sleepy
8e72eef09c
- Rename gamma to glm5 and model to minimax-m2.7 - Add model_comparison/ directory with head-to-head analyses - Sanitize all session.jsonl files: remove absolute paths and usernames - Remove __pycache__ artifacts - Add .gitignore
509 lines
20 KiB
Python
509 lines
20 KiB
Python
"""
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KV-Cache Optimizations
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======================
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Three production-grade optimizations for the base KV-cache:
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1. PagedAttention — block-based virtual memory for the cache
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2. Chunked Prefill — split long prompts into fixed-size chunks
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3. Cache Quantization — compress K/V to lower precision
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Each optimisation is a drop-in wrapper around the base KVCache
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interface, keeping the same update / get_kv contract.
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"""
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from __future__ import annotations
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import math
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from typing import List, Tuple, Optional, Dict
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import numpy as np
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from kv_cache import KVCache
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# ══════════════════════════════════════════════════════════════════════
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# OPTIMIZATION 1: PAGED ATTENTION
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# ══════════════════════════════════════════════════════════════════════
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#
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# Problem
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# -------
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# The base cache pre-allocates (B, H, S_max, D) per layer. If S_max
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# is large (e.g. 128 k tokens) this wastes enormous memory for short
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# sequences and fragments GPU memory when sequences finish at different
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# times.
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#
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# Solution (cf. vLLM / PagedAttention)
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# -------
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# Divide the cache into fixed-size *blocks* (pages) of BLOCK_SIZE tokens.
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# A per-sequence page table maps virtual positions → physical block ids.
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# Blocks are allocated from a pool — freed when a sequence finishes and
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# immediately reusable by a new sequence.
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#
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# Memory layout (physical):
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# k_pool: (NUM_BLOCKS, H, BLOCK_SIZE, D)
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# v_pool: (NUM_BLOCKS, H, BLOCK_SIZE, D)
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#
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# Per-sequence metadata:
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# page_table: list[list[int]] — page_table[b] = [block_0, block_1, ...]
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# seq_lens: list[int]
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#
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# GPU mapping: the page table lives in GPU memory and is indexed by a
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# custom CUDA kernel that performs the gather from scattered blocks.
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# On CPU we simulate it with index arithmetic.
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# ══════════════════════════════════════════════════════════════════════
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class PagedKVCache:
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"""
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Block-scattered KV cache inspired by vLLM's PagedAttention.
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Unlike the base KVCache which pre-allocates a contiguous (B, H, S_max, D)
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tensor, PagedKVCache allocates a fixed pool of blocks and assigns them
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on demand. This eliminates:
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- memory waste from over-provisioning S_max
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- fragmentation from variable-length sequences
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- the need for a single contiguous S_max allocation
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"""
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def __init__(
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self,
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num_blocks: int,
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block_size: int,
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num_heads: int,
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head_dim: int,
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max_num_seqs: int,
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dtype: np.dtype = np.float32,
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):
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self.num_blocks = num_blocks
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self.block_size = block_size
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self.H = num_heads
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self.D = head_dim
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self.dtype = dtype
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# Physical block pool — shapes (num_blocks, H, block_size, D)
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self.k_pool = np.zeros(
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(num_blocks, num_heads, block_size, head_dim), dtype=dtype
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)
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self.v_pool = np.zeros(
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(num_blocks, num_heads, block_size, head_dim), dtype=dtype
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)
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# Free-list of available block indices
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self.free_blocks: List[int] = list(range(num_blocks))
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# Per-sequence bookkeeping
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self.page_tables: List[List[int]] = [] # seq_id → list of block ids
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self.seq_lens: List[int] = [] # seq_id → current length
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self.max_num_seqs = max_num_seqs
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# ── sequence lifecycle ───────────────────────────────────────────
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def add_sequence(self) -> int:
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"""Register a new sequence; returns its id."""
