[Hybrid] A simpler algorithm to find kernel_block_size

tests/v1/worker/test_gpu_model_runner.py

@@ -6,6 +6,7 @@
 import torch
 
 from vllm.attention import Attention
+from vllm.attention.backends.abstract import MultipleOf
 from vllm.config import (
     CacheConfig,
     ModelConfig,
@@ -34,6 +35,7 @@
 from vllm.v1.sample.metadata import SamplingMetadata
 from vllm.v1.worker.gpu_input_batch import InputBatch
 from vllm.v1.worker.gpu_model_runner import GPUModelRunner
+from vllm.v1.worker.utils import AttentionGroup
 
 BLOCK_SIZE = 16
 NUM_BLOCKS = 10
@@ -181,6 +183,57 @@ def _is_req_state_block_table_match(model_runner, req_id: str) -> bool:
     ).all()
 
 
+def _make_mock_backend_for_kernel_block_size(
+    supported_sizes: list[int | MultipleOf],
+):
+    class _MockBackend:
+        @staticmethod
+        def get_supported_kernel_block_size():
+            return supported_sizes
+
+    return _MockBackend()
+
+
+def _make_kv_cache_spec() -> FullAttentionSpec:
+    return FullAttentionSpec(block_size=1, num_kv_heads=1, head_size=1, dtype="float16")
+
+
+def test_select_common_block_size_prefers_manager_block_size():
+    backend_a = _make_mock_backend_for_kernel_block_size([MultipleOf(32)])
+    backend_b = _make_mock_backend_for_kernel_block_size([64, MultipleOf(16)])
+    attn_groups = [
+        AttentionGroup(backend_a, [], [], _make_kv_cache_spec(), 0),
+        AttentionGroup(backend_b, [], [], _make_kv_cache_spec(), 0),
+    ]
+
+    selected_size = GPUModelRunner.select_common_block_size(128, attn_groups)
+    assert selected_size == 128
+
+
+def test_select_common_block_size_uses_largest_shared_int():
+    backend_a = _make_mock_backend_for_kernel_block_size([128, 64])
+    backend_b = _make_mock_backend_for_kernel_block_size([64, 32])
+    attn_groups = [
+        AttentionGroup(backend_a, [], [], _make_kv_cache_spec(), 0),
+        AttentionGroup(backend_b, [], [], _make_kv_cache_spec(), 0),
+    ]
+
+    selected_size = GPUModelRunner.select_common_block_size(256, attn_groups)
+    assert selected_size == 64
+
+
+def test_select_common_block_size_no_valid_option():
+    backend_a = _make_mock_backend_for_kernel_block_size([64])
+    backend_b = _make_mock_backend_for_kernel_block_size([MultipleOf(16)])
+    attn_groups = [
+        AttentionGroup(backend_a, [], [], _make_kv_cache_spec(), 0),
+        AttentionGroup(backend_b, [], [], _make_kv_cache_spec(), 0),
+    ]
+
+    with pytest.raises(ValueError):
+        GPUModelRunner.select_common_block_size(48, attn_groups)
+
+
 def test_update_states_new_request(model_runner, dist_init):
     req_id = "req_0"
 

vllm/v1/worker/gpu_model_runner.py

@@ -3978,6 +3978,7 @@ def get_attn_backends_for_group(
 
         def create_attn_groups(
             attn_backends_map: dict[AttentionGroupKey, list[str]],
+            kv_cache_group_id: int,
         ) -> list[AttentionGroup]:
             attn_groups: list[AttentionGroup] = []
             for (attn_backend, kv_cache_spec), layer_names in attn_backends_map.items():
@@ -3987,6 +3988,7 @@ def create_attn_groups(
                     kv_cache_spec,
                     self.vllm_config,
                     self.device,
+                    kv_cache_group_id,
                     num_metadata_builders=1
                     if not self.parallel_config.enable_dbo
                     else 2,
@@ -4005,8 +4007,8 @@ def create_attn_groups(
         # Resolve cudagraph_mode before actually initialize metadata_builders
         self._check_and_update_cudagraph_mode(attention_backend_set)
 
