block_manager.py

"""A block manager that manages token blocks."""
from typing import Dict, List, Optional, Set, Tuple

from cacheflow.block import PhysicalTokenBlock
from cacheflow.sequence import Sequence, SequenceGroup, SequenceStatus
from cacheflow.utils import Device


class BlockAllocator:
    """Manages free physical token blocks for a device.

    The allocator maintains a list of free blocks and allocates a block when
    requested. When a block is freed, its reference count is decremented. If
    the reference count becomes zero, the block is added back to the free list.
    """

    def __init__(
        self,
        device: Device,
        block_size: int,
        num_blocks: int,
    ) -> None:
        self.device = device
        self.block_size = block_size
        self.num_blocks = num_blocks

        # Initialize the free blocks.
        self.free_blocks: List[PhysicalTokenBlock] = []
        for i in range(num_blocks):
            block = PhysicalTokenBlock(
                device=device, block_number=i, block_size=block_size)
            self.free_blocks.append(block)

    def allocate(self) -> PhysicalTokenBlock:
        if not self.free_blocks:
            raise ValueError("Out of memory! No free blocks are available.")
        block = self.free_blocks.pop()
        block.ref_count = 1
        return block

    def free(self, block: PhysicalTokenBlock) -> None:
        if block.ref_count == 0:
            raise ValueError(f"Double free! {block} is already freed.")
        block.ref_count -= 1
        if block.ref_count == 0:
            self.free_blocks.append(block)

    def get_num_free_blocks(self) -> int:
        return len(self.free_blocks)


# Mapping: logical block number -> physical block.
BlockTable = List[PhysicalTokenBlock]


class BlockSpaceManager:
    """Manages the mapping between logical and physical token blocks."""

    def __init__(
        self,
        block_size: int,
        num_gpu_blocks: int,
        num_cpu_blocks: int,
        watermark: float = 0.01,
    ) -> None:
        self.block_size = block_size
        self.num_total_gpu_blocks = num_gpu_blocks
        self.num_total_cpu_blocks = num_cpu_blocks
        self.watermark = watermark
        assert watermark >= 0.0

        self.watermark_blocks = int(watermark * num_gpu_blocks)
        self.gpu_allocator = BlockAllocator(Device.GPU, block_size,
                                            num_gpu_blocks)
        self.cpu_allocator = BlockAllocator(Device.CPU, block_size,
                                            num_cpu_blocks)
        # Mapping: seq_id -> BlockTable.
        self.block_tables: Dict[int, BlockTable] = {}

    def can_allocate(self, seq_group: SequenceGroup) -> bool:
        # FIXME(woosuk): Here we assume that all sequences in the group share
        # the same prompt. This may not be true for preempted sequences.
        seq = seq_group.get_seqs()[0]
        num_required_blocks = len(seq.logical_token_blocks)
        num_free_gpu_blocks = self.gpu_allocator.get_num_free_blocks()
        # Use watermark to avoid frequent cache eviction.
        return num_free_gpu_blocks - num_required_blocks >= self.watermark_blocks

    def allocate(self, seq_group: SequenceGroup) -> None:
        # NOTE: Here we assume that all sequences in the group have the same prompt.
        seq = seq_group.get_seqs()[0]

        # Allocate new physical token blocks that will store the prompt tokens.
        block_table: BlockTable = []
        for _ in range(len(seq.logical_token_blocks)):
            block = self.gpu_allocator.allocate()
            # Set the reference counts of the token blocks.
            block.ref_count = seq_group.num_seqs()
            block_table.append(block)

        # Assign the block table for each sequence.
        for seq in seq_group.get_seqs():
            self.block_tables[seq.seq_id] = block_table.copy()

    def can_append_slot(self, seq_group: SequenceGroup) -> bool:
        # Simple heuristic: If there is at least one free block
        # for each sequence, we can append.
        num_free_gpu_blocks = self.gpu_allocator.get_num_free_blocks()
        num_seqs = seq_group.num_seqs(status=SequenceStatus.RUNNING)
        return num_seqs <= num_free_gpu_blocks

    def append_slot(self, seq: Sequence) -> Optional[Tuple[int, int]]:
        """Allocate a physical slot for a new token."""
        logical_blocks = seq.logical_token_blocks
        block_table = self.block_tables[seq.seq_id]

        if len(block_table) < len(logical_blocks):
            # The sequence has a new logical block.
            # Allocate a new physical block.
            block = self.gpu_allocator.allocate()
            block_table.append(block)
            return None

