Python源码阅读-内存管理机制（二）

arena

arena: 多个pool聚合的结果

arena size

pool的大小默认值位4KB

arena的大小默认值256KB, 能放置 256/4=64 个pool

obmalloc.c中代码

1	#define ARENA_SIZE              (256 << 10)     /* 256KB */

arena 结构

一个完整的arena = arena_object + pool集合

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typedef uchar block;   /* Record keeping for arenas. */ struct arena_object {     /* The address of the arena, as returned by malloc.  Note that 0      * will never be returned by a successful malloc, and is used      * here to mark an arena_object that doesn't correspond to an      * allocated arena.      */     uptr address;       /* Pool-aligned pointer to the next pool to be carved off. */     block* pool_address;       /* The number of available pools in the arena:  free pools + never-      * allocated pools.      */     uint nfreepools;       /* The total number of pools in the arena, whether or not available. */     uint ntotalpools;       /* Singly-linked list of available pools. */     // 单链表, 可用pool集合     struct pool_header* freepools;       /* Whenever this arena_object is not associated with an allocated      * arena, the nextarena member is used to link all unassociated      * arena_objects in the singly-linked `unused_arena_objects` list.      * The prevarena member is unused in this case.      *      * When this arena_object is associated with an allocated arena      * with at least one available pool, both members are used in the      * doubly-linked `usable_arenas` list, which is maintained in      * increasing order of `nfreepools` values.      *      * Else this arena_object is associated with an allocated arena      * all of whose pools are in use.  `nextarena` and `prevarena`      * are both meaningless in this case.      */     // arena链表     struct arena_object* nextarena;     struct arena_object* prevarena; };

arena_object的作用

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1. 与其他arena连接, 组成双向链表 2. 维护arena中可用的pool, 单链表 3. 其他信息

pool_header 与 arena_object

1 2	pool_header和管理的blocks内存是一块连续的内存 => pool_header被申请时, 其管理的block集合的内存一并被申请 arena_object和其管理的内存是分离的 => arena_object被申请时, 其管理的pool集合的内存没有被申请, 而是在某一时刻建立的联系

arena的两种状态

arena存在两种状态: 未使用(没有建立联系)/可用(建立了联系)

全局由两个链表维护着

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/* The head of the singly-linked, NULL-terminated list of available * arena_objects. */ // 单链表 static struct arena_object* unused_arena_objects = NULL;   /* The head of the doubly-linked, NULL-terminated at each end, list of * arena_objects associated with arenas that have pools available. */ // 双向链表 static struct arena_object* usable_arenas = NULL;

arena的初始化

首先, 来看下初始化相关的一些参数定义

代码obmalloc.c

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/* Array of objects used to track chunks of memory (arenas). */ // arena_object 数组 static struct arena_object* arenas = NULL;   /* Number of slots currently allocated in the `arenas` vector. */ // 当前arenas中管理的arena_object的个数, 初始化时=0 static uint maxarenas = 0;   /* How many arena_objects do we initially allocate? * 16 = can allocate 16 arenas = 16 * ARENA_SIZE = 4MB before growing the * `arenas` vector. */ // 初始化时申请的arena_object个数 #define INITIAL_ARENA_OBJECTS 16   /* Number of arenas allocated that haven't been free()'d. */ static size_t narenas_currently_allocated = 0;   /* The head of the singly-linked, NULL-terminated list of available * arena_objects. */ // 未使用状态arena的单链表 static struct arena_object* unused_arena_objects = NULL;   /* The head of the doubly-linked, NULL-terminated at each end, list of * arena_objects associated with arenas that have pools available. */ // 可用状态arena的双向链表 static struct arena_object* usable_arenas = NULL;

