Python内存管理解析
Python的内存管理架构
基本分层
在Objects/obmalloc.c源码中, 给了一个分层划分
_____ ______ ______ ________
[ int ] [ dict ] [ list ] ... [ string ] Python core |
+3 | <----- Object-specific memory -----> | <-- Non-object memory --> |
_______________________________ | |
[ Python's object allocator ] | |
+2 | ####### Object memory ####### | <------ Internal buffers ------> |
______________________________________________________________ |
[ Python's raw memory allocator (PyMem_ API) ] |
+1 | <----- Python memory (under PyMem manager's control) ------> | |
__________________________________________________________________
[ Underlying general-purpose allocator (ex: C library malloc) ]
0 | <------ Virtual memory allocated for the python process -------> |
=========================================================================
_______________________________________________________________________
[ OS-specific Virtual Memory Manager (VMM) ]
-1 | <--- Kernel dynamic storage allocation & management (page-based) ---> |
__________________________________ __________________________________
[ ] [ ]
-2 | <-- Physical memory: ROM/RAM --> | | <-- Secondary storage (swap) --> |可以看到
layer 3: Object-specific memory(int/dict/list/string....)
Python 实现并维护
更高抽象层次的内存管理策略, 主要是各类特定对象的缓冲池机制. 具体见前面几篇涉及的内存分配机制
layer 2: Python's object allocator
Python 实现并维护
实现了创建/销毁Python对象的接口(PyObject_New/Del), 涉及对象参数/引用计数等
layer 1: Python's raw memory allocator (PyMem_ API)
Python 实现并维护, 包装了第0层的内存管理接口, 提供统一的raw memory管理接口
封装的原因: 不同操作系统 C 行为不一定一致, 保证可移植性, 相同语义相同行为
layer 0: Underlying general-purpose allocator (ex: C library malloc)
操作系统提供的内存管理接口, 由操作系统实现并管理, Python不能干涉这一层的行为
第三层layer 3前面已经介绍过了, 几乎每种常用的数据类型都伴有一套缓冲池机制.
在这里, 我们关注的是layer 2/1
简要介绍下layer 1, 然后重点关注layer
2, 这才是重点！！！主要就这两层是python的主要操作。
layer 1: PyMem_ API
PyMem_ API是对操作系统内存管理接口进行的封装
查看pymem.h可以看到
// Raw memory interface
// 这里存在三个宏定义, 宏可以避免一次函数调用的开销, 提高运行效率
// 不允许非配空间大小为0的内存空间
#define PyMem_MALLOC(n) ((size_t)(n) > (size_t)PY_SSIZE_T_MAX ? NULL
: malloc((n) ? (n) : 1))
#define PyMem_REALLOC(p, n) ((size_t)(n) > (size_t)PY_SSIZE_T_MAX ? NULL
: realloc((p), (n) ? (n) : 1))
#define PyMem_FREE free
// 这里做了三个函数的声明, 平台独立的 malloc/realloc/free
PyAPI_FUNC(void *) PyMem_Malloc(size_t);
PyAPI_FUNC(void *) PyMem_Realloc(void *, size_t);
PyAPI_FUNC(void) PyMem_Free(void *);
// ============================================================
// Type-oriented memory interface
// 这里还有三个类型相关的内存接口, 批量分配/重分配 n 个 类型为 type内存
#define PyMem_New(type, n)
( ((size_t)(n) > PY_SSIZE_T_MAX / sizeof(type)) ? NULL :
( (type *) PyMem_Malloc((n) * sizeof(type)) ) )
