57 lines
2.0 KiB
Python
57 lines
2.0 KiB
Python
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"""Fixed-size block allocator for the classical paged KV cache.
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One BlockAllocator per layer per "pool kind" (classical / indexer).
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Total blocks are sized at engine startup. Blocks are recycled on
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request completion.
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Cudagraph-safety: allocation can't happen inside a captured graph
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(allocation rate is per-request not per-token). The contract is:
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- acquire() called between graph captures.
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- release() called between graph captures.
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- read access (via block table) happens INSIDE captured graphs.
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"""
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from __future__ import annotations
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import torch
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class BlockAllocator:
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def __init__(
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self,
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num_total_blocks: int,
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device: str = "cuda",
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):
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self.num_total_blocks = num_total_blocks
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self.device = device
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# Free-list as a GPU stack: ids[0..top-1] holds free block IDs.
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# `top` lives in pinned host memory so we can read it without a
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# device sync (it's modified only between graph captures).
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self.free_ids = torch.arange(
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num_total_blocks, dtype=torch.int32, device=device,
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)
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self.top_cpu = torch.tensor([num_total_blocks], dtype=torch.int32, pin_memory=True)
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@property
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def num_free(self) -> int:
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return int(self.top_cpu[0])
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def acquire(self, n: int) -> torch.Tensor:
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"""Return a tensor of `n` block IDs. Called between captures."""
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top = int(self.top_cpu[0])
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if n > top:
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raise RuntimeError(
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f"KV cache OOM: requested {n} blocks, {top} available "
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f"(of {self.num_total_blocks} total)"
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)
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new_top = top - n
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ids = self.free_ids[new_top:top].clone() # snapshot
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self.top_cpu[0] = new_top
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return ids
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def release(self, ids: torch.Tensor) -> None:
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"""Return blocks to the free list. Called between captures."""
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n = ids.numel()
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top = int(self.top_cpu[0])
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self.free_ids[top:top + n] = ids.to(device=self.device)
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self.top_cpu[0] = top + n
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