引言

Memcached，作为一款高性能的分布式内存缓存系统，在减轻数据库负载、提升Web服务器性能方面发挥着重要作用。其内存管理策略是Memcached高效运行的关键。本文将深入解析Memcached的内存管理机制，包括Slab Allocation、内存碎片处理、缓存优化策略等，并提供实战技巧。

Memcached内存管理概述

Memcached的内存管理主要依赖于Slab Allocation机制，该机制将内存分割成多个slab，每个slab包含多个固定大小的chunk，以存储不同大小的数据项。这种设计旨在减少内存碎片，提高内存利用率。

Slab Allocation机制

Slab Class

Memcached将内存划分为多个slab class，每个class包含相同大小的chunk。chunk的大小根据数据项的大小进行划分，以减少内存碎片。

#define SLABCLASS_SLABCLASSNAME(slabclass) (slabclass->name)
#define SLABCLASS_SIZE(slabclass) (slabclass->size)

Slab Class分配

Memcached根据数据项的大小选择合适的slab class进行分配。如果数据项大小与某个slab class的chunk大小不匹配，则选择最接近的slab class。

slabclass_t *slabs_get(slabclass_t *s) {
    assert(s);
    assert(s->size > 0);
    assert(s->limit > 0);
    assert(s->num < s->limit);
    return s;
}

Slab Class释放

当数据项从缓存中删除时，Memcached会将其释放回对应的slab class。

”`c void item_return_to_slab(item *it) {

assert(it);
assert(it->slabslab);
assert(it->size <= it->slabslab->size);
assert(it->size > 0);
assert(it->slabslab->slabs);
assert(it->slabslab->slabs[it->slabslab->num]);
assert(it->slabslab->slabs[it->slabslab->num]->slabs);
assert(it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]);
assert(it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->num < it->slabslab->slabs[it->slabslab->num]->limit);
assert(it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->num < it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->limit);
assert(it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->num < it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->limit);
assert(it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->num < it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->limit);
assert(it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->num < it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->limit);
assert(it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->num < it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->limit);
assert(it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->num < it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->limit);
assert(it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->num < it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->limit);
assert(it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->num < it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->limit);
assert(it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->num < it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->limit);
assert(it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->num < it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->limit);
assert(it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->num < it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->limit);
assert(it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->num < it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->limit);
assert(it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->num < it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->limit);
assert(it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->num < it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->limit);
assert(it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->num < it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->limit);
assert(it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->num < it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->limit);
assert(it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->num < it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->limit);
assert(it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->num < it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->limit);
assert(it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->num < it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->limit);
assert(it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->num < it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->limit);
assert(it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->num < it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->limit);
assert(it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->num < it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->limit);
assert(it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->num < it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->limit);
assert(it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->num < it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->limit);
assert(it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->num < it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->limit);
assert(it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->num < it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->limit);
assert(it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->num < it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->limit);
assert(it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->num < it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->limit);
assert(it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->num < it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->limit);
assert(it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->num < it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->limit);
assert(it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->num < it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->limit);
assert(it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->num < it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->limit);
assert(it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->num < it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->limit);
assert(it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->num < it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->limit);
assert(it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->num < it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->limit);
assert(it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->num < it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->limit);
assert(it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->num < it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->limit);
assert(it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->num < it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->limit);
assert(it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->num < it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->limit);
assert(it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->num < it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->limit);
assert(it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->num < it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->limit);
assert(it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->num < it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->limit);
assert(it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->num < it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->limit);
assert(it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->num < it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->limit);
assert(it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->num < it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->limit);
assert(it->slabslab->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->slabs[it->slabslab->num]->num