#define PyMem_NEW(type, n)
( ((size_t)(n) > PY_SSIZE_T_MAX / sizeof(type)) ? NULL :
( (type *) PyMem_MALLOC((n) * sizeof(type)) ) )
#define PyMem_Resize(p, type, n)
( (p) = ((size_t)(n) > PY_SSIZE_T_MAX / sizeof(type)) ? NULL :
(type *) PyMem_Realloc((p), (n) * sizeof(type)) )
#define PyMem_RESIZE(p, type, n)
( (p) = ((size_t)(n) > PY_SSIZE_T_MAX / sizeof(type)) ? NULL :
(type *) PyMem_REALLOC((p), (n) * sizeof(type)) )object.c中,
我们关注实现,
三个实现的函数调用了对应的宏
// 使用 C 写Python扩展模块时使用函数而不是对应的宏
void *
PyMem_Malloc(size_t nbytes)
{
return PyMem_MALLOC(nbytes);
}
void *
PyMem_Realloc(void *p, size_t nbytes)
{
return PyMem_REALLOC(p, nbytes);
}
void
PyMem_Free(void *p)
{
PyMem_FREE(p);
}这些接口都相对简单
好了, 结束, 开始关注layer 2: Python's object allocator
Python 的内存分配策略
先来看Objects/obmalloc.c中的一段注释
/*
* "Memory management is where the rubber meets the road -- if we do the wrong
* thing at any level, the results will not be good. And if we don't make the
* levels work well together, we are in serious trouble." (1)
*
* (1) Paul R. Wilson, Mark S. Johnstone, Michael Neely, and David Boles,
* "Dynamic Storage Allocation: A Survey and Critical Review",
* in Proc. 1995 Int'l. Workshop on Memory Management, September 1995.
*/Python引入了内存池机制, 用于管理对小块内存的申请和释放
逻辑
1. 如果要分配的内存空间大于 SMALL_REQUEST_THRESHOLD bytes(512 bytes), 将直接使用layer 1的内存分配接口进行分配
2. 否则, 使用不同的block来满足分配需求整个小块内存池可以视为一个层次结构
1 2 3 4 | 1. 内存池(概念上的, 标识Python对于整个小块内存分配和释放的内存管理机制) 2. arena 3. pool 4. block |
block
Python内存的最小单位, 所有block长度都是8字节对齐的
注意这里block只是一个概念, 在源代码中并没有实体存在.
不同类型block, 对应不同内存大小, 这个内存大小的值被称为size class.
不同长度的block
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*
Request
in
bytes
Size
of
allocated
block Size
class
idx
*
----------------------------------------------------------------
* 1-8
8
0
* 9-16
16
1
*
17-24
24
2
*
25-32
32
3
*
33-40
40
4
*
41-48
48
5
*
49-56
56
6
*
57-64
64
7
*
65-72
72
8
* ...
...
...
* 497-504
504 62
* 505-512
512 63
*
* 0,
SMALL_REQUEST_THRESHOLD
+
1
and
up:
routed
to
the
underlying
* allocator.
*/
|
例如
1 | 申请一块大小28字节的内存, 实际从内存中划到32字节的一个block (从size class index为3的pool里面划出) |
图示:
注意: 这里有个Size class idx, 这个主要为了后面pool中用到
size class和size class index之间的转换
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#define ALIGNMENT
8 /* must be 2^N */
#define ALIGNMENT_SHIFT
3
#define ALIGNMENT_MASK (ALIGNMENT
- 1)
//
size class index => size class
#define INDEX2SIZE(I) (((uint)(I)
+ 1) size class index
size
=
(uint)(nbytes
-
1)
>>
ALIGNMENT_SHIFT;
/*
即
(8
- 1) / 8 = 0
(16
- 8) / 8 = 1
*/
|
pool
pool管理block, 一个pool管理着一堆有固定大小的内存块
本质: pool管理着一大块内存, 它有一定的策略, 将这块大的内存划分为多个大小一致的小块内存.