然后, 看下obmalloc.c中arena初始化的代码

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/* Allocate a new arena.  If we run out of memory, return NULL.  Else * allocate a new arena, and return the address of an arena_object * describing the new arena.  It's expected that the caller will set * `usable_arenas` to the return value. */ static struct arena_object* new_arena(void) {     struct arena_object* arenaobj;     uint excess;        /* number of bytes above pool alignment */     void *address;     int err;       // 判断是否需要扩充"未使用"的arena_object列表     if (unused_arena_objects == NULL) {         uint i;         uint numarenas;         size_t nbytes;           /* Double the number of arena objects on each allocation.          * Note that it's possible for `numarenas` to overflow.          */         // 确定需要申请的个数, 首次初始化, 16, 之后每次翻倍         numarenas = maxarenas ? maxarenas  1 : INITIAL_ARENA_OBJECTS;         if (numarenas  maxarenas)             return NULL;                /* overflow */  //溢出了           ....           nbytes = numarenas * sizeof(*arenas);         // 申请内存         arenaobj = (struct arena_object *)realloc(arenas, nbytes);         if (arenaobj == NULL)             return NULL;         arenas = arenaobj;           /* We might need to fix pointers that were copied.  However,          * new_arena only gets called when all the pages in the          * previous arenas are full.  Thus, there are *no* pointers          * into the old array. Thus, we don't have to worry about          * invalid pointers.  Just to be sure, some asserts:          */         assert(usable_arenas == NULL);         assert(unused_arena_objects == NULL);           // 初始化         /* Put the new arenas on the unused_arena_objects list. */         for (i = maxarenas; i  numarenas; ++i) {             arenas[i].address = 0;              /* mark as unassociated */             // 新申请的一律为0, 标识着这个arena处于"未使用"             arenas[i].nextarena = i  numarenas - 1 ?                                    &arenas[i+1] : NULL;         }           // 将其放入unused_arena_objects链表中         // unused_arena_objects 为新分配内存空间的开头         /* Update globals. */         unused_arena_objects = &arenas[maxarenas];           // 更新数量         maxarenas = numarenas;     }       /* Take the next available arena object off the head of the list. */     assert(unused_arena_objects != NULL);       // 从unused_arena_objects中, 获取一个未使用的object     arenaobj = unused_arena_objects;     unused_arena_objects = arenaobj->nextarena; // 更新链表       // 开始处理这个 arenaobject       assert(arenaobj->address == 0);     // 申请内存, 256KB, 内存地址赋值给arena的address. 这块内存可用 #ifdef ARENAS_USE_MMAP     address = mmap(NULL, ARENA_SIZE, PROT_READ|PROT_WRITE,                    MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);     err = (address == MAP_FAILED); #else     address = malloc(ARENA_SIZE);     err = (address == 0); #endif     if (err) {         /* The allocation failed: return NULL after putting the          * arenaobj back.          */         arenaobj->nextarena = unused_arena_objects;         unused_arena_objects = arenaobj;         return NULL;     }     arenaobj->address = (uptr)address;       ++narenas_currently_allocated;       // 设置pool集合相关信息     arenaobj->freepools = NULL;  // 设置为NULL, 只有在释放一个pool的时候才有用     /* pool_address  first pool-aligned address in the arena        nfreepools  number of whole pools that fit after alignment */     arenaobj->pool_address = (block*)arenaobj->address;     arenaobj->nfreepools = ARENA_SIZE / POOL_SIZE;       assert(POOL_SIZE * arenaobj->nfreepools == ARENA_SIZE);       // 将pool的起始地址调整为系统页的边界     // 申请到 256KB, 放弃了一些内存, 而将可使用的内存边界pool_address调整到了与系统页对齐     excess = (uint)(arenaobj->address & POOL_SIZE_MASK);     if (excess != 0) {         --arenaobj->nfreepools;         arenaobj->pool_address += POOL_SIZE - excess;     }     arenaobj->ntotalpools = arenaobj->nfreepools;       return arenaobj; }

图示: 初始化arenas数组, 初始化后的所有arena都在unused_arena_objects单链表里面

图示: 从arenas取一个arena进行初始化

没有可用的arena?