        # We want to append the token to the last physical block.
        last_block = block_table[-1]
        assert last_block.device == Device.GPU
        if last_block.ref_count == 1:
            # Not shared with other sequences. Appendable.
            return None
        else:
            # The last block is shared with other sequences.
            # Copy on Write: Allocate a new block and copy the tokens.
            new_block = self.gpu_allocator.allocate()
            block_table[-1] = new_block
            self.gpu_allocator.free(last_block)
            return last_block.block_number, new_block.block_number

    def fork(self, parent_seq: Sequence, child_seq: Sequence) -> None:
        # NOTE: fork does not allocate a new physical block.
        # Thus, it is always safe from OOM.
        src_block_table = self.block_tables[parent_seq.seq_id]
        self.block_tables[child_seq.seq_id] = src_block_table.copy()
        for block in src_block_table:
            block.ref_count += 1

    def _get_physical_blocks(self, seq_group: SequenceGroup) -> List[PhysicalTokenBlock]:
        # NOTE: Here, we assume that the physical blocks are only shared by
        # the sequences in the same group.
        blocks: Set[PhysicalTokenBlock] = set()
        for seq in seq_group.get_seqs():
            if SequenceStatus.is_finished(seq.status):
                continue
            block_table = self.block_tables[seq.seq_id]
            for block in block_table:
                blocks.add(block)
        return list(blocks)

    def can_swap_in(self, seq_group: SequenceGroup) -> bool:
        blocks = self._get_physical_blocks(seq_group)
        num_swapped_seqs = seq_group.num_seqs(status=SequenceStatus.SWAPPED)
        num_free_blocks = self.gpu_allocator.get_num_free_blocks()
        # NOTE: Conservatively, we assume that every sequence will allocate
        # at least one free block right after the swap-in.
        # NOTE: This should match the logic in can_append_slot().
        num_required_blocks = len(blocks) + num_swapped_seqs
        return num_free_blocks - num_required_blocks >= self.watermark_blocks

    def swap_in(self, seq_group: SequenceGroup) -> Dict[int, int]:
        # CPU block -> GPU block.
        mapping: Dict[PhysicalTokenBlock, PhysicalTokenBlock] = {}
        for seq in seq_group.get_seqs():
            if SequenceStatus.is_finished(seq.status):
                continue
            new_block_table: BlockTable = []
            block_table = self.block_tables[seq.seq_id]

            for cpu_block in block_table:
                if cpu_block in mapping:
                    gpu_block = mapping[cpu_block]
                    gpu_block.ref_count += 1
                else:
                    gpu_block = self.gpu_allocator.allocate()
                    mapping[cpu_block] = gpu_block
                new_block_table.append(gpu_block)
                # Free the CPU block swapped in to GPU.
                self.cpu_allocator.free(cpu_block)
            self.block_tables[seq.seq_id] = new_block_table

        block_number_mapping = {
            cpu_block.block_number: gpu_block.block_number
            for cpu_block, gpu_block in mapping.items()
        }
        return block_number_mapping

    def can_swap_out(self, seq_group: SequenceGroup) -> bool:
        blocks = self._get_physical_blocks(seq_group)
        return len(blocks) <= self.cpu_allocator.get_num_free_blocks()

    def swap_out(self, seq_group: SequenceGroup) -> Dict[int, int]:
        # GPU block -> CPU block.
        mapping: Dict[PhysicalTokenBlock, PhysicalTokenBlock] = {}
        for seq in seq_group.get_seqs():
            if SequenceStatus.is_finished(seq.status):
                continue
            new_block_table: BlockTable = []
            block_table = self.block_tables[seq.seq_id]

            for gpu_block in block_table:
                if gpu_block in mapping:
                    cpu_block = mapping[gpu_block]
                    cpu_block.ref_count += 1
                else:
                    cpu_block = self.cpu_allocator.allocate()
                    mapping[gpu_block] = cpu_block
                new_block_table.append(cpu_block)
                # Free the GPU block swapped out to CPU.
                self.gpu_allocator.free(gpu_block)
            self.block_tables[seq.seq_id] = new_block_table

        block_number_mapping = {
            gpu_block.block_number: cpu_block.block_number
            for gpu_block, cpu_block in mapping.items()
        }
        return block_number_mapping

    def _free_block_table(self, block_table: BlockTable) -> None:
        for block in block_table:
            if block.device == Device.GPU:
                self.gpu_allocator.free(block)
            else:
                self.cpu_allocator.free(block)

    def free(self, seq: Sequence) -> None:
        block_table = self.block_tables[seq.seq_id]
        self._free_block_table(block_table)
        del self.block_tables[seq.seq_id]

    def reset(self) -> None:
        for block_table in self.block_tables.values():
            self._free_block_table(block_table)
        self.block_tables.clear()

    def get_block_table(self, seq: Sequence) -> List[int]:
        block_table = self.block_tables[seq.seq_id]
        return [block.block_number for block in block_table]

    def get_num_free_gpu_blocks(self) -> int:
        return self.gpu_allocator.get_num_free_blocks()

    def get_num_free_cpu_blocks(self) -> int:
        return self.cpu_allocator.get_num_free_blocks()