iOS-内存管理分析（下）
2021-09-22 本文已影响0人
似水流年_9ebe
前言
iOS-内存管理分析（上）以及iOS-内存管理分析（中）分析了内存的分区，布局，散列表，弱引用计数表等相关知识，这篇文章继续分析内存管理。
1 自动释放池初探
在我们的demo工程的main文件代码如下
#import <Foundation/Foundation.h>
int main(int argc, const char * argv[]) {
@autoreleasepool {
NSLog(@"Hello, World!");
}
return 0;
}
我们通过clang命令，看下底层，我们在main.cpp找到如下代码
int main(int argc, const char * argv[]) {
/* */ { __AtAutoreleasePool __autoreleasepool;
NSLog((NSString *)&__NSConstantStringImpl__var_folders_r7_rnm6hs1x2jg8bqy7sjwsskx00000gn_T_main_1cecfd_mi_0);
}
return 0;
}
@autoreleasepool被注释了，替换的是*__AtAutoreleasePool __autoreleasepool; *这个。
__AtAutoreleasePool是一个结构体，如下
struct __AtAutoreleasePool {
__AtAutoreleasePool() {atautoreleasepoolobj = objc_autoreleasePoolPush();}
~__AtAutoreleasePool() {objc_autoreleasePoolPop(atautoreleasepoolobj);}
void * atautoreleasepoolobj;
};
这里有一个构造和析构函数，构造函数创建一个objc_autoreleasePoolPush对象，释构函数调用objc_autoreleasePoolPop释入这个objc_autoreleasePoolPush对象。
当然我们还可以通过汇编分析流程，这里不再叙述。
2 自动释放池结构
objc_autoreleasePoolPush与objc_autoreleasePoolPop的底层到底是什么，做了什么操作，我们接下来分析下。
我们在objc(818)的源码中搜索下objc_autoreleasePoolPush这个函数，找到如下代码
void *
objc_autoreleasePoolPush(void)
{
return AutoreleasePoolPage::push();
}
调用了AutoreleasePoolPage它的push()函数，我们进去看下，如下
class AutoreleasePoolPage : private AutoreleasePoolPageData
AutoreleasePoolPage继承于AutoreleasePoolPageData。
Autorelease pool implementation
A thread's autorelease pool is a stack of pointers.
Each pointer is either an object to release, or POOL_BOUNDARY which is
an autorelease pool boundary.
A pool token is a pointer to the POOL_BOUNDARY for that pool. When
the pool is popped, every object hotter than the sentinel is released.
The stack is divided into a doubly-linked list of pages. Pages are added
and deleted as necessary.
Thread-local storage points to the hot page, where newly autoreleased
objects are stored.
自动释放池与线程有关联的。POOL_BOUNDARY就是哨兵对象边（自动释放池边界），不能无限制出栈，如果没有这个边界，不断的出栈，会出现访问野指针。
这里说了自动释放池是一个双向链表。
我们再来看下AutoreleasePoolPageData
struct AutoreleasePoolPageData
{
#if SUPPORT_AUTORELEASEPOOL_DEDUP_PTRS
struct AutoreleasePoolEntry {
uintptr_t ptr: 48;
uintptr_t count: 16;
static const uintptr_t maxCount = 65535; // 2^16 - 1
};
static_assert((AutoreleasePoolEntry){ .ptr = MACH_VM_MAX_ADDRESS }.ptr == MACH_VM_MAX_ADDRESS, "MACH_VM_MAX_ADDRESS doesn't fit into AutoreleasePoolEntry::ptr!");
#endif
magic_t const magic; // 16
__unsafe_unretained id *next; // 8
pthread_t const thread; // 8
AutoreleasePoolPage * const parent; // 8
AutoreleasePoolPage *child; // 8
uint32_t const depth; // 4
uint32_t hiwat; // 4
AutoreleasePoolPageData(__unsafe_unretained id* _next, pthread_t _thread, AutoreleasePoolPage* _parent, uint32_t _depth, uint32_t _hiwat)
: magic(), next(_next), thread(_thread),
parent(_parent), child(nil),
depth(_depth), hiwat(_hiwat)
{
}
};
- magic⽤来校验AutoreleasePoolPage的结构是否完整；
- next指向最新添加的autoreleased对象的下⼀个位置，初始化时指向begin()；
- thread指向当前线程；
- parent指向⽗结点，第⼀个结点的parent值为nil；
- child指向⼦结点，最后⼀个结点的child值为nil；
- depth代表深度，从0开始，往后递增1；
- hiwat代表highwatermark最⼤⼊栈数量标记
我们改下main.m源码，压栈一个对象，
#import <Foundation/Foundation.h>
extern void _objc_autoreleasePoolPrint(void);
int main(int argc, const char * argv[]) {
@autoreleasepool {
NSObject *objc = [[[NSObject alloc] init] autorelease];
_objc_autoreleasePoolPrint();
}
return 0;
}
这里一定要把arc关掉。
运行项目，如下
objc[27889]: AUTORELEASE POOLS for thread 0x1000ebe00
objc[27889]: 2 releases pending.
