ -157,6 +158,10 @@ class TritonAttentionBackend(AttentionBackend):
     def get_supported_dtypes(cls) -> list[torch.dtype]:
         return [torch.float16, torch.bfloat16, torch.float32]
 
+    @staticmethod
+    def get_supported_kernel_block_size() -> list[Union[int, MultipleOf]]:
+        return [MultipleOf(16)]
+
     @classmethod
     def validate_head_size(cls, head_size: int) -> None:
         # Triton Attention supports any head size above 32

vllm/v1/attention/backends/xformers.py

@@ -3,7 +3,7 @@
 """Attention layer with XFormersAttention."""
 
 from dataclasses import dataclass
-from typing import Optional
+from typing import Optional, Union
 
 import torch
 
@@ -12,6 +12,7 @@ from vllm.attention.backends.abstract import (
     AttentionImpl,
     AttentionMetadata,
     AttentionType,
+    MultipleOf,
 )
 from vllm.attention.ops.triton_unified_attention import unified_attention
 from vllm.config import VllmConfig
@@ -80,6 +81,10 @@ class XFormersAttentionBackend(AttentionBackend):
             256,
         ]
 
+    @staticmethod
+    def get_supported_kernel_block_size() -> list[Union[int, MultipleOf]]:
+        return [MultipleOf(16)]
+
     @classmethod
     def validate_head_size(cls, head_size: int) -> None:
         supported_head_sizes = cls.get_supported_head_sizes()

vllm/v1/worker/block_table.py

@@ -22,22 +22,64 @@ class BlockTable:
         max_num_batched_tokens: int,
         pin_memory: bool,
         device: torch.device,
+        kernel_block_size: int,
     ):
-        self.block_size = block_size
+        """
+        Args:
+            block_size: Block size used for KV cache memory allocation
+            max_num_reqs: Maximum number of concurrent requests supported.
+            max_num_blocks_per_req: Maximum number of blocks per request.
+            max_num_batched_tokens: Maximum number of tokens in a batch.
+            pin_memory: Whether to pin memory for faster GPU transfers.
+            device: Target device for the block table.
+            kernel_block_size: The block_size of underlying attention kernel.
+                Will be the same as `block_size` if `block_size` is supported
+                by the attention kernel.
+        """
         self.max_num_reqs = max_num_reqs
-        self.max_num_blocks_per_req = max_num_blocks_per_req
         self.max_num_batched_tokens = max_num_batched_tokens
         self.pin_memory = pin_memory
         self.device = device
 
+        if kernel_block_size == block_size:
+            # Standard case: allocation and computation use same block size
+            # No block splitting needed, direct mapping
+            self.block_size = block_size
+            self.blocks_per_kv_block = 1
+            self.use_hybrid_blocks = False
+        else:
+            # Hybrid case: allocation block size differs from kernel block size
+            # Memory blocks are subdivided to match kernel requirements
+            # Example: 32-token memory blocks with 16-token kernel blocks
+            # → Each memory block corresponds to 2 kernel blocks
+            if block_size % kernel_block_size != 0:
+                raise ValueError(
+                    f"kernel_block_size {kernel_block_size} must divide "
+                    f"kv_manager_block_size size {block_size} evenly"
+                )
+
+            self.block_size = kernel_block_size
+            self.blocks_per_kv_block = block_size // kernel_block_size
+            self.use_hybrid_blocks = True
+
+        self.max_num_blocks_per_req = max_num_blocks_per_req * self.blocks_per_kv_block
+
         self.block_table = self._make_buffer(
-            max_num_reqs, max_num_blocks_per_req, dtype=torch.int32
+            self.max_num_reqs, self.max_num_blocks_per_req, dtype=torch.int32
         )
         self.num_blocks_per_row = np.zeros(max_num_reqs, dtype=np.int32)
 