-        for attn_backends_map in attention_backend_maps:
-            self.attn_groups.append(create_attn_groups(attn_backends_map))
+        for i, attn_backend_map in enumerate(attention_backend_maps):
+            self.attn_groups.append(create_attn_groups(attn_backend_map, i))
 
         # Calculate reorder batch threshold (if needed)
         self.calculate_reorder_batch_threshold()
@@ -4156,104 +4158,96 @@ def calculate_reorder_batch_threshold(self) -> None:
             return
         self.reorder_batch_threshold = reduce(min_none_high, reorder_batch_thresholds)
 
-    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]
+    @staticmethod
+    def select_common_block_size(
+        kv_manager_block_size: int, attn_groups: list[AttentionGroup]
     ) -> int:
         """
-        Select common block size for all backends.
+        Select a block size that is supported by all backends and is a factor of
+        kv_manager_block_size.
+
+        If kv_manager_block_size is supported by all backends, return it directly.
+        Otherwise, return the max supported size.
 
         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
+            The selected block size
 
         Raises:
-            ValueError: If no common block size found
+            ValueError: If no valid 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
+        def block_size_is_supported(
+            backends: list[type[AttentionBackend]], block_size: int
+        ) -> bool:
+            """
+            Check if the block size is supported by all backends.
+            """
+            for backend in backends:
+                is_supported = False
+                for supported_size in backend.get_supported_kernel_block_size():
+                    if isinstance(supported_size, int):
+                        if block_size == supported_size:
+                            is_supported = True
+                    elif isinstance(supported_size, MultipleOf):
+                        if block_size % supported_size.base == 0:
+                            is_supported = True
+                    else:
+                        raise ValueError(f"Unknown supported size: {supported_size}")
+                if not is_supported:
+                    return False
+            return True
+
+        backends = [group.backend for group in attn_groups]
+
+        # Case 1: if the block_size of kv cache manager is supported by all backends,
+        # return it directly
+        if block_size_is_supported(backends, kv_manager_block_size):
+            return kv_manager_block_size
+
+        # Case 2: otherwise, the block_size must be an `int`-format supported size of
+        # at least one backend. Iterate over all `int`-format supported sizes in
+        # descending order and return the first one that is supported by all backends.
+        # Simple proof:
+        # If the supported size b is in MultipleOf(x_i) format for all attention
+        # backends i, and b a factor of kv_manager_block_size, then
+        # kv_manager_block_size also satisfies MultipleOf(x_i) for all i. We will
+        # return kv_manager_block_size in case 1.
+        all_int_supported_sizes = set(
+            supported_size
+            for backend in backends
+            for supported_size in backend.get_supported_kernel_block_size()
+            if isinstance(supported_size, int)
+        )
 
-        return max(common_supported_sizes)
+        for supported_size in sorted(all_int_supported_sizes, reverse=True):
+            if kv_manager_block_size % supported_size != 0:
+                continue
+            if block_size_is_supported(backends, supported_size):
+                return supported_size
+        raise ValueError(f"No common block size for {kv_manager_block_size}. ")
 
-    def may_reinitialize_input_batch(self, kv_cache_config: KVCacheConfig) -> None:
+    def may_reinitialize_input_batch(
+        self, kv_cache_config: KVCacheConfig, kernel_block_sizes: list[int]
+    ) -> None:
         """
         Re-initialize the input batch if the block sizes are different from
         `[self.cache_config.block_size]`. This usually happens when there
         are multiple KV cache groups.
 