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assert len(self.page_tables) < self.max_num_seqs, "too many sequences"
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seq_id = len(self.page_tables)
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self.page_tables.append([])
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self.seq_lens.append(0)
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return seq_id
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def finish_sequence(self, seq_id: int) -> None:
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"""Release all blocks held by a finished sequence."""
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for block_id in self.page_tables[seq_id]:
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self.free_blocks.append(block_id)
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self.page_tables[seq_id] = []
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self.seq_lens[seq_id] = 0
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# ── block allocation ─────────────────────────────────────────────
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def _ensure_blocks(self, seq_id: int, total_tokens: int) -> None:
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"""Allocate enough blocks for `total_tokens` positions."""
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blocks_needed = math.ceil(total_tokens / self.block_size)
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current = len(self.page_tables[seq_id])
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while current < blocks_needed:
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if not self.free_blocks:
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raise RuntimeError(
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f"Out of blocks! Need {blocks_needed}, have {self.num_blocks} total. "
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f"Free: {len(self.free_blocks)}"
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)
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self.page_tables[seq_id].append(self.free_blocks.pop(0))
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current += 1
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# ── update (write K, V) ──────────────────────────────────────────
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def update(
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self,
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seq_id: int,
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new_k: np.ndarray,
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new_v: np.ndarray,
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) -> None:
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"""
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Write new tokens for a single sequence.
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new_k, new_v : shape (H, T, D)
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"""
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T = new_k.shape[1]
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old_len = self.seq_lens[seq_id]
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new_len = old_len + T
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self._ensure_blocks(seq_id, new_len)
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for t in range(T):
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global_pos = old_len + t
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block_idx = global_pos // self.block_size
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offset = global_pos % self.block_size
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phys_block = self.page_tables[seq_id][block_idx]
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self.k_pool[phys_block, :, offset, :] = new_k[:, t, :]
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self.v_pool[phys_block, :, offset, :] = new_v[:, t, :]
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self.seq_lens[seq_id] = new_len
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# ── retrieval (gather scattered blocks) ──────────────────────────
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def get_kv(self, seq_id: int) -> Tuple[np.ndarray, np.ndarray]:
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"""
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Gather keys/values for a sequence from scattered blocks.
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Returns (H, S_valid, D) arrays for keys and values.
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"""
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S = self.seq_lens[seq_id]
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num_full_blocks = S // self.block_size
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remainder = S % self.block_size
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k_parts = []
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v_parts = []
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for i in range(num_full_blocks):
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phys = self.page_tables[seq_id][i]
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k_parts.append(self.k_pool[phys]) # (H, block_size, D)
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v_parts.append(self.v_pool[phys])
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if remainder > 0:
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phys = self.page_tables[seq_id][num_full_blocks]
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k_parts.append(self.k_pool[phys, :, :remainder, :])
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v_parts.append(self.v_pool[phys, :, :remainder, :])
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if not k_parts:
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H, D = self.H, self.D
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return np.empty((H, 0, D), dtype=self.dtype), np.empty(
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(H, 0, D), dtype=self.dtype
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)
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return np.concatenate(k_parts, axis=1), np.concatenate(v_parts, axis=1)
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# ── memory stats ─────────────────────────────────────────────────
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def memory_bytes(self) -> int:
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return self.k_pool.nbytes + self.v_pool.nbytes
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def utilization(self) -> float:
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"""Fraction of blocks currently in use."""
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used = self.num_blocks - len(self.free_blocks)
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return used / self.num_blocks
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def __repr__(self) -> str:
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used = self.num_blocks - len(self.free_blocks)
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return (
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f"PagedKVCache(blocks={used}/{self.num_blocks}, "
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f"block_size={self.block_size}, H={self.H}, D={self.D}, "
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f"seqs={len(self.page_tables)}, "
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f"mem={self.memory_bytes() / 1e6:.1f} MB)"
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)
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# ══════════════════════════════════════════════════════════════════════
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# OPTIMIZATION 2: CHUNKED PREFILL
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# ══════════════════════════════════════════════════════════════════════
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#
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# Problem
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# -------
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# During the prefill phase the entire prompt is processed in one shot.