pool size
在Python中, 一个pool的大小通常为一个系统内存页. 4kB
1 2 3 4 5 6 7 | obmalloc.c #define SYSTEM_PAGE_SIZE (4 * 1024) #define SYSTEM_PAGE_SIZE_MASK (SYSTEM_PAGE_SIZE - 1) #define POOL_SIZE SYSTEM_PAGE_SIZE /* must be 2^N */ #define POOL_SIZE_MASK SYSTEM_PAGE_SIZE_MASK |
pool组成
pool的4kB内存 = pool_header + block集合(N多大小一样的block)
pool_header
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/*
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 */
-
size
class
index
uint
nextoffset; /*
bytes to virgin block */
uint
maxnextoffset;
/*
largest valid nextoffset */
};
|
pool_header的作用
1 2 3 4 | 1. 与其他pool链接, 组成双向链表 2. 维护pool中可用的block, 单链表 3. 保存 szidx , 这个和该pool中block的大小有关系, (block size=8, szidx=0), (block size=16, szidx=1)...用于内存分配时匹配到拥有对应大小block的pool 4. arenaindex, 后面说 |
结构图:
pool初始化
从内存中初始化一个全新的空的pool
Objects/obmalloc.c的
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void
*
PyObject_Malloc(size_t
nbytes)
{
...
init_pool:
//
1. 连接到 used_pools 双向链表, 作为表头
//
注意, 这里 usedpools[0] 保存着 block size = 8 的所有used_pools的表头
/*
Frontlink to used pools. */
next
=
usedpools[size
+
size];
/*
== prev */
pool->nextpool
=
next;
pool->prevpool
=
next;
next->nextpool
=
pool;
next->prevpool
=
pool;
pool->ref.count
=
1;
//
如果已经初始化过了...这里看初始化, 跳过
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.
*/
//
开始初始化pool_header
//
这里 size = (uint)(nbytes - 1) >> ALIGNMENT_SHIFT; 其实是Size class idx, 即szidx
pool->szidx
=
size;
//
计算获得每个block的size
size
=
INDEX2SIZE(size);
//
注意 #define POOL_OVERHEAD ROUNDUP(sizeof(struct pool_header))
//
bp => 初始化为pool + pool_header size, 跳过pool_header的内存
bp
=
(block
*)pool
+
POOL_OVERHEAD;
//
计算偏移量, 这里的偏移量是绝对值
//
#define POOL_SIZE SYSTEM_PAGE_SIZE /* must be 2^N */
//
POOL_SIZE = 4kb, POOL_OVERHEAD = pool_header size
//
下一个偏移位置: pool_header size + 2 * size
pool->nextoffset
=
POOL_OVERHEAD
+
(size
maxnextoffset
=
POOL_SIZE
-
size;
//
freeblock指向 bp + size = pool_header size + size
pool->freeblock
=
bp
+
size;
//
赋值NULL
*(block
**)(pool->freeblock)
=
NULL;
UNLOCK();
return
(void
*)bp;
}
|
初始化后的图
pool进行block分配 – 0 总体代码
总体分配的代码如下
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 | 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; // 指针指向空闲block起始位置 assert(bp != NULL); // 代码-1 // 调整 pool->freeblock (假设A节点)指向链表下一个, 即bp首字节指向的下一个节点(假设B节点) , 如果此时!= NULL // 表示 A节点可用, 直接返回 if ((pool->freeblock = *(block **)bp) != NULL) { UNLOCK(); return (void *)bp; } // 代码-2 /* * Reached the end of the free list, try to extend it. */ // 有足够的空间, 分配一个, pool->freeblock 指向后移 if (pool->nextoffset maxnextoffset) { /* There is room for another block. */ // 变更位置信息 pool->freeblock = (block*)pool + pool->nextoffset; pool->nextoffset += INDEX2SIZE(size); *(block **)(pool->freeblock) = NULL; // 注意, 指向NULL UNLOCK(); // 返回bp return (void *)bp; } // 代码-3 /* Pool is full, unlink from used pools. */ // 满了, 需要从下一个pool获取 next = pool->nextpool; pool = pool->prevpool; next->prevpool = pool; pool->nextpool = next; UNLOCK(); return (void *)bp; } |