         Args:
             kv_cache_config: The KV cache configuration.
+            kernel_block_sizes: The kernel block sizes for each KV cache group.
         """
         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)
         ]
 
-        # 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
         ]:
@@ -4354,7 +4348,7 @@ def _prepare_kernel_block_sizes(self, kv_cache_config: KVCacheConfig) -> list[in
                 # 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(
+                selected_kernel_size = self.select_common_block_size(
                     kv_manager_block_size, attn_groups
                 )
                 kernel_block_sizes.append(selected_kernel_size)
@@ -4372,6 +4366,7 @@ def _reshape_kv_cache_tensors(
         self,
         kv_cache_config: KVCacheConfig,
         kv_cache_raw_tensors: dict[str, torch.Tensor],
+        kernel_block_sizes: list[int],
     ) -> dict[str, torch.Tensor]:
         """
         Reshape the KV cache tensors to the desired shape and dtype.
@@ -4380,6 +4375,7 @@ def _reshape_kv_cache_tensors(
             kv_cache_config: The KV cache config
             kv_cache_raw_tensors: The KV cache buffer of each layer, with
                 correct size but uninitialized shape.
+            kernel_block_sizes: The kernel block sizes for each KV cache group.
         Returns:
             Dict[str, torch.Tensor]: A map between layer names to their
             corresponding memory buffer for KV cache.
@@ -4389,6 +4385,10 @@ def _reshape_kv_cache_tensors(
         for group in self._kv_cache_spec_attn_group_iterator():
             kv_cache_spec = group.kv_cache_spec
             attn_backend = group.backend
+            if group.kv_cache_group_id == len(kernel_block_sizes):
+                # There may be a last group for layers without kv cache.
+                continue
+            kernel_block_size = kernel_block_sizes[group.kv_cache_group_id]
             for layer_name in group.layer_names:
                 if layer_name in self.runner_only_attn_layers:
                     continue
@@ -4397,24 +4397,21 @@ def _reshape_kv_cache_tensors(
                 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
+                    num_blocks_per_kv_block = (
+                        kv_cache_spec.block_size // kernel_block_size
                     )
-                    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(
                         kernel_num_blocks,
-                        kernel_size,
+                        kernel_block_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()  # noqa: E501
+                        kv_cache_stride_order = attn_backend.get_kv_cache_stride_order()
                         assert len(kv_cache_stride_order) == len(kv_cache_shape)
                     except (AttributeError, NotImplementedError):
                         kv_cache_stride_order = tuple(range(len(kv_cache_shape)))
@@ -4497,13 +4494,15 @@ def _update_hybrid_attention_mamba_layout(
                     )
 
     def initialize_kv_cache_tensors(
-        self, kv_cache_config: KVCacheConfig
+        self, kv_cache_config: KVCacheConfig, kernel_block_sizes: list[int]
     ) -> dict[str, torch.Tensor]:
         """
         Initialize the memory buffer for KV cache.
 
         Args:
             kv_cache_config: The KV cache config
+            kernel_block_sizes: The kernel block sizes for each KV cache group.
+
         Returns:
             Dict[str, torch.Tensor]: A map between layer names to their
             corresponding memory buffer for KV cache.
@@ -4512,7 +4511,7 @@ def initialize_kv_cache_tensors(
         kv_cache_raw_tensors = self._allocate_kv_cache_tensors(kv_cache_config)
         # Change the memory buffer to the desired shape
         kv_caches = self._reshape_kv_cache_tensors(
-            kv_cache_config, kv_cache_raw_tensors
+            kv_cache_config, kv_cache_raw_tensors, kernel_block_sizes
         )
 
         # Set up cross-layer KV cache sharing
@@ -4571,9 +4570,17 @@ def initialize_kv_cache(self, kv_cache_config: KVCacheConfig) -> None:
         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)
+        # The kernel block size for all KV cache groups. For example, if
+        # kv_cache_manager uses block_size 256 for a given group, but the attention
+        # backends for that group only supports block_size 64, we will return
+        # kernel_block_size 64 and split the 256-token-block to 4 blocks with 64
+        # tokens each.
+        kernel_block_sizes = self._prepare_kernel_block_sizes(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)
+        self.may_reinitialize_input_batch(kv_cache_config, kernel_block_sizes)
+        kv_caches = self.initialize_kv_cache_tensors(
+            kv_cache_config, kernel_block_sizes
+        )
 
         if self.speculative_config and self.speculative_config.use_eagle():
             assert isinstance(self.drafter, EagleProposer)