此时

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// 判断成立     if (unused_arena_objects == NULL) {         ....         // 确定需要申请的个数, 首次初始化, 16, 之后每次翻倍         numarenas = maxarenas ? maxarenas << 1 : INITIAL_ARENA_OBJECTS;

然后, 假设第一次分配了16个, 发现没有arena之后, 第二次处理结果: numarenas = 32

即, 数组扩大了一倍

arena分配

new了一个全新的 arena之后,

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void *   PyObject_Malloc(size_t nbytes)   {             // 刚开始没有可用的arena             if (usable_arenas == NULL) {               // new一个, 作为双向链表的表头               usable_arenas = new_arena();               if (usable_arenas == NULL) {                   UNLOCK();                   goto redirect;               }                 usable_arenas->nextarena =                   usable_arenas->prevarena = NULL;              }             .......             // 从arena中获取一个pool           pool = (poolp)usable_arenas->pool_address;           assert((block*)pool <= (block*)usable_arenas->address +                                  ARENA_SIZE - POOL_SIZE);           pool->arenaindex = usable_arenas - arenas;           assert(&arenas[pool->arenaindex] == usable_arenas);           pool->szidx = DUMMY_SIZE_IDX;             // 更新 pool_address 向下一个节点           usable_arenas->pool_address += POOL_SIZE;           // 可用节点数量-1           --usable_arenas->nfreepools;   }

图示: 从全新的arena中获取一个pool

假设arena是旧的, 怎么分配的pool

1 2	pool = usable_arenas->freepools;           if (pool != NULL) {

这个arena->freepools是何方神圣?

当arena中一整块pool被释放的时候

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void   PyObject_Free(void *p)   {                 struct arena_object* ao;               uint nf;  /* ao->nfreepools */                 /* Link the pool to freepools.  This is a singly-linked                * list, and pool->prevpool isn't used there.               */               ao = &arenas[pool->arenaindex];               pool->nextpool = ao->freepools;               ao->freepools = pool;               nf = ++ao->nfreepools;

也就是说, 在pool整块被释放的时候, 会将pool加入到arena->freepools作为单链表的表头, 然后, 在从非全新arena中分配pool时, 优先从arena->freepools里面取, 如果取不到, 再从arena内存块里面获取

图示

一个arena满了之后呢

很自然, 从下一个arena中获取

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void *   PyObject_Malloc(size_t nbytes)   {             // 当发现用完了最后一个pool!!!!!!!!!!!           // nfreepools = 0           if (usable_arenas->nfreepools == 0) {               assert(usable_arenas->nextarena == NULL ||                      usable_arenas->nextarena->prevarena ==                      usable_arenas);               /* Unlink the arena:  it is completely allocated. */                 // 找到下一个节点!               usable_arenas = usable_arenas->nextarena;               // 右下一个               if (usable_arenas != NULL) {                   usable_arenas->prevarena = NULL; // 更新下一个节点的prevarens                   assert(usable_arenas->address != 0);               }               // 没有下一个, 此时 usable_arenas = NULL, 下次进行内存分配的时候, 就会从arenas数组中取一个             }     }

注意: 这里有个逻辑, 就是每分配一个pool, 就检查是不是用到了最后一个, 如果是, 需要变更usable_arenas到下一个可用的节点, 如果没有可用的, 那么下次进行内存分配的时候, 会判定从arenas数组中取一个

arena回收

内存分配和回收最小单位是block, 当一个block被回收的时候, 可能触发pool被回收, pool被回收, 将会触发arena的回收机制

四种情况

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1. arena中所有pool都是闲置的(empty), 将arena内存释放, 返回给操作系统 2. 如果arena中之前所有的pool都是占用的(used), 现在释放了一个pool(empty), 需要将 arena加入到usable_arenas, 会加入链表表头 3. 如果arena中empty的pool个数n, 则从useable_arenas开始寻找可以插入的位置. 将arena插入. (useable_arenas是一个有序链表, 按empty pool的个数, 保证empty pool数量越多, 被使用的几率越小, 最终被整体释放的机会越大) 4. 其他情况, 不对arena 进行处理

具体可以看PyObject_Free的代码

内存分配步骤

好的, 到这里, 我们已经知道了block和pool的关系(包括pool怎么管理block的), 以及arena和pool的关系(怎么从arena中拉到可用的pool)

那么, 在分析PyObject_Malloc(size_t nbytes)如何进行内存分配的时候, 我们就刨除掉这些管理代码

关注: 如何寻找得到一块可用的nbytes的block内存

其实代码那么多, 寻址得到对应的block也就这么几行代码, 其他代码都是pool没有, 找arena, 申请arena, arena没有, 找arenas, 最终的到一块pool, 初始化, 返回第一个block