         self.slot_mapping = self._make_buffer(
             self.max_num_batched_tokens, dtype=torch.int64
         )
+
+        if self.use_hybrid_blocks:
+            self._kernel_block_arange = np.arange(0, self.blocks_per_kv_block).reshape(
+                1, -1
+            )
+        else:
+            self._kernel_block_arange = None
+
         try:
             self.dcp_world_size = get_dcp_group().world_size
             self.dcp_rank = get_dcp_group().rank_in_group
@@ -53,6 +95,10 @@ class BlockTable:
     ) -> None:
         if not block_ids:
             return
+
+        if self.use_hybrid_blocks:
+            block_ids = self._map_to_kernel_blocks(np.array(block_ids))
+
         num_blocks = len(block_ids)
         start = self.num_blocks_per_row[row_idx]
         self.num_blocks_per_row[row_idx] += num_blocks
@@ -94,6 +140,7 @@ class BlockTable:
                 req_indices * self.max_num_blocks_per_req
                 + positions // virtual_block_size
             )
+
             block_numbers = self.block_table.np.ravel()[block_table_indices]
             # Use virtual_block_size for mask calculation, which marks local
             # tokens.
@@ -111,6 +158,7 @@ class BlockTable:
             block_table_indices = (
                 req_indices * self.max_num_blocks_per_req + positions // self.block_size
             )
+
             block_numbers = self.block_table.np.ravel()[block_table_indices]
             block_offsets = positions % self.block_size
             np.add(
@@ -129,6 +177,31 @@ class BlockTable:
         self.block_table.gpu.fill_(0)
         self.block_table.cpu.fill_(0)
 
+    def _map_to_kernel_blocks(self, kv_manager_block_ids: np.ndarray) -> np.ndarray:
+        """Convert kv_manager_block_id IDs to kernel block IDs.
+
+        Example:
+            # kv_manager_block_ids: 32 tokens,
+            # Kernel block size: 16 tokens
+            # blocks_per_kv_block = 2
+            >>> kv_manager_block_ids = np.array([0, 1, 2])
+            >>> Result: [0, 1, 2, 3, 4, 5]
+
+            # Each kv_manager_block_id maps to 2 kernel block id:
+            # kv_manager_block_id 0 → kernel block id [0, 1]
+            # kv_manager_block_id 1 → kernel block id [2, 3]
+            # kv_manager_block_id 2 → kernel block id [4, 5]
+        """
+        if not self.use_hybrid_blocks:
+            return kv_manager_block_ids
+
+        kernel_block_ids = (
+            kv_manager_block_ids.reshape(-1, 1) * self.blocks_per_kv_block
+            + self._kernel_block_arange
+        )
+
+        return kernel_block_ids.reshape(-1)
+
     def get_device_tensor(self, num_reqs: int) -> torch.Tensor:
         """Returns the device tensor of the block table."""
         return self.block_table.gpu[:num_reqs]
@@ -160,6 +233,7 @@ class MultiGroupBlockTable:
         pin_memory: bool,
         device: torch.device,
         block_sizes: list[int],
+        kernel_block_sizes: list[int],
         num_speculative_tokens: int = 0,
     ) -> None:
         # Note(hc): each dcp rank only store
@@ -172,6 +246,12 @@ class MultiGroupBlockTable:
             # DCP might not be initialized in testing
             dcp_world_size = 1
 
+        if len(kernel_block_sizes) != len(block_sizes):
+            raise ValueError(
+                f"kernel_block_sizes length ({len(kernel_block_sizes)}) "
+                f"must match block_sizes length ({len(block_sizes)})"
+            )
+
         self.block_tables = [
             BlockTable(
                 block_size,
@@ -183,8 +263,9 @@ class MultiGroupBlockTable:
                 max_num_batched_tokens,
                 pin_memory,
                 device,
+                kernel_block_size,
             )
-            for block_size in block_sizes
+            for block_size, kernel_block_size in zip(block_sizes, kernel_block_sizes)
         ]
 
     def append_row(self, block_ids: tuple[list[int], ...], row_idx: int) -> None:

