Jemalloc Mempool and Adaptation for CPU HASHTABLE

[ArronHZG]

Jemalloc Mempool and Adaptation for CPU HASHTABLE

1 Current Status

Code: https://github.com/pytorch/FBGEMM/tree/main/fbgemm_gpu/src/dram_kv_embedding_cache

In the current implementation, the value structure of the CPU hashtable uses std::vector, which directly requests memory from the system via std::allocator. Without memory pool management, frequent allocations incur significant system call overhead.

2 Proposed Solution

2.1 Overview

Given that the memory requirements for embedding allocations are known upfront, we can leverage this information to customize bin sizes more appropriately.

Our scenario (embedding hashtable) does not involve frequent memory deallocations (only during ID eviction) but requires frequent allocations. Freed memory can be directly reused for new allocations, minimizing fragmentation concerns.

Compared to jemalloc, our approach avoids complex Buddy/Slob algorithms, memory block merging, and multi-scale slot designs. The core idea is:

Implement dedicated memory pool management for each shard of fixed emb_dim hashtables (including merged tables)
Reuse existing lock mechanisms from SynchronizedShardedMap for concurrency control.
Create a "lock-free per-table memory pool" design that minimizes code invasiveness.

Design features:
Leverages existing SharedMutexWritePriority from SynchronizedShardedMap
Memory pool operations share the same critical section with hashtable insertions

template <typename K, typename V, typename M = folly::SharedMutexWritePriority>
class SynchronizedShardedMap {
 public:
   ...
 private:
  std::vector<folly::Synchronized<folly::F14FastMap<K, V>, M>> shards_;
  // New memory pool with same count as F14FastMap
  std::vector<FixedBlockPool> mempool_; 
};

2.2 FixedBlockPool Design

Three-Level Structure Model

• Pool: Manages multiple chunks, maintains global free list, interfaces with PMR
• Chunk: Preallocated contiguous memory (e.g., 1024 blocks), divided into equal-sized blocks with alignment
• Block: Minimum allocation unit. Stores next-block pointer in first sizeof(void*) bytes when free

Core Data Structures

• Free List: Singly-linked list using head-of-block pointers (O(1) alloc/dealloc)
• Chunk List: Stores pointers to allocated chunks for destruction-time cleanup

Workflow

• Allocate: Take head block from free list. Allocate new chunk when empty.
• Deallocate: Return block to free list head.

Alignment

Ensure block addresses meet alignment requirements (power-of-2 alignment, block size multiples)

2.3 Implementation Details

Chunk Handling

• Minimum chunk size: sizeof(void*) to store link pointers
• Chunk splitting: Fixed-size blocks with zero metadata overhead
• Memory alignment: Enforced during chunk allocation

Custom memory_resource Class

• Inherits from std::pmr::memory_resource
• Parameters: block_size, blocks_per_chunk, alignment
• Maintains free list and chunk tracking

Special Case Handling

For allocations ≤8 bytes (sizeof(void*)): Requires additional handling to prevent metadata overwrite

3 Benefit Analysis

Comparative advantages vs baseline & jemalloc:

1. vs Jemalloc

   • Tighter integration with SynchronizedShardedMap through lock-free per-table memory pool design
   • Avoids multi-threaded resource contention overhead via shared lock mechanism

2. Free Block Management

   • O(1) allocation/deallocation via embedded pointer single-linked list
   • Uses block's own memory for free list pointers

3. std::pmr Advantages

   • Enforces chunk alignment (cache-line friendly)
   • Reduces CPU cache-miss penalties
   • Built-in memory resource chaining support

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