PHP原理之内存管理中难懂的几个点

本文主要是介绍PHP原理之内存管理中难懂的几个点，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！

PHP的内存管理, 分为俩大部分, 第一部分是PHP自身的内存管理, 这部分主要的内容就是引用计数, 写时复制, 等等面向应用的层面的管理. 而第二部分就是今天我要介绍的, zend_alloc中描写的关于PHP自身的内存管理, 包括它是如何管理可用内存, 如何分配内存等.

另外, 为什么要写这个呢, 因为之前并没有任何资料来介绍PHP内存管理中使用的策略, 数据结构, 或者算法. 而在我们平时开发扩展, 修复PHP的bug的时候, 却对这一部分的知识需要有一个良好的理解. PHP开发组内的很多朋友也对这块不是很清楚, 所以我觉得有必要专门写一下.

一些基本的概念, 我就不赘述了, 因为看代码很容易能看懂, 我这里就主要介绍几个看代码没那么容易看懂的点, 为什么这么说呢, 呵呵, 我在写文章之前, 查找了下已有的资料, 已避免重复功, 其中看到了TIPI项目对这部分的描述, 发现其中错误很多. 所以, 我想这部分就是看代码也没那么容易看懂的点

目前, 英文版的介绍也在撰写中: Zend MM

Zend Memory Manager, 以下简称Zend MM, 是PHP中内存管理的逻辑. 其中有一个关键数据结构: zend_mm_heap:

Zend MM把内存非为小块内存和大块内存俩种, 区别对待, 对于小块内存, 这部分是最最常用的, 所以追求高性能. 而对于大块内存, 则追求的是稳妥, 尽量避免内存浪费.

所以, 对于小块内存, PHP还引入了cache机制:

Zend MM 希望通过cache尽量做到, 一次定位就能查找分配.

而一个不容易看懂的点是free_buckets的申明:

Q: 为什么free_buckets数组的长度是ZEND_MM_NUMBER_BUCKET个?

A: 这是因为, PHP在这处使用了一个技巧, 用一个定长的数组来存储ZEND_MM_NUMBER_BUCKET个zend_mm_free_block, 如上图中红色框所示. 对于一个没有被使用的free_buckets的元素, 唯一有用的数据结构就是next_free_block和prev_free_block, 所以, 为了节省内存, PHP并没有分配ZEND_MM_NUMBER_BUCKET * sizeof(zend_mm_free_block)大小的内存, 而只是用了ZEND_MM_NUMBER_BUCKET * (sizeof(*next_free_block) + sizeof(*prev_free_block))大小的内存..

我们来看ZEND_MM_SMALL_FREE_BUCKET宏的定义:

<ol style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline; list-style-type: none;"><li style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;"><span class="sh_preproc" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(255, 210, 167);">#define</span> <span class="sh_function" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(255, 210, 167);">ZEND_MM_SMALL_FREE_BUCKET</span><span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">(</span>heap<span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">,</span> index<span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">)</span> <span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">\</span></li><li style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">    <span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">(</span>zend_mm_free_block<span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">*)</span> <span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">((</span><span class="sh_type" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(255, 255, 182);">char</span><span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">*)&</span>heap<span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">-></span>free_buckets<span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">[</span>index <span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">*</span> <span class="sh_number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(255, 115, 253);">2</span><span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">]</span> <span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">+</span> <span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">\</span></li><li style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">        <span class="sh_keyword" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(150, 203, 254);">sizeof</span><span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">(</span>zend_mm_free_block<span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">*)</span> <span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">*</span> <span class="sh_number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(255, 115, 253);">2</span> <span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">-</span> <span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">\</span></li><li style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">        <span class="sh_keyword" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(150, 203, 254);">sizeof</span><span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">(</span>zend_mm_small_free_block<span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">))</span></li></ol>

之后, Zend MM 保证只会使用prev和next俩个指针, 所以不会造成内存读错..

那么, 第二个不容易看懂的点, 就是PHP对large_free_buckets的管理, 先介绍分配(TIPI项目组对此部分的描述有些含糊不清):

<ol style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline; list-style-type: none;"><li style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;"><span class="sh_keyword" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(150, 203, 254);">static</span> <span class="sh_usertype" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">zend_mm_free_block</span><span class="sh_normal" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;"> </span><span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">*</span><span class="sh_function" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(255, 210, 167);">zend_mm_search_large_block</span><span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">(</span><span class="sh_usertype" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">zend_mm_heap</span><span class="sh_normal" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;"> </span><span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">*</span>heap<span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">,</span> <span class="sh_usertype" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">size_t</span><span class="sh_normal" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;"> </span>true_size<span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">)</span></li></ol>

large_free_buckets可以说是一个建树和双向列表的结合:

large_free_buckets使用一个宏来决定某个大小的内存, 落在什么index上:

<ol style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline; list-style-type: none;"><li style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;"><span class="sh_preproc" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(255, 210, 167);">#define</span> <span class="sh_function" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(255, 210, 167);">ZEND_MM_LARGE_BUCKET_INDEX</span><span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">(</span>S<span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">)</span> <span class="sh_function" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(255, 210, 167);">zend_mm_high_bit</span><span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">(</span>S<span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">)</span></li></ol>

zend_mm_high_bit获取true_size中最高位1的序号(zend_mm_high_bit), 对应的汇编指令是bsr(此处, TIPI项目错误的说明为: “这个hash函数用来计算size的位数，返回值为size二进码中1的个数-1″).

也就是说, 每一个在large_free_buckets中的元素, 都保持着指向一个大小为在对应index处为1的size的内存块的指针. 诶, 有点绕口, 举个例子:

比如对于large_free_buckets[2], 就只会保存, 大小在0b1000到0b1111大小的内存. 再比如: large_free_buckets[6], 就保存着大小为0b10000000到0b11111111大小的内存的指针.