vllm/v1/worker/gpu_input_batch.py

@@ -78,6 +78,7 @@ class InputBatch:
         pin_memory: bool,
         vocab_size: int,
         block_sizes: list[int],  # The block_size of each kv cache group
+        kernel_block_sizes: list[int],
         logitsprocs: Optional[LogitsProcessors] = None,
         is_spec_decode: bool = False,
         is_pooling_model: bool = False,
@@ -132,6 +133,7 @@ class InputBatch:
             pin_memory=pin_memory,
             device=device,
             block_sizes=block_sizes,
+            kernel_block_sizes=kernel_block_sizes,
             num_speculative_tokens=num_speculative_tokens,
         )
 

vllm/v1/worker/gpu_model_runner.py

@@ -19,7 +19,7 @@ from typing_extensions import TypeAlias
 
 import vllm.envs as envs
 from vllm.attention import Attention, AttentionType
-from vllm.attention.backends.abstract import AttentionBackend
+from vllm.attention.backends.abstract import AttentionBackend, MultipleOf
 from vllm.attention.layer import MLAAttention
 from vllm.attention.layers.chunked_local_attention import ChunkedLocalAttention
 from vllm.compilation.counter import compilation_counter
@@ -359,6 +359,7 @@ class GPUModelRunner(LoRAModelRunnerMixin, KVConnectorModelRunnerMixin):
             pin_memory=self.pin_memory,
             vocab_size=self.model_config.get_vocab_size(),
             block_sizes=[self.cache_config.block_size],
+            kernel_block_sizes=[self.cache_config.block_size],
             is_spec_decode=bool(self.vllm_config.speculative_config),
             logitsprocs=build_logitsprocs(
                 self.vllm_config,
@@ -4050,6 +4051,86 @@ class GPUModelRunner(LoRAModelRunnerMixin, KVConnectorModelRunnerMixin):
                 else:
                     self.reorder_batch_threshold = reorder_batch_threshold_i
 