objc[27889]: [0x10200c000] ................ PAGE (hot) (cold)
objc[27889]: [0x10200c038] ################ POOL 0x10200c038
objc[27889]: [0x10200c040] 0x10060dfc0 NSObject
objc[27889]: ##############
Program ended with exit code: 0
这里可以看到NSObject对象是压入栈了。
这里有两个对象的原因，是因为哨兵对象也是一个对象。
3 自动释放池的创建和压栈
自动释放池的创建和压栈是如何进行的，我们分析它的源码看下。
我们看下AutoreleasePoolPage::push();的源码，如下
static inline void *push()
{
id *dest;
if (slowpath(DebugPoolAllocation)) {
// Each autorelease pool starts on a new pool page.
dest = autoreleaseNewPage(POOL_BOUNDARY);
} else {
dest = autoreleaseFast(POOL_BOUNDARY);
}
ASSERT(dest == EMPTY_POOL_PLACEHOLDER || *dest == POOL_BOUNDARY);
return dest;
}
我们再来看下autoreleaseFast这个函数，源码如下
static inline id *autoreleaseFast(id obj)
{
AutoreleasePoolPage *page = hotPage();
if (page && !page->full()) {
return page->add(obj);
} else if (page) {
return autoreleaseFullPage(obj, page);
} else {
return autoreleaseNoPage(obj);
}
}
这里判断页面是否存在并且没有满，直接add进去。
如果满了调用autoreleaseFullPage处理，如果没有这个页面，调用autoreleaseNoPage创建。
我们再来看下autoreleaseNoPage，如下
static __attribute__((noinline))
id *autoreleaseNoPage(id obj)
{
// "No page" could mean no pool has been pushed
// or an empty placeholder pool has been pushed and has no contents yet
ASSERT(!hotPage());
bool pushExtraBoundary = false;
if (haveEmptyPoolPlaceholder()) {
// We are pushing a second pool over the empty placeholder pool
// or pushing the first object into the empty placeholder pool.
// Before doing that, push a pool boundary on behalf of the pool
// that is currently represented by the empty placeholder.
pushExtraBoundary = true;
}
else if (obj != POOL_BOUNDARY && DebugMissingPools) {
// We are pushing an object with no pool in place,
// and no-pool debugging was requested by environment.
_objc_inform("MISSING POOLS: (%p) Object %p of class %s "
"autoreleased with no pool in place - "
"just leaking - break on "
"objc_autoreleaseNoPool() to debug",
objc_thread_self(), (void*)obj, object_getClassName(obj));
objc_autoreleaseNoPool(obj);
return nil;
}
else if (obj == POOL_BOUNDARY && !DebugPoolAllocation) {
// We are pushing a pool with no pool in place,
// and alloc-per-pool debugging was not requested.
// Install and return the empty pool placeholder.
return setEmptyPoolPlaceholder();
}
// We are pushing an object or a non-placeholder'd pool.
// Install the first page.