pool进行block分配 – 1 刚开始
内存块尚未分配完, 且此时不存在回收的block, 全新进来的时候, 分配第一块block
|
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|
(pool->freeblock
=
*(block
**)bp)
==
NULL
|
所以进入的逻辑是代码-2
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 | bp = pool->freeblock; // 指针指向空闲block起始位置 ..... // 代码-2 /* * Reached the end of the free list, try to extend it. */ // 有足够的空间, 分配一个, pool->freeblock 指向后移 if (pool->nextoffset maxnextoffset) { /* There is room for another block. */ // 变更位置信息 pool->freeblock = (block*)pool + pool->nextoffset; pool->nextoffset += INDEX2SIZE(size); *(block **)(pool->freeblock) = NULL; // 注意, 指向NULL UNLOCK(); // 返回bp return (void *)bp; } |
结果图示
pool进行block分配 – 2 回收了某几个block
回收涉及的代码
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void
PyObject_Free(void
*p)
{
poolp
pool;
block
*lastfree;
poolp
next,
prev;
uint
size;
pool
=
POOL_ADDR(p);
if
(Py_ADDRESS_IN_RANGE(p,
pool))
{
/*
We allocated this address. */
LOCK();
/*
Link p to the start of the pool's freeblock list. Since
* the pool had at least the p block outstanding, the pool
* wasn't empty (so it's already in a usedpools[] list, or
* was full and is in no list -- it's not in the freeblocks
* list in any case).
*/
assert(pool->ref.count
>
0); /*
else it was empty */
//
p被释放, p的第一个字节值被设置为当前freeblock的值
*(block
**)p
=
lastfree
=
pool->freeblock;
//
freeblock被更新为指向p的首地址
pool->freeblock
=
(block
*)p;
//
相当于往list中头插入了一个节点
...
}
}
|
没释放一个block, 该block就会变成 pool->freeblock 的头节点, 而单链表一个节点如何指向下一个节点呢? 通过赋值, 节点内存空间保存着下个节点的地址, 最后一个节点指向NULL(知道上面代码-1的判断条件了吧>_
假设已经连续分配了5块, 第1块和第4块被释放
此时内存图示
此时再一个block分配调用进来, 执行分配, 进入的逻辑是代码-1
1 2 3 4 5 6 7 8 | bp = pool->freeblock; // 指针指向空闲block起始位置 // 代码-1 // 调整 pool->freeblock (假设A节点)指向链表下一个, 即bp首字节指向的下一个节点(假设B节点) , 如果此时!= NULL // 表示 A节点可用, 直接返回 if ((pool->freeblock = *(block **)bp) != NULL) { UNLOCK(); return (void *)bp; } |
pool进行block分配 – 3 pool用完了
pool中内存空间都用完了, 进入代码-3
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bp
=
pool->freeblock;
//
指针指向空闲block起始位置
//
代码-3
/*
Pool is full, unlink from used pools. */ //
满了, 需要从下一个pool获取
next
=
pool->nextpool;
pool
=
pool->prevpool;
next->prevpool
=
pool;
pool->nextpool
=
next;
UNLOCK();
return
(void
*)bp;
|
