Detailed explanation of the storage layer in PHP memory pool (1)

Detailed explanation of the storage layer in PHP memory pool (1)_PHP tutorial

WBOY

Release： 2016-07-22 09:02:19

Original

989 people have browsed it

PHP’s memory manager is hierarchical. This manager has three layers: storage layer, heap layer and emalloc/efree layer. The storage layer actually applies for memory from the system through functions such as malloc() and mmap(), and releases the requested memory through the free() function.

The memory blocks usually applied for by the storage layer are relatively large. The large memory applied here does not refer to the large memory required by the storage layer structure. It is just that when the heap layer calls the allocation method of the storage layer, it applies for memory in the segment format. Relatively large, the role of the storage layer is to make the memory allocation method transparent to the heap layer.

First look at the structure of the storage layer:

<ol class="dp-xml">
<li class="alt"><span><span>/* Heaps with user defined storage */  </span></span></li>
<li><span>typedef struct _zend_mm_storage zend_mm_storage;  </span></li>
<li class="alt"><span>   </span></li>
<li><span>typedef struct _zend_mm_segment {  </span></li>
<li class="alt"><span>    size_t    size;  </span></li>
<li><span>    struct _zend_mm_segment *next_segment;  </span></li>
<li class="alt"><span>} zend_mm_segment;  </span></li>
<li><span>   </span></li>
<li class="alt"><span>typedef struct _zend_mm_mem_handlers {  </span></li>
<li><span>    const char *name;  </span></li>
<li class="alt"><span>    zend_mm_storage* (*init)(void *params);    //初始化函数  </span></li>
<li><span>    void (*dtor)(zend_mm_storage *storage);    //析构函数  </span></li>
<li class="alt"><span>    void (*compact)(zend_mm_storage *storage);  </span></li>
<li><span>    zend_mm_segment* (*_alloc)(zend_mm_storage *storage, size_t size);    //内存分配函数  </span></li>
<li class="alt"><span>    zend_mm_segment* (*_realloc)(zend_mm_storage *storage, zend_mm_segment *ptr, size_t size);    //重新分配内存函数  </span></li>
<li><span>    void (*_free)(zend_mm_storage *storage, zend_mm_segment *ptr);    //释放内存函数  </span></li>
<li class="alt"><span>} zend_mm_mem_handlers;  </span></li>
<li><span>   </span></li>
<li class="alt"><span>struct _zend_mm_storage {  </span></li>
<li><span>    const zend_mm_mem_handlers *handlers;    //处理函数集  </span></li>
<li class="alt"><span>    void *data;  </span></li>
<li><span>};  </span></li>
</ol>

Copy after login

The memory allocation method, the function called is the processing function set in the _zend_mm_storage structure, and the memory is expressed in the form of segments.

4 memory schemes

PHP has 4 memory allocation schemes in the storage layer: malloc, win32, mmap_anon, mmap_zero. By default, malloc is used to allocate memory. If the ZEND_WIN32 macro is set, it is the Windows version and HeapAlloc is called to allocate memory. The remaining two memory schemes are anonymous memory mapping, and PHP's memory scheme can be modified by setting variables.

The official description is as follows:

The Zend MM can be tweaked using ZEND_MM_MEM_TYPE and ZEND_MM_SEG_SIZE environment variables. Default values are “malloc” and “256K”.Dependent on target system you can also use “mmap_anon”, “mmap_zero” and “win32 ″ storage managers.

In the code, for these four memory allocation schemes, each processing function in zend_mm_mem_handlers is implemented respectively. A simple description of the matching code is as follows:

<ol class="dp-xml">
<li class="alt"><span><span>/* 使用mmap内存映射函数分配内存 写入时拷贝的私有映射，并且匿名映射，映射区不与任何文件关联。*/  </span></span></li>
<li><span># define ZEND_MM_MEM_MMAP_ANON_DSC {"mmap_anon", zend_mm_mem_dummy_init, zend_mm_mem_dummy_dtor, zend_mm_mem_dummy_compact, zend_mm_mem_mmap_anon_alloc, zend_mm_mem_mmap_realloc, zend_mm_mem_mmap_free}  </span></li>
<li class="alt"><span> </span></li>
<li><span>/* 使用mmap内存映射函数分配内存 写入时拷贝的私有映射，并且映射到/dev/zero。*/  </span></li>
<li class="alt"><span># define ZEND_MM_MEM_MMAP_ZERO_DSC {"mmap_zero", zend_mm_mem_mmap_zero_init, zend_mm_mem_mmap_zero_dtor, zend_mm_mem_dummy_compact, zend_mm_mem_mmap_zero_alloc, zend_mm_mem_mmap_realloc, zend_mm_mem_mmap_free}  </span></li>
<li><span> </span></li>
<li class="alt"><span>/* 使用HeapAlloc分配内存 windows版本 关于这点，注释中写的是VirtualAlloc() to allocate memory，实际在程序中使用的是HeapAlloc*/  </span></li>
<li><span># define ZEND_MM_MEM_WIN32_DSC {"win32", zend_mm_mem_win32_init, zend_mm_mem_win32_dtor, zend_mm_mem_win32_compact, zend_mm_mem_win32_alloc, zend_mm_mem_win32_realloc, zend_mm_mem_win32_free}  </span></li>
<li class="alt"><span> </span></li>
<li><span>/* 使用malloc分配内存 默认为此种分配 如果有加ZEND_WIN32宏，则使用win32的分配方案*/  </span></li>
<li class="alt"><span># define ZEND_MM_MEM_MALLOC_DSC {"malloc", zend_mm_mem_dummy_init, zend_mm_mem_dummy_dtor, zend_mm_mem_dummy_compact, zend_mm_mem_malloc_alloc, zend_mm_mem_malloc_realloc, zend_mm_mem_malloc_free}  </span></li>
<li><span> </span></li>
<li class="alt"><span>static const zend_mm_mem_handlers mem_handlers[] = {  </span></li>
<li><span>#ifdef HAVE_MEM_WIN32  </span></li>
<li class="alt"><span>    ZEND_MM_MEM_WIN32_DSC,  </span></li>
<li><span>#endif  </span></li>
<li class="alt"><span>#ifdef HAVE_MEM_MALLOC  </span></li>
<li><span>    ZEND_MM_MEM_MALLOC_DSC,  </span></li>
<li class="alt"><span>#endif  </span></li>
<li><span>#ifdef HAVE_MEM_MMAP_ANON  </span></li>
<li class="alt"><span>    ZEND_MM_MEM_MMAP_ANON_DSC,  </span></li>
<li><span>#endif  </span></li>
<li class="alt"><span>#ifdef HAVE_MEM_MMAP_ZERO  </span></li>
<li><span>    ZEND_MM_MEM_MMAP_ZERO_DSC,  </span></li>
<li class="alt"><span>#endif  </span></li>
<li><span>    {NULL, NULL, NULL, NULL, NULL, NULL}  </span></li>
<li class="alt"><span>};  </span></li>
</ol>

Copy after login