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# For a prompt of length S this means an O(S²) attention matrix which
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# can blow up memory and latency (e.g. S=32 k → 1 billion elements).
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#
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# Solution
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# --------
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# Split the prompt into chunks of CHUNK_SIZE tokens. Process each
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# chunk sequentially, writing its K/V into the cache. Subsequent
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# chunks attend to all previously cached chunks *plus* their own
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# positions (causal masking within the current chunk).
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#
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# This reduces peak memory from O(S²) to O(CHUNK_SIZE × S) and
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# allows overlapping prefill of one request with decode of others.
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# ══════════════════════════════════════════════════════════════════════
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class ChunkedPrefillCache:
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"""
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Wrapper around KVCache that processes long prompts in chunks.
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Instead of filling the entire prompt at once (O(S²) memory),
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we iterate over chunks of size C:
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- Each chunk's K/V is written to the cache
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- Attention for chunk i sees positions [0 .. i*C + C)
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- Peak attention memory: O(C × i*C) instead of O(S²)
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"""
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def __init__(
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self,
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base_cache: KVCache,
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chunk_size: int = 512,
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):
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self.cache = base_cache
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self.chunk_size = chunk_size
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def prefill(
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self,
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all_k: np.ndarray,
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all_v: np.ndarray,
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w_q: np.ndarray,
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w_k: np.ndarray,
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w_v: np.ndarray,
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w_o: np.ndarray,
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) -> np.ndarray:
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"""
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Process a long prompt in chunks.
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all_k, all_v : (B, H, S, D) — keys and values for the full prompt
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w_q, w_k, w_v, w_o : projection matrices
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Returns the output of the *last* chunk (B, 1, d_model) which
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is needed for predicting the next token.
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"""
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B, H, S, D = all_k.shape
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chunk_size = self.chunk_size
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num_chunks = math.ceil(S / chunk_size)
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last_output = None
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for c in range(num_chunks):
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start = c * chunk_size
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end = min(start + chunk_size, S)
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T = end - start
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# Write this chunk's K, V into the cache
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chunk_k = all_k[:, :, start:end, :] # (B, H, T, D)
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chunk_v = all_v[:, :, start:end, :]
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self.cache.update(chunk_k, chunk_v)
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# Now compute attention: queries from this chunk vs all cached K,V
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# For simplicity, return the last-position output
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from kv_cache import multi_head_attention_with_cache
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# Reconstruct a fake q_new in (B, T, d_model) space
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# In a real model q would come from the embedding of chunk tokens
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# Here we simulate by just using the chunk's K projected through w_q
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d_model = w_q.shape[0]
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# We only need the last position for autoregressive output
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q_single = np.random.randn(B, 1, d_model).astype(all_k.dtype)
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last_output = multi_head_attention_with_cache(
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q_single, self.cache, w_q, w_k, w_v, w_o
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)
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return last_output
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# ══════════════════════════════════════════════════════════════════════
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# OPTIMIZATION 3: KV CACHE QUANTIZATION (INT8 / INT4)
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# ══════════════════════════════════════════════════════════════════════
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#
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# Problem
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# -------
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# For long contexts the cache grows linearly with sequence length.
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# A 32-layer, 32-head, 128-dim model at batch=1 and seq=65 k uses:
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# 2 × 32 × 32 × 128 × 65536 × 4 bytes ≈ 68 GB (!!!)
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#
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# Solution
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# --------
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# Quantize cached K/V to lower precision on-the-fly:
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# - INT8: store scale + quantized values → 2× memory reduction
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# - INT4: store scale + quantized values → 4× memory reduction
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#
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# During attention, dequantize back to FP32 before matmul.
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# This trades a small accuracy loss for massive memory savings.
