KV Cache Optimization

KV Cache Optimization in LLMIR

KV cache management is one of the core optimizations in LLMIR, focusing on efficient handling of key-value pairs in attention mechanisms for transformer-based LLMs.

The KV Cache Challenge ¶

In large language model inference, the key-value cache stores computed key and value tensors from previous tokens to avoid redundant computation. As sequence lengths grow, efficiently managing this cache becomes critical for:

• Memory efficiency
• Computation speed
• Support for longer contexts
• Dynamic batch handling

PagedAttention in LLMIR ¶

LLMIR implements a paged attention mechanism inspired by vLLM’s approach, which treats the KV cache as blocks of memory rather than a continuous buffer:

Block-based KV Cache ¶

// Define a paged KV cache type
!kv_cache_t = !llm.paged_kv_cache<f16, 12, 16, 64, 16, 4096>

Key parameters:

• Element type: f16
• Number of layers: 12
• Number of heads: 16
• Head dimension: 64
• Block size: 16 (tokens per block)
• Maximum sequence length: 4096

Key Cache Operations ¶

LLMIR provides specialized operations for managing the KV cache:

// Append new key-value pairs to the cache
%new_kv, %block_indices = llm.append_kv %kv_cache, %keys, %values, %seq_ids {
  block_size = 16 : i32,
  max_seq_len = 4096 : i32
} : (!kv_cache_t, tensor<2x1x16x64xf16>, tensor<2x1x16x64xf16>, tensor<2xi32>) 
    -> (!kv_cache_t, tensor<2x1xi32>)

// Perform paged attention with the KV cache
%output = llm.paged_attention %query, %new_kv, %block_indices, %seq_lens {
  num_heads = 16 : i32,
  head_dim = 64 : i32,
  scale = 0.125 : f32
} : (tensor<2x1x16x64xf16>, !kv_cache_t, tensor<2x128xi32>, tensor<2xi32>) 
    -> tensor<2x1x16x64xf16>

Runtime Implementation ¶

The LLMIR runtime library will include an efficient implementation of the paged KV cache:

// C++ Runtime API (Planned)
class PagedKVCache {
public:
  PagedKVCache(int numLayers, int numHeads, int headDim, 
               int blockSize, int maxSeqLen, ElementType type);
  
  // Append new KV pairs to the cache
  void appendKV(void* keyPtr, void* valuePtr, int batchSize, 
                int seqLen, int* seqIds, int* blockIndices);
  
  // Lookup existing KV pairs
  void lookupKV(int* blockIndices, int* seqLens, int batchSize,
                void* outputKeys, void* outputValues);
  
private:
  // Block-based memory manager
  BlockAllocator allocator_;
  // Maps sequence IDs to block indices
  std::unordered_map<int, std::vector<int>> seqToBlocks_;
  // ... other implementation details
};

Optimization Passes ¶

LLMIR will include several optimization passes for the KV cache:

1. BlockifyKVCachePass: Convert continuous KV caches to block-based representations
2. PagedAttentionRewritePass: Rewrite standard attention to use paged attention
3. KVCacheAllocationOptimizationPass: Optimize memory allocation for KV caches
4. KVCachePruningPass: Remove unused or stale entries in the KV cache
5. KVCacheShardingPass: Support sharded KV caches for large models

Memory Management ¶

The KV cache implementation uses block-based memory management to:

• Allocate memory in fixed-size blocks
• Efficiently handle varying sequence lengths
• Minimize memory fragmentation
• Enable fast memory reuse for finished sequences

Future Enhancements ¶

As part of LLMIR’s roadmap, the KV cache optimization will be enhanced with:

• Support for multi-head attention variants
• Quantized KV cache for reduced memory footprint
• Distributed KV cache for multi-device inference
• Cache eviction policies for memory-constrained environments

This feature is currently under development as part of Phase 2 of the LLMIR project.