如果有的情况, 用现成的

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pool = usedpools[size + size]; if pool可用:     pool 没满, 取一个block返回     pool 满了, 从下一个pool取一个block返回 否则:     获取arena, 从里面初始化一个pool, 拿到第一个block, 返回

从上面这个判断逻辑来看, 内存分配其实主要操作的是pool, 跟arena并不是基本的操作单元(只是用来管理pool的)

结论: 进行内存分配和销毁, 所有操作都是在pool上进行的

usedpools 是什么鬼? 其实是可用pool缓冲池, 后面说

内存池

arena 内存池的大小

取决于用户, Python提供的编译符号, 用于决定是否控制

obmalloc.c

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#ifdef WITH_MEMORY_LIMITS #ifndef SMALL_MEMORY_LIMIT #define SMALL_MEMORY_LIMIT      (64 * 1024 * 1024)      /* 64 MB -- more? */ #endif #endif   #ifdef WITH_MEMORY_LIMITS #define MAX_ARENAS              (SMALL_MEMORY_LIMIT / ARENA_SIZE) #endif

具体使用中, python并不直接与arenas和arena打交道, 当Python申请内存时, 最基本的操作单元并不是arena, 而是pool

问题: pool中所有block的size一样, 但是在arena中, 每个pool的size都可能不一样, 那么最终这些pool是怎么维护的? 怎么根据大小找到需要的block所在的pool? => usedpools

pool在内存池中的三种状态

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1. used状态: pool中至少有一个block已经被使用, 并且至少有一个block未被使用. 这种状态的pool受控于Python内部维护的usedpool数组   2. full状态: pool中所有的block都已经被使用, 这种状态的pool在arena中, 但不在arena的freepools链表中 处于full的pool各自独立, 不会被链表维护起来   3. empty状态: pool中所有block都未被使用, 处于这个状态的pool的集合通过其pool_header中的nextpool构成一个链表, 链表的表头是arena_object中的freepools

usedpools

usedpools数组: 维护着所有处于used状态的pool, 当申请内存的时候, 会通过usedpools寻找到一块可用的(处于used状态的)pool, 从中分配一个block

结构:

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#define SMALL_REQUEST_THRESHOLD 512   // 512/8 = 64   #define NB_SMALL_SIZE_CLASSES   (SMALL_REQUEST_THRESHOLD / ALIGNMENT)     #define PTA(x)  ((poolp )((uchar *)&(usedpools[2*(x)]) - 2*sizeof(block *)))   #define PT(x)   PTA(x), PTA(x)     // 2 * ((64 + 7) / 8) * 8 = 128, 大小为128的数组   static poolp usedpools[2 * ((NB_SMALL_SIZE_CLASSES + 7) / 8) * 8] = {       PT(0), PT(1), PT(2), PT(3), PT(4), PT(5), PT(6), PT(7)   #if NB_SMALL_SIZE_CLASSES > 8       , PT(8), PT(9), PT(10), PT(11), PT(12), PT(13), PT(14), PT(15)   #if NB_SMALL_SIZE_CLASSES > 16       , PT(16), PT(17), PT(18), PT(19), PT(20), PT(21), PT(22), PT(23)   #if NB_SMALL_SIZE_CLASSES > 24       , PT(24), PT(25), PT(26), PT(27), PT(28), PT(29), PT(30), PT(31)   #if NB_SMALL_SIZE_CLASSES > 32       , PT(32), PT(33), PT(34), PT(35), PT(36), PT(37), PT(38), PT(39)   #if NB_SMALL_SIZE_CLASSES > 40       , PT(40), PT(41), PT(42), PT(43), PT(44), PT(45), PT(46), PT(47)   #if NB_SMALL_SIZE_CLASSES > 48       , PT(48), PT(49), PT(50), PT(51), PT(52), PT(53), PT(54), PT(55)   #if NB_SMALL_SIZE_CLASSES > 56       , PT(56), PT(57), PT(58), PT(59), PT(60), PT(61), PT(62), PT(63)   #if NB_SMALL_SIZE_CLASSES > 64   #error "NB_SMALL_SIZE_CLASSES should be less than 64"   #endif /* NB_SMALL_SIZE_CLASSES > 64 */   #endif /* NB_SMALL_SIZE_CLASSES > 56 */   #endif /* NB_SMALL_SIZE_CLASSES > 48 */   #endif /* NB_SMALL_SIZE_CLASSES > 40 */   #endif /* NB_SMALL_SIZE_CLASSES > 32 */   #endif /* NB_SMALL_SIZE_CLASSES > 24 */   #endif /* NB_SMALL_SIZE_CLASSES > 16 */   #endif /* NB_SMALL_SIZE_CLASSES >  8 */   };     即     // 得到usedpools数组 static poolp usedpools[128] = {    PTA(0), PTA(0), PTA(1), PTA(1), PTA(2), PTA(2), PTA(3), PTA(3),    ....    PTA(63), PTA(63) }