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#
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# GPU mapping:
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# - Store quantized data in INT8/INT4 tensors
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# - Dequantize in registers before the QK^T matmul
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# - Or use specialized kernels (e.g. FP8 attention in Hopper GPUs)
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# ══════════════════════════════════════════════════════════════════════
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class QuantizedKVCache:
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"""
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KV cache with on-the-fly quantization to a target bit-width.
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Internally stores:
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k_quant : uint8 array (packed)
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k_scale : float32 per-(batch, head, token) scale factor
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v_quant : uint8 array (packed)
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v_scale : float32 per-(batch, head, token) scale factor
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Supports INT8 (bits=8) and INT4 (bits=4, stored 2-per-byte).
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"""
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def __init__(
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self,
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batch_size: int,
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max_seq_len: int,
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num_heads: int,
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head_dim: int,
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bits: int = 8,
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):
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assert bits in (4, 8), "Only INT8 and INT4 are supported"
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self.B = batch_size
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self.S_max = max_seq_len
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self.H = num_heads
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self.D = head_dim
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self.bits = bits
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# Per-token scale factors and zero points: (B, H, S_max)
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self.k_scale = np.zeros((batch_size, num_heads, max_seq_len), dtype=np.float32)
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self.v_scale = np.zeros((batch_size, num_heads, max_seq_len), dtype=np.float32)
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self.k_zp = np.zeros((batch_size, num_heads, max_seq_len), dtype=np.float32)
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self.v_zp = np.zeros((batch_size, num_heads, max_seq_len), dtype=np.float32)
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if bits == 8:
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self.k_quant = np.zeros(
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(batch_size, num_heads, max_seq_len, head_dim), dtype=np.uint8
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)
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self.v_quant = np.zeros(
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(batch_size, num_heads, max_seq_len, head_dim), dtype=np.uint8
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)
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else:
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# INT4: pack 2 values per byte → head_dim / 2 bytes per token
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assert head_dim % 2 == 0, "head_dim must be even for INT4 packing"
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self.k_quant = np.zeros(
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(batch_size, num_heads, max_seq_len, head_dim // 2), dtype=np.uint8
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)
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self.v_quant = np.zeros(
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(batch_size, num_heads, max_seq_len, head_dim // 2), dtype=np.uint8
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)
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self.seq_lens: List[int] = [0] * batch_size
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# ── quantization helpers ─────────────────────────────────────────
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def _quantize_token(self, vec: np.ndarray) -> Tuple[np.ndarray, np.float32]:
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"""Quantize a 1-D vector to unsigned integers + scale."""
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vmin = np.min(vec)
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vmax = np.max(vec)
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max_int = (1 << self.bits) - 1
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scale = (vmax - vmin) / max_int if max_int > 0 else 1.0
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zero_point = vmin # shift so min maps to 0
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quantized = np.clip(np.round((vec - zero_point) / (scale + 1e-8)), 0, max_int).astype(np.uint8)
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return quantized, np.float32(scale), np.float32(zero_point)
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def _pack_int4(self, vec: np.ndarray) -> np.ndarray:
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"""Pack a uint8 vector of 0..15 values into nibbles."""
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packed = np.zeros(len(vec) // 2, dtype=np.uint8)
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for i in range(len(vec) // 2):
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packed[i] = (vec[2 * i] << 4) | vec[2 * i + 1]
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return packed
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def _unpack_int4(self, packed: np.ndarray) -> np.ndarray:
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"""Unpack nibbles back to a full uint8 vector."""
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out = np.zeros(len(packed) * 2, dtype=np.uint8)
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for i in range(len(packed)):
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out[2 * i] = (packed[i] >> 4) & 0x0F
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out[2 * i + 1] = packed[i] & 0x0F
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return out
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# ── dequantize for attention ─────────────────────────────────────
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def _dequantize_token(
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self, quant: np.ndarray, scale: np.float32, zero_point: np.float32
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) -> np.ndarray:
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"""Dequantize back to float32."""
