PHP的記憶體管理器是分層(hierarchical)的。這個管理器共有三層:儲存層(storage)、堆(heap)層和 emalloc/efree 層。儲存層透過 malloc()、mmap() 等函數向系統真正的申請內存,並透過free()函數釋放所申請的內存。
儲存層通常申請的記憶體區塊都比較大,這裡申請的記憶體大並不是指storage層結構所需的記憶體大,只是堆疊透過呼叫儲存層的分配方法時,其以段的格式申請的記憶體比較大,儲存層的作用是將記憶體分配的方式對堆層透明化。
首先看storage層的結構:
<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>
記憶體的分配方式,呼叫的函數是_zend_mm_storage結構中的處理函數集,而記憶體是以段的形式表現的。
4種記憶體方案
PHP在儲存層共有4種記憶體分配方案: malloc,win32,mmap_anon,mmap_zero。預設使用malloc分配內存,如果設定了ZEND_WIN32宏,則為windows版本,調用HeapAlloc分配內存,剩下兩種內存方案為匿名內存映射,並且PHP的內存方案可以通過設置變量來修改。
官方說明如下:
The Zend MM can be tweaked using ZEND_MM_MEM_TYPE and ZEND_MM_SEG_SIZE environment variables. Default values are “getpunues are “ ”, “mmap_zero” and “win32″ storage managers.
在程式碼中,對於這4種記憶體分配方案,分別對應實現了zend_mm_mem_handlers中的各個處理函數。配合程式碼的簡單說明如下:
<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>
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