+    def _find_compatible_block_sizes(
+        self,
+        kv_manager_block_size: int,
+        backend_cls: type[AttentionBackend],
+        return_all: bool = False,
+    ) -> list[int]:
+        """
+        Find compatible block sizes for a backend.
+
+        Args:
+            kv_manager_block_size: Physical block size of KV cache
+            backend_cls: Attention backend class
+            return_all: Return all compatible sizes if True, max size if False
+
+        Returns:
+            Compatible block size(s) based on return_all parameter
+
+        Raises:
+            ValueError: If no compatible block size found
+        """
+        supported_block_size = backend_cls.get_supported_kernel_block_size()
+        compatible_sizes = []
+
+        for block_size in supported_block_size:
+            if isinstance(block_size, int):
+                if kv_manager_block_size % block_size == 0:
+                    compatible_sizes.append(block_size)
+            elif (
+                isinstance(block_size, MultipleOf)
+                and kv_manager_block_size % block_size.base == 0
+            ):
+                compatible_sizes.append(kv_manager_block_size)
+
+        if not compatible_sizes:
+            raise ValueError(f"No compatible block size for {kv_manager_block_size}")
+
+        return compatible_sizes if return_all else [max(compatible_sizes)]
+
+    def _select_common_block_size(
+        self, kv_manager_block_size: int, attn_groups: list[AttentionGroup]
+    ) -> int:
+        """
+        Select common block size for all backends.
+
+        Args:
+            kv_manager_block_size: Block size of KV cache
+            attn_groups: List of attention groups
+
+        Returns:
+            Block size supported by all backends,
+            prioritizing cache_config.block_size
+
+        Raises:
+            ValueError: If no common block size found
+        """
+        all_backend_supports = []
+
+        for attn_group in attn_groups:
+            compatible_sizes = self._find_compatible_block_sizes(
+                kv_manager_block_size, attn_group.backend, return_all=True
+            )
+            supported_sizes = sorted(list(set(compatible_sizes)), reverse=True)
+            all_backend_supports.append(set(supported_sizes))
+
+        common_supported_sizes = set.intersection(*all_backend_supports)
+
+        if not common_supported_sizes:
+            error_msg = f"No common block size for {kv_manager_block_size}. "
+            for i, attn_group in enumerate(attn_groups):
+                supported = all_backend_supports[i]
+                error_msg += (
+                    f"Backend {attn_group.backend} supports: {sorted(supported)}. "
+                )
+            raise ValueError(error_msg)
+
+        if self.cache_config.block_size in common_supported_sizes:
+            return self.cache_config.block_size
+
+        return max(common_supported_sizes)
+
     def may_reinitialize_input_batch(self, kv_cache_config: KVCacheConfig) -> None:
         """
         Re-initialize the input batch if the block sizes are different from
@@ -4062,8 +4143,15 @@ class GPUModelRunner(LoRAModelRunnerMixin, KVConnectorModelRunnerMixin):
         block_sizes = [
             kv_cache_group.kv_cache_spec.block_size
             for kv_cache_group in kv_cache_config.kv_cache_groups
+            if not isinstance(kv_cache_group.kv_cache_spec, EncoderOnlyAttentionSpec)
         ]
-        if block_sizes != [self.cache_config.block_size]:
+
+        # Generate kernel_block_sizes that matches each block_size
+        kernel_block_sizes = self._prepare_kernel_block_sizes(kv_cache_config)
+
+        if block_sizes != [self.cache_config.block_size] or kernel_block_sizes != [
+            self.cache_config.block_size
+        ]:
             assert self.cache_config.cpu_offload_gb == 0, (
                 "Cannot re-initialize the input batch when CPU weight "
                 "offloading is enabled. See https://github.com/vllm-project/vllm/pull/18298 "  # noqa: E501
@@ -4077,6 +4165,7 @@ class GPUModelRunner(LoRAModelRunnerMixin, KVConnectorModelRunnerMixin):
                 pin_memory=self.pin_memory,
                 vocab_size=self.model_config.get_vocab_size(),
                 block_sizes=block_sizes,
+                kernel_block_sizes=kernel_block_sizes,
                 is_spec_decode=bool(self.vllm_config.speculative_config),
                 logitsprocs=self.input_batch.logitsprocs,
                 is_pooling_model=self.is_pooling_model,
@@ -4128,6 +4217,46 @@ class GPUModelRunner(LoRAModelRunnerMixin, KVConnectorModelRunnerMixin):
         for attn_groups in self.attn_groups:
             yield from attn_groups
 
