cache_engine.py

# SPDX-License-Identifier: Apache-2.0
# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
"""CacheEngine class for managing the KV cache."""
from typing import List

import torch

from vllm.attention import get_attn_backend
from vllm.config import CacheConfig, DeviceConfig, ModelConfig, ParallelConfig
from vllm.logger import init_logger
from vllm.utils import (STR_DTYPE_TO_TORCH_DTYPE, LayerBlockType,
                        get_dtype_size, is_pin_memory_available)

logger = init_logger(__name__)


class CacheEngine:
    """Manages the KV cache.

    This class is responsible for initializing and managing the GPU and CPU KV
    caches. It also provides methods for performing KV cache operations, such
    as swapping and copying.
    """

    def __init__(
        self,
        cache_config: CacheConfig,
        model_config: ModelConfig,
        parallel_config: ParallelConfig,
        device_config: DeviceConfig,
    ) -> None:
        self.cache_config = cache_config
        self.model_config = model_config
        self.parallel_config = parallel_config
        self.device_config = device_config

        self.head_size = model_config.get_head_size()
        # Models like Jamba, have mixed typed layers, E.g Mamba
        self.num_attention_layers = model_config.get_num_layers_by_block_type(
            parallel_config, LayerBlockType.attention)
        self.num_kv_heads = model_config.get_num_kv_heads(parallel_config)

        self.block_size = cache_config.block_size
        self.num_gpu_blocks = cache_config.num_gpu_blocks
        if self.num_gpu_blocks:
            self.num_gpu_blocks //= parallel_config.pipeline_parallel_size
        self.num_cpu_blocks = cache_config.num_cpu_blocks
        if self.num_cpu_blocks:
            self.num_cpu_blocks //= parallel_config.pipeline_parallel_size

        if cache_config.cache_dtype == "auto":
            self.dtype = model_config.dtype
        else:
            self.dtype = STR_DTYPE_TO_TORCH_DTYPE[cache_config.cache_dtype]

        # Get attention backend.
        self.attn_backend = get_attn_backend(self.head_size,
                                             model_config.dtype,
                                             cache_config.cache_dtype,
                                             self.block_size,
                                             model_config.is_attention_free,
                                             use_mla=model_config.use_mla)

        # Initialize the cache.
        self.gpu_cache = self._allocate_kv_cache(
            self.num_gpu_blocks, self.device_config.device_type)
        self.cpu_cache = self._allocate_kv_cache(self.num_cpu_blocks, "cpu")

    def _allocate_kv_cache(
        self,
        num_blocks: int,
        device: str,
    ) -> List[torch.Tensor]:
        """Allocates KV cache on the specified device."""
        kv_cache_generic_shape = self.attn_backend.get_kv_cache_shape(
            num_blocks, self.block_size, self.num_kv_heads, self.head_size)
        pin_memory = is_pin_memory_available() if device == "cpu" else False
        kv_cache: List[torch.Tensor] = []
        try:
            kv_cache_stride_order = self.attn_backend.get_kv_cache_stride_order(
            )
        except (AttributeError, NotImplementedError):
            kv_cache_stride_order = tuple(range(len(kv_cache_generic_shape)))

        # The allocation respects the backend-defined stride order to ensure
        # the semantic remains consistent for each backend. We first obtain the
        # generic kv cache shape and then permute it according to the stride
        # order which could result in a non-contiguous tensor.
        kv_cache_allocation_shape = tuple(kv_cache_generic_shape[i]
                                          for i in kv_cache_stride_order)

        for _ in range(self.num_attention_layers):
            # null block in CpuGpuBlockAllocator requires at least that
            # block to be zeroed-out.
            # We zero-out everything for simplicity.
            layer_kv_cache = torch.zeros(
                kv_cache_allocation_shape,
                dtype=self.dtype,
                pin_memory=pin_memory,
                device=device).permute(*kv_cache_stride_order)

            # view back to (TOTAL_PAGES, PAGE_SIZE, entry_shape...) for cases
            # when entry_shape is higher than 1D
            kv_cache.append(layer_kv_cache)
        return kv_cache

    def swap_in(self, src_to_dst: torch.Tensor) -> None:
        for i in range(self.num_attention_layers):
            self.attn_backend.swap_blocks(self.cpu_cache[i], self.gpu_cache[i],
                                          src_to_dst)

    def swap_out(self, src_to_dst: torch.Tensor) -> None:
        for i in range(self.num_attention_layers):
            self.attn_backend.swap_blocks(self.gpu_cache[i], self.cpu_cache[i],
                                          src_to_dst)

    def copy(self, src_to_dsts: torch.Tensor) -> None:
        self.attn_backend.copy_blocks(self.gpu_cache, src_to_dsts)

    @staticmethod
    def get_cache_block_size(
        cache_config: CacheConfig,
        model_config: ModelConfig,
        parallel_config: ParallelConfig,
    ) -> int:
        head_size = model_config.get_head_size()
        num_heads = model_config.get_num_kv_heads(parallel_config)
        num_attention_layers = model_config.get_num_layers_by_block_type(
            parallel_config, LayerBlockType.attention)

        if cache_config.cache_dtype == "auto":
            dtype = model_config.dtype
        else:
            dtype = STR_DTYPE_TO_TORCH_DTYPE[cache_config.cache_dtype]

        key_cache_entry = num_heads * head_size

        # For MLA there is no value cache, since the latent vector
        # is joint keys and values.
        value_cache_entry = key_cache_entry if not model_config.use_mla else 0
        total = num_attention_layers * cache_config.block_size * \
            (key_cache_entry + value_cache_entry)

        dtype_size = get_dtype_size(dtype)
        return dtype_size * total