获取下一个pool(链表上每个pool的block size都是一致的)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 | void * PyObject_Malloc(size_t nbytes) { ... init_pool: // 1. 连接到 used_pools 双向链表, 作为表头 // 注意, 这里 usedpools[0] 保存着 block size = 8 的所有used_pools的表头 /* Frontlink to used pools. */ next = usedpools[size + size]; /* == prev */ pool->nextpool = next; pool->prevpool = next; next->nextpool = pool; next->prevpool = pool; pool->ref.count = 1; // 如果已经初始化过了...这里看初始化, 跳过 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. */ // 开始初始化pool_header // 这里 size = (uint)(nbytes - 1) >> ALIGNMENT_SHIFT; 其实是Size class idx, 即szidx pool->szidx = size; // 计算获得每个block的size size = INDEX2SIZE(size); // 注意 #define POOL_OVERHEAD ROUNDUP(sizeof(struct pool_header)) // bp => 初始化为pool + pool_header size, 跳过pool_header的内存 bp = (block *)pool + POOL_OVERHEAD; // 计算偏移量, 这里的偏移量是绝对值 // #define POOL_SIZE SYSTEM_PAGE_SIZE /* must be 2^N */ // POOL_SIZE = 4kb, POOL_OVERHEAD = pool_header size // 下一个偏移位置: pool_header size + 2 * size pool->nextoffset = POOL_OVERHEAD + (size maxnextoffset = POOL_SIZE - size; // freeblock指向 bp + size = pool_header size + size pool->freeblock = bp + size; // 赋值NULL *(block **)(pool->freeblock) = NULL; UNLOCK(); return (void *)bp; } |
初始化后的图
pool进行block分配 – 0 总体代码
总体分配的代码如下
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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;
//
指针指向空闲block起始位置
assert(bp
!=
NULL);
//
代码-1
//
调整 pool->freeblock (假设A节点)指向链表下一个, 即bp首字节指向的下一个节点(假设B节点) , 如果此时!= NULL
//
表示 A节点可用, 直接返回
if
((pool->freeblock
=
*(block
**)bp)
!=
NULL)
{
UNLOCK();
return
(void
*)bp;
}
//
代码-2
/*
* Reached the end of the free list, try to extend it.
*/
//
有足够的空间, 分配一个, pool->freeblock 指向后移
if
(pool->nextoffset
maxnextoffset)
{
/*
There is room for another block. */
//
变更位置信息
pool->freeblock
=
(block*)pool
+
pool->nextoffset;
pool->nextoffset
+=
INDEX2SIZE(size);
*(block
**)(pool->freeblock)
=
NULL;
//
注意, 指向NULL
UNLOCK();
//
返回bp
return
(void
*)bp;
}
//
代码-3
/*
Pool is full, unlink from used pools. */ //
满了, 需要从下一个pool获取
next
=
pool->nextpool;
pool
=
pool->prevpool;
next->prevpool
=
pool;
pool->nextpool
=
next;
UNLOCK();
return
(void
*)bp;
}
|
pool进行block分配 – 1 刚开始
内存块尚未分配完, 且此时不存在回收的block, 全新进来的时候, 分配第一块block
1 | (pool->freeblock = *(block **)bp) == NULL |
所以进入的逻辑是代码-2
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bp
=
pool->freeblock;
//
指针指向空闲block起始位置
.....
//
代码-2
/*
* Reached the end of the free list, try to extend it.
*/
//
有足够的空间, 分配一个, pool->freeblock 指向后移
if
(pool->nextoffset
maxnextoffset)
{
/*
There is room for another block. */
//
变更位置信息
pool->freeblock
=
(block*)pool
+
pool->nextoffset;
pool->nextoffset
+=
INDEX2SIZE(size);
*(block
**)(pool->freeblock)
=
NULL;
//
注意, 指向NULL
UNLOCK();
//
返回bp
return
(void
*)bp;
}
|
结果图示
pool进行block分配 – 2 回收了某几个block
回收涉及的代码
