Python源码阅读-内存管理机制(二),python源码,未经作者许可,禁止转载!
Python源码阅读-内存管理机制(二),python源码,未经作者许可,禁止转载!
本文作者: 编橙之家 - wklken 。未经作者许可,禁止转载!欢迎加入编橙之家 专栏作者。
Python 的内存分配策略
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
#define ARENA_SIZE (256 << 10) /* 256KB */
arena 结构
一个完整的arena = arena_object + pool集合
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
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
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
/* 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
/* 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
/* 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
// 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,
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
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
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
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
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
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
#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
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:
#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
)
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
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
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
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…..
后面还有内存相关的就是垃圾收集了, 后面再说了吧
wklken
2015-08-29
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