解开看(obmalloc.c)

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typedef uchar block;     /* Pool for small blocks. */   struct pool_header {       union { block *_padding;               uint count; } ref;          /* number of allocated blocks    */       block *freeblock;                   /* pool's free list head         */       struct pool_header *nextpool;       /* next pool of this size class  */       struct pool_header *prevpool;       /* previous pool       ""        */       uint arenaindex;                    /* index into arenas of base adr */       uint szidx;                         /* block size class index        */       uint nextoffset;                    /* bytes to virgin block         */       uint maxnextoffset;                 /* largest valid nextoffset      */   };   typedef struct pool_header *poolp;   usedpools[0] = PTA(0) = ((poolp )((uchar *)

为了看懂这步的trick, 心好累>_

直接上图

new一个pool时维护

init获得的情况, 其实就是将刚刚从arena中获取的pool加入到 usedpools 对应的双向链表中, 然后初始化, 然后返回block

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init_pool:               /* Frontlink to used pools. */                 // 1. 获取得到usedpools链表头               next = usedpools[size + size]; /* == prev */                 // 2. 将新的pool加入到双向链表               pool->nextpool = next;               pool->prevpool = next;               next->nextpool = pool;               next->prevpool = pool;               pool->ref.count = 1;                 // 3. 后面的是具体pool和block的了               if (pool->szidx == size) {                   /* Luckily, this pool last contained blocks                    * of the same size class, so its header                    * and free list are already initialized.                    */                   bp = pool->freeblock;                   pool->freeblock = *(block **)bp;                   UNLOCK();                   return (void *)bp;               }               /*                * Initialize the pool header, set up the free list to                * contain just the second block, and return the first                * block.                */               pool->szidx = size;               size = INDEX2SIZE(size);               bp = (block *)pool + POOL_OVERHEAD;               pool->nextoffset = POOL_OVERHEAD + (size maxnextoffset = POOL_SIZE - size;               pool->freeblock = bp + size;               *(block **)(pool->freeblock) = NULL;               UNLOCK();               return (void *)bp;   // here           }

从现有pool中获取block

从现有的pool, 其实就是 usedpools得到双向链表头部, 判断是不是空链表, 不是的话代表有可用的pool, 直接从里面获取

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if ((nbytes - 1) > ALIGNMENT_SHIFT;           pool = usedpools[size + size];             // 注意这里的判断, pool != pool-> nextpool 表示得到的链表不是空的           if (pool != pool->nextpool) {               /*                * There is a used pool for this size class.                * Pick up the head block of its free list.                */               ++pool->ref.count;               bp = pool->freeblock;               assert(bp != NULL);               if ((pool->freeblock = *(block **)bp) != NULL) {                   UNLOCK();                   return (void *)bp;               }               /*                * Reached the end of the free list, try to extend it.                */               if (pool->nextoffset maxnextoffset) {                   /* There is room for another block. */                   pool->freeblock = (block*)pool +                                     pool->nextoffset;                   pool->nextoffset += INDEX2SIZE(size);                   *(block **)(pool->freeblock) = NULL;                   UNLOCK();                   return (void *)bp;               }               /* Pool is full, unlink from used pools. */               next = pool->nextpool;               pool = pool->prevpool;               next->prevpool = pool;               pool->nextpool = next;               UNLOCK();               return (void *)bp;   // here           }

全局结构

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先这样吧, Python中整个内存池基本结构和机制大概如此, 是不是发现有好多数组/链表等等, 在分配/回收上处理下做成各种池…..