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 | void PyObject_Free(void *p) { poolp pool; block *lastfree; poolp next, prev; uint size; pool = POOL_ADDR(p); if (Py_ADDRESS_IN_RANGE(p, pool)) { /* We allocated this address. */ LOCK(); /* Link p to the start of the pool's freeblock list. Since * the pool had at least the p block outstanding, the pool * wasn't empty (so it's already in a usedpools[] list, or * was full and is in no list -- it's not in the freeblocks * list in any case). */ assert(pool->ref.count > 0); /* else it was empty */ // p被释放, p的第一个字节值被设置为当前freeblock的值 *(block **)p = lastfree = pool->freeblock; // freeblock被更新为指向p的首地址 pool->freeblock = (block *)p; // 相当于往list中头插入了一个节点 ... } } |
没释放一个block, 该block就会变成 pool->freeblock 的头节点, 而单链表一个节点如何指向下一个节点呢? 通过赋值, 节点内存空间保存着下个节点的地址, 最后一个节点指向NULL(知道上面代码-1的判断条件了吧>_
假设已经连续分配了5块, 第1块和第4块被释放
此时内存图示
此时再一个block分配调用进来, 执行分配, 进入的逻辑是代码-1
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bp
=
pool->freeblock;
//
指针指向空闲block起始位置
//
代码-1
//
调整 pool->freeblock (假设A节点)指向链表下一个, 即bp首字节指向的下一个节点(假设B节点) , 如果此时!= NULL
//
表示 A节点可用, 直接返回
if
((pool->freeblock
=
*(block
**)bp)
!=
NULL)
{
UNLOCK();
return
(void
*)bp;
}
|
pool进行block分配 – 3 pool用完了
pool中内存空间都用完了, 进入代码-3
1 2 3 4 5 6 7 8 9 10 | bp = pool->freeblock; // 指针指向空闲block起始位置 // 代码-3 /* Pool is full, unlink from used pools. */ // 满了, 需要从下一个pool获取 next = pool->nextpool; pool = pool->prevpool; next->prevpool = pool; pool->nextpool = next; UNLOCK(); return (void *)bp; |
获取下一个pool(链表上每个pool的block size都是一致的)
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void
*
PyObject_Malloc(size_t
nbytes)
{
...
init_pool:
//
1. 连接到 used_pools 双向链表, 作为表头
//
注意, 这里 usedpools[0] 保存着 block size = 8 的所有used_pools的表头
/*
Frontlink to used pools. */
next
=
usedpools[size
+
size];
/*
== prev */
pool->nextpool
=
next;
pool->prevpool
=
next;
next->nextpool
=
pool;
next->prevpool
=
pool;
pool->ref.count
=
1;
//
如果已经初始化过了...这里看初始化, 跳过
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.
*/
//
开始初始化pool_header
//
这里 size = (uint)(nbytes - 1) >> ALIGNMENT_SHIFT; 其实是Size class idx, 即szidx
pool->szidx
=
size;
//
计算获得每个block的size
size
=
INDEX2SIZE(size);
//
注意 #define POOL_OVERHEAD ROUNDUP(sizeof(struct pool_header))
//
bp => 初始化为pool + pool_header size, 跳过pool_header的内存
bp
=
(block
*)pool
+
POOL_OVERHEAD;
//
计算偏移量, 这里的偏移量是绝对值
//
#define POOL_SIZE SYSTEM_PAGE_SIZE /* must be 2^N */
//
POOL_SIZE = 4kb, POOL_OVERHEAD = pool_header size
//
下一个偏移位置: pool_header size + 2 * size
pool->nextoffset
=
POOL_OVERHEAD
+
(size
maxnextoffset
=
POOL_SIZE
-
size;
//
freeblock指向 bp + size = pool_header size + size
pool->freeblock
=
bp
+
size;
//
赋值NULL
*(block
**)(pool->freeblock)
=
NULL;
UNLOCK();
return
(void
*)bp;
}
|
初始化后的图
pool进行block分配 – 0 总体代码
总体分配的代码如下
