龙兴平 (MSN: lxp8@sina.com) 第 4 页 共 83 页 2010-4-27
freelists for each class whenever an object is freed. See the Bonwick paper in Resources for more
details. Memcached currently generates slab classes for all power-of-two sizes from 64 bytes to
1MB, and it allocates an object of the smallest size that can hold a submitted item. As a result of
using a slab allocator, we can guarantee performance over any length of time. Indeed, we've had
production Memcached servers up for 4–5 months at a time, averaging 7,000 queries/second,
without problems and maintaining consistently low CPU usage.
Another key requirement for Memcached was that it be lockless. All objects are multiversioned
internally and reference counted, so no client can block any other client's actions. If one client is
updating an object stored in Memcached while a dozen others are downloading it, even with one
client on a lossy network connection dropping half its packets, nobody has to wait for anybody
else.
A final optimization worth noting is that the protocol allows fetching multiple keys at once. This
is useful if your application knows it needs to load a few hundred keys. Instead of retrieving them
all sequentially, which would take a fraction of a second in network round-trips, the application
can fetch them all in one request. When necessary, the client libraries automatically split
multi-key loads from the application into separate parallel multi-key loads to the Memcached
instances. Alternatively, applications can provide explicit hash values with keys to keep groups of
data on the same instance. That also saves the client library a bit of CPU time by not needing to
calculate hash values.
Ref：
http://www.linuxjournal.com/article/7451?page=0,2
Memcached 是 danga.com（运营 LiveJournal 的技术团队）开发的一套分布式内存对象缓存
系统，用于在动态系统中减少数据库负载，提升性能。关于这个东西，相信很多人都用过，
本文意在通过对 memcached 的实现及代码分析，获得对这个出色的开源软件更深入的了解，
并可以根据我们的需要对其进行更进一步的优化。末了将通过对 BSM_Memcache 扩展的分
析，加深对 memcached 的使用方式理解。
本文的部分内容可能需要比较好的数学基础作为辅助。
◎Memcached 是什么
在阐述这个问题之前，我们首先要清楚它“不是什么”。很多人把它当作和 SharedMemory
那种形式的存储载体来使用，虽然 memcached 使用了同样的“Key=>Value”方式组织数据，
但是它和共享内存、APC 等本地缓存有非常大的区别。Memcached 是分布式的，也就是说
它不是本地的。它基于网络连接（当然它也可以使用 localhost）方式完成服务，本身它是一
个独立于应用的程序或守护进程（Daemon 方式）。
Memcached 使用 libevent 库实现网络连接服务，理论上可以处理无限多的连接，但是它和
Apache 不同，它更多的时候是面向稳定的持续连接的，所以它实际的并发能力是有限制的。
在保守情况下 memcached 的最大同时连接数为 200，这和 Linux 线程能力有关系，这个数值
YuFS2019-04-17资源分不多，对个人帮忙还可以，应该给4分