+    def _prepare_kernel_block_sizes(self, kv_cache_config: KVCacheConfig) -> list[int]:
+        """
+        Generate kernel_block_sizes that matches each block_size.
+
+        For attention backends that support virtual block splitting,
+        use the supported block sizes from the backend.
+        For other backends (like Mamba), use the same block size (no splitting).
+
+        Args:
+            kv_cache_config: The KV cache configuration.
+
+        Returns:
+            list[int]: List of kernel block sizes for each cache group.
+        """
+        kernel_block_sizes = []
+        for kv_cache_group_id, kv_cache_group in enumerate(
+            kv_cache_config.kv_cache_groups
+        ):
+            if isinstance(kv_cache_group.kv_cache_spec, EncoderOnlyAttentionSpec):
+                continue
+            elif isinstance(kv_cache_group.kv_cache_spec, AttentionSpec):
+                # This is an attention backend that supports virtual
+                # block splitting. Get the supported block sizes from
+                # all backends in the group.
+                attn_groups = self.attn_groups[kv_cache_group_id]
+                kv_manager_block_size = kv_cache_group.kv_cache_spec.block_size
+                selected_kernel_size = self._select_common_block_size(
+                    kv_manager_block_size, attn_groups
+                )
+                kernel_block_sizes.append(selected_kernel_size)
+            elif isinstance(kv_cache_group.kv_cache_spec, MambaSpec):
+                # This is likely Mamba or other non-attention cache,
+                # no splitting.
+                kernel_block_sizes.append(kv_cache_group.kv_cache_spec.block_size)
+            else:
+                raise NotImplementedError(
+                    f"unknown kv cache spec {kv_cache_group.kv_cache_spec}"
+                )
+        return kernel_block_sizes
+
     def _reshape_kv_cache_tensors(
         self,
         kv_cache_config: KVCacheConfig,
@@ -4157,16 +4286,24 @@ class GPUModelRunner(LoRAModelRunnerMixin, KVConnectorModelRunnerMixin):
                 num_blocks = raw_tensor.numel() // kv_cache_spec.page_size_bytes
                 if isinstance(kv_cache_spec, AttentionSpec):
                     has_attn = True
+                    kv_manager_block_size = kv_cache_spec.block_size
+                    kernel_size_list = self._find_compatible_block_sizes(
+                        kv_manager_block_size, attn_backend, return_all=False
+                    )
+                    kernel_size = kernel_size_list[0]
+                    num_blocks_per_kv_block = kv_manager_block_size // kernel_size
+                    kernel_num_blocks = num_blocks * num_blocks_per_kv_block
+
                     kv_cache_shape = attn_backend.get_kv_cache_shape(
-                        num_blocks,
-                        kv_cache_spec.block_size,
+                        kernel_num_blocks,
+                        kernel_size,
                         kv_cache_spec.num_kv_heads,
                         kv_cache_spec.head_size,
                         cache_dtype_str=self.cache_config.cache_dtype,
                     )
                     dtype = kv_cache_spec.dtype
                     try:
-                        kv_cache_stride_order = attn_backend.get_kv_cache_stride_order()
+                        kv_cache_stride_order = attn_backend.get_kv_cache_stride_order()  # noqa: E501
                         assert len(kv_cache_stride_order) == len(kv_cache_shape)
                     except (AttributeError, NotImplementedError):
                         kv_cache_stride_order = tuple(range(len(kv_cache_shape)))
@@ -4320,10 +4457,11 @@ class GPUModelRunner(LoRAModelRunnerMixin, KVConnectorModelRunnerMixin):
         """
         kv_cache_config = deepcopy(kv_cache_config)
         self.kv_cache_config = kv_cache_config
-        self.may_reinitialize_input_batch(kv_cache_config)
         self.may_add_encoder_only_layers_to_kv_cache_config()
         self.maybe_add_kv_sharing_layers_to_kv_cache_groups(kv_cache_config)
         self.initialize_attn_backend(kv_cache_config)
+        # Reinitialize need to after initialize_attn_backend
+        self.may_reinitialize_input_batch(kv_cache_config)
         kv_caches = self.initialize_kv_cache_tensors(kv_cache_config)
 
         if self.speculative_config and self.speculative_config.use_eagle():

vllm/v1/worker/tpu_input_batch.py

@@ -27,6 +27,7 @@ class InputBatch:
         pin_memory: bool,
         vocab_size: int,
         block_sizes: list[int],  # The block_size of each kv cache group
+        kernel_block_sizes: list[int],
     ):
         self.max_num_reqs = max_num_reqs
         self.max_model_len = max_model_len
@@ -68,6 +69,7 @@ class InputBatch:
             pin_memory=pin_memory,
             device=device,
             block_sizes=block_sizes,
+            kernel_block_sizes=kernel_block_sizes,
         )
 
         # Sampling-related.

vllm/v1/worker/tpu_model_runner.py

@@ -259,6 +259,7 @@ class TPUModelRunner(LoRAModelRunnerMixin, KVConnectorModelRunnerMixin):
             pin_memory=self.pin_memory,
             vocab_size=self.model_config.get_vocab_size(),
             block_sizes=[self.block_size],
+            kernel_block_sizes=[self.cache_config.block_size],
         )
 
         # Cached torch/numpy tensor
@@ -1788,6 +1789,9 @@ class TPUModelRunner(LoRAModelRunnerMixin, KVConnectorModelRunnerMixin):
                 block_sizes=[
                     kv_cache_config.kv_cache_groups[0].kv_cache_spec.block_size
                 ],
+                kernel_block_sizes=[
+                    kv_cache_config.kv_cache_groups[0].kv_cache_spec.block_size
+                ],
             )
         # Verify dtype compatibility between block_table_cpu and input_batch
         assert (