这样, 再分配内存的时候, Zend MM就可以快速定位到最可能适合的区域来查找. 提高性能.

而, 每一个元素又同时是一个双向列表, 保持着同样size的内存块, 而左右孩子(child[0]和child[1])分别代表着键值0和1, 这个键值是指什么呢?

我们来举个例子, 比如我向PHP申请一个true_size为0b11010大小的内存, 经过一番步骤以后, 没有找到合适的内存, PHP进入了zend_mm_search_large_block的逻辑来在large_free_buckets中寻找合适的内存:

1. 首先, 计算true_size对应的index, 计算方法如之前描述的ZEND_MM_LARGE_BUCKET_INDEX

2. 然后在一个位图结构中, 判断是否存在一个大于true_size的可用内存已经存在于large_free_buckets, 如果不存在就返回:

<ol style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline; list-style-type: none;"><li style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;"><span class="sh_usertype" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">size_t</span><span class="sh_normal" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;"> </span>bitmap <span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">=</span> heap<span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">-></span>large_free_bitmap <span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">>></span> index<span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">;</span></li><li style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;"><span class="sh_keyword" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(150, 203, 254);">if</span> <span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">(</span>bitmap <span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">==</span> <span class="sh_number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(255, 115, 253);">0</span><span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">)</span> <span class="sh_cbracket" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">{</span></li><li style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">   <span class="sh_keyword" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(150, 203, 254);">return</span> NULL<span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">;</span></li><li style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;"><span class="sh_cbracket" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">}</span></li></ol>

3. 判断, free_buckets[index]是否存在可用的内存:

<ol style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline; list-style-type: none;"><li style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;"><span class="sh_keyword" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(150, 203, 254);">if</span> <span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">(</span><span class="sh_function" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(255, 210, 167);">UNEXPECTED</span><span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">((</span>bitmap <span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">&</span> <span class="sh_number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(255, 115, 253);">1</span><span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">)</span> <span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">!=</span> <span class="sh_number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(255, 115, 253);">0</span><span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">))</span></li></ol>

4. 如果存在, 则从free_buckets[index]开始, 寻找最合适的内存, 步骤如下:

4.1. 从free_buckets[index]开始, 如果free_buckets[index]当前的内存大小和true_size相等, 则寻找结束, 成功返回.

4.2. 查看true_size对应index后(true_size << (ZEND_MM_NUM_BUCKETS - index))的当前最高位, 如果为1. 则在free_buckets[index]->child[1]下面继续寻找, 如果free_buckets[index]->child[1]不存在, 则跳出. 如果true_size的当前最高位为0, 则在free_buckets[index]->child[0]下面继续寻找, 如果free_buckets[index]->child[0]不存在, 则在free_buckets[index]->child[1]下面寻找最小内存(因为此时可以保证, 在free_buckets[index]->child[1]下面的内存都是大于true_size的)

4.3. 出发点变更为2中所述的child, 左移一位ture_size.

5. 如果上述逻辑并没有找到合适的内存, 则寻找最小的”大块内存”:

<ol style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline; list-style-type: none;"><li style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">   <span class="sh_comment" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(124, 124, 124);">/* Search for smallest "large" free block */</span></li><li style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">    best_fit <span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">=</span> p <span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">=</span> heap<span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">-></span>large_free_buckets<span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">[</span>index <span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">+</span> <span class="sh_function" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(255, 210, 167);">zend_mm_low_bit</span><span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">(</span>bitmap<span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">)];</span></li><li style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">    <span class="sh_keyword" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(150, 203, 254);">while</span> <span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">((</span>p <span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">=</span> p<span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">-></span>child<span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">[</span>p<span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">-></span>child<span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">[</span><span class="sh_number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(255, 115, 253);">0</span><span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">]</span> <span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">!=</span> NULL<span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">]))</span> <span class="sh_cbracket" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">{</span></li><li style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">        <span class="sh_keyword" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(150, 203, 254);">if</span> <span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">(</span><span class="sh_function" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(255, 210, 167);">ZEND_MM_FREE_BLOCK_SIZE</span><span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">(</span>p<span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">)</span> <span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;"><</span> <span class="sh_function" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; vertical-align: baseline; color: rgb(255, 210, 167);">ZEND_MM_FREE_BLOCK_SIZE</span><span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">(</span>best_fit<span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">))</span> <span class="sh_cbracket" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">{</span></li><li style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">            best_fit <span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">=</span> p<span class="sh_symbol" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">;</span></li><li style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">        <span class="sh_cbracket" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">}</span></li><li style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">    <span class="sh_cbracket" style="margin: 0px; padding: 0px; border: 0px; font-weight: inherit; font-style: inherit; font-family: inherit; vertical-align: baseline;">}</span></li></ol>

注意上面的逻辑, (p = p->child[p->child[0] != NULL]), PHP在尽量寻找最小的内存.

为什么说, large_free_buckets是个键树呢, 从上面的逻辑我们可以看出, PHP把一个size, 按照二进制的0,1做键, 把内存大小信息反应到了键树上, 方便了快速查找.

另外, 还有一个rest_buckets, 这个结构是个双向列表, 用来保存一些PHP分配后剩下的内存, 避免无意义的把剩余内存插入free_buckets带来的性能问题(此处, TIPI项目错误的描述为: “这是一个只有两个元素的数组。而我们常用的插入和查找操作是针对第一个元素，即heap->rest_buckets[0]“).

作者: Laruence( )
本文地址: http://www.laruence.com/2011/11/09/2277.html