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if self.bits == 4:
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unpacked = self._unpack_int4(quant)
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else:
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unpacked = quant.astype(np.float32)
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return unpacked * (scale + 1e-8) + zero_point
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# ── update ───────────────────────────────────────────────────────
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def update(
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self,
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new_k: np.ndarray,
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new_v: np.ndarray,
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) -> None:
|
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"""
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Quantize and store new K/V tokens.
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new_k, new_v : (B, H, T, D) float32
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"""
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T = new_k.shape[2]
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for b in range(self.B):
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pos = self.seq_lens[b]
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for h in range(self.H):
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for t in range(T):
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k_vec = new_k[b, h, t, :]
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v_vec = new_v[b, h, t, :]
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k_q, k_s, k_z = self._quantize_token(k_vec)
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v_q, v_s, v_z = self._quantize_token(v_vec)
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self.k_scale[b, h, pos + t] = k_s
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self.v_scale[b, h, pos + t] = v_s
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self.k_zp[b, h, pos + t] = k_z
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self.v_zp[b, h, pos + t] = v_z
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if self.bits == 8:
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self.k_quant[b, h, pos + t, :] = k_q
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self.v_quant[b, h, pos + t, :] = v_q
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else:
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self.k_quant[b, h, pos + t, :] = self._pack_int4(k_q)
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self.v_quant[b, h, pos + t, :] = self._pack_int4(v_q)
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self.seq_lens[b] += T
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# ── retrieval ────────────────────────────────────────────────────
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|
||
def get_kv(self, batch_idx: int) -> Tuple[np.ndarray, np.ndarray]:
|
||
"""
|
||
Dequantize and return (H, S_valid, D) arrays.
|
||
"""
|
||
S = self.seq_lens[batch_idx]
|
||
k_out = np.zeros((self.H, S, self.D), dtype=np.float32)
|
||
v_out = np.zeros((self.H, S, self.D), dtype=np.float32)
|
||
|
||
for h in range(self.H):
|
||
for t in range(S):
|
||
scale_k = self.k_scale[batch_idx, h, t]
|
||
scale_v = self.v_scale[batch_idx, h, t]
|
||
zp_k = self.k_zp[batch_idx, h, t]
|
||
zp_v = self.v_zp[batch_idx, h, t]
|
||
|
||
if self.bits == 8:
|
||
k_q = self.k_quant[batch_idx, h, t, :]
|
||
v_q = self.v_quant[batch_idx, h, t, :]
|
||
else:
|
||
k_q = self.k_quant[batch_idx, h, t, :]
|
||
v_q = self.v_quant[batch_idx, h, t, :]
|
||
|
||
k_out[h, t, :] = self._dequantize_token(k_q, scale_k, zp_k)
|
||
v_out[h, t, :] = self._dequantize_token(v_q, scale_v, zp_v)
|
||
|
||
return k_out, v_out
|
||
|
||
# ── memory savings ───────────────────────────────────────────────
|
||
|
||
def memory_bytes(self) -> int:
|
||
return (
|
||
self.k_quant.nbytes + self.v_quant.nbytes
|
||
+ self.k_scale.nbytes + self.v_scale.nbytes
|
||
+ self.k_zp.nbytes + self.v_zp.nbytes
|
||
)
|
||
|
||
def savings_vs_fp32(self) -> float:
|
||
"""Ratio of this cache's memory to an equivalent FP32 cache."""
|
||
fp32_bytes = (
|
||
2 * self.B * self.H * self.S_max * self.D * 4 # 2 arrays × 4 bytes
|
||
)
|
||
return self.memory_bytes() / fp32_bytes
|
||
|
||
def __repr__(self) -> str:
|
||
return (
|
||
f"QuantizedKVCache(INT{self.bits}, B={self.B}, H={self.H}, "
|
||
f"S_max={self.S_max}, D={self.D}, "
|
||
f"mem={self.memory_bytes() / 1e6:.1f} MB, "
|
||
f"savings={self.savings_vs_fp32():.2f}x vs FP32)"
|
||
)
|