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 | 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; // 指针指向空闲block起始位置 assert(bp != NULL); // 代码-1 // 调整 pool->freeblock (假设A节点)指向链表下一个, 即bp首字节指向的下一个节点(假设B节点) , 如果此时!= NULL // 表示 A节点可用, 直接返回 if ((pool->freeblock = *(block **)bp) != NULL) { UNLOCK(); return (void *)bp; } // 代码-2 /* * Reached the end of the free list, try to extend it. */ // 有足够的空间, 分配一个, pool->freeblock 指向后移 if (pool->nextoffset maxnextoffset) { /* There is room for another block. */ // 变更位置信息 pool->freeblock = (block*)pool + pool->nextoffset; pool->nextoffset += INDEX2SIZE(size); *(block **)(pool->freeblock) = NULL; // 注意, 指向NULL UNLOCK(); // 返回bp return (void *)bp; } // 代码-3 /* Pool is full, unlink from used pools. */ // 满了, 需要从下一个pool获取 next = pool->nextpool; pool = pool->prevpool; next->prevpool = pool; pool->nextpool = next; UNLOCK(); return (void *)bp; } |
pool进行block分配 – 1 刚开始
内存块尚未分配完, 且此时不存在回收的block, 全新进来的时候, 分配第一块block
|
1
|
(pool->freeblock
=
*(block
**)bp)
==
NULL
|
所以进入的逻辑是代码-2
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 | bp = pool->freeblock; // 指针指向空闲block起始位置 ..... // 代码-2 /* * Reached the end of the free list, try to extend it. */ // 有足够的空间, 分配一个, pool->freeblock 指向后移 if (pool->nextoffset maxnextoffset) { /* There is room for another block. */ // 变更位置信息 pool->freeblock = (block*)pool + pool->nextoffset; pool->nextoffset += INDEX2SIZE(size); *(block **)(pool->freeblock) = NULL; // 注意, 指向NULL UNLOCK(); // 返回bp return (void *)bp; } |
结果图示
pool进行block分配 – 2 回收了某几个block
回收涉及的代码
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void
PyObject_Free(void
*p)
{
poolp
pool;
block
*lastfree;
poolp
next,
prev;
uint
size;
pool
=
POOL_ADDR(p);
if
(Py_ADDRESS_IN_RANGE(p,
pool))
{
/*
We allocated this address. */
LOCK();
/*
Link p to the start of the pool's freeblock list. Since
* the pool had at least the p block outstanding, the pool
* wasn't empty (so it's already in a usedpools[] list, or
* was full and is in no list -- it's not in the freeblocks
* list in any case).
*/
assert(pool->ref.count
>
0); /*
else it was empty */
//
p被释放, p的第一个字节值被设置为当前freeblock的值
*(block
**)p
=
lastfree
=
pool->freeblock;
//
freeblock被更新为指向p的首地址
pool->freeblock
=
(block
*)p;
//
相当于往list中头插入了一个节点
...
}
}
|
没释放一个block, 该block就会变成 pool->freeblock 的头节点, 而单链表一个节点如何指向下一个节点呢? 通过赋值, 节点内存空间保存着下个节点的地址, 最后一个节点指向NULL(知道上面代码-1的判断条件了吧>_
假设已经连续分配了5块, 第1块和第4块被释放
此时内存图示
此时再一个block分配调用进来, 执行分配, 进入的逻辑是代码-1
1 2 3 4 5 6 7 8 | bp = pool->freeblock; // 指针指向空闲block起始位置 // 代码-1 // 调整 pool->freeblock (假设A节点)指向链表下一个, 即bp首字节指向的下一个节点(假设B节点) , 如果此时!= NULL // 表示 A节点可用, 直接返回 if ((pool->freeblock = *(block **)bp) != NULL) { UNLOCK(); return (void *)bp; } |
pool进行block分配 – 3 pool用完了
pool中内存空间都用完了, 进入代码-3
|
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|
bp
=
pool->freeblock;
//
指针指向空闲block起始位置
//
代码-3
/*
Pool is full, unlink from used pools. */ //
满了, 需要从下一个pool获取
next
=
pool->nextpool;
pool
=
pool->prevpool;
next->prevpool
=
pool;
pool->nextpool
=
next;
UNLOCK();
return
(void
*)bp;
|
获取下一个pool(链表上每个pool的block size都是一致的)
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