AutoreleasePoolPage *page = new AutoreleasePoolPage(nil);
setHotPage(page);
// Push a boundary on behalf of the previously-placeholder'd pool.
if (pushExtraBoundary) {
page->add(POOL_BOUNDARY);
}
// Push the requested object or pool.
return page->add(obj);
}
在这里
AutoreleasePoolPage *page = new AutoreleasePoolPage(nil);
setHotPage(page);
创建新页面，并设置成hot页面，之后再add对象
我们再看下AutoreleasePoolPage
AutoreleasePoolPage(AutoreleasePoolPage *newParent) :
AutoreleasePoolPageData(begin(),
objc_thread_self(),
newParent,
newParent ? 1+newParent->depth : 0,
newParent ? newParent->hiwat : 0)
{
if (objc::PageCountWarning != -1) {
checkTooMuchAutorelease();
}
if (parent) {
parent->check();
ASSERT(!parent->child);
parent->unprotect();
parent->child = this;
parent->protect();
}
protect();
}
这里设置双向链表的数据。
我们看下的begin如下
id * begin() {
return (id *) ((uint8_t *)this+sizeof(*this));
}
sizeof(*this)这时56的大小，来源于autorealeaPoolDat成员变量
magic_t const magic;//16
__unsafe_unretained id *next;//8
pthread_t const thread; //8
AutoreleasePoolPage * const parent; //8
AutoreleasePoolPage *child; //8
uint32_t const depth;//4
uint32_t hiwat;//4
magic_t是结构体，有一个uint32_4 m[4]占用16字节。
objc_thread_self(),获取当前线程。
自动释放池的成员变量也是占用空间的。
POOL 0x10080c038上方的这个是我们的哨兵对象。
4 自动释放池的满页临界和压栈出栈
当我们的页面满的时候，是如何处理的，结构又有什么变化呢，我们继续分析下源码才能得到。
我们知道内存是分页的加载的，为什么这么做呢？
假如不分页的话会出现以下情况
- 不断的压栈出栈，会对这张表操作非常频繁，也就是说对内存操作频繁
- 压栈出栈的时候，会对整张表进行加锁解锁，非常消耗性能
- 所有的对象存在这张表，非常复杂，管理复杂
- 如果某个局部发生问题，整张表都会影响
而分页的方式就不会有这些问题
内存分页的好处 - 内存可以不连续
- 性能大大的提高
我们一张表可以存放多少内存， 我们探索一下。
我们在改下我们的main.m文件
#import <Foundation/Foundation.h>
extern void _objc_autoreleasePoolPrint(void);
int main(int argc, const char * argv[]) {
@autoreleasepool {
for (int i = 0; i<4; i++) {
NSObject *objc = [[[NSObject alloc] init] autorelease];
}
_objc_autoreleasePoolPrint();
}
return 0;
}
我们运行项目，看下效果，如下所示
objc[29930]: AUTORELEASE POOLS for thread 0x1000ebe00
objc[29930]: 5 releases pending.
objc[29930]: [0x101009000] ................ PAGE (hot) (cold)
objc[29930]: [0x101009038] ################ POOL 0x101009038
objc[29930]: [0x101009040] 0x101a4b020 NSObject
objc[29930]: [0x101009048] 0x101a3f2d0 NSObject
objc[29930]: [0x101009050] 0x101a3ebf0 NSObject
objc[29930]: [0x101009058] 0x101a3dac0 NSObject
objc[29930]: ##############
KCObjcBuild was compiled with optimization - stepping may behave oddly; variables may not be available.
我们可以看到没有产生分页的效果，我们再把循环改下505，再看下效果
objc[29984]: AUTORELEASE POOLS for thread 0x1000ebe00
objc[29984]: 506 releases pending.
objc[29984]: [0x10480d000] ................ PAGE (full) (cold)
objc[29984]: [0x10480d038] ################ POOL 0x10480d038
objc[29984]: [0x10480d040] 0x10132a9f0 NSObject
objc[29984]: [0x10480d048] 0x101328f20 NSObject
objc[29984]: [0x10480d050] 0x101328b70 NSObject
objc[29984]: [0x10480d058] 0x101328bd0 NSObject
objc[29984]: [0x10480d060] 0x101327b10 NSObject
objc[29984]: [0x10480d068] 0x101327b50 NSObject
objc[29984]: [0x10480d070] 0x101327100 NSObject
objc[29984]: [0x10480d078] 0x101325fd0 NSObject
objc[29984]: [0x10480d080] 0x101326010 NSObject
objc[29984]: [0x10480d088] 0x101326040 NSObject
objc[29984]: [0x10480d090] 0x101325ae0 NSObject
objc[29984]: [0x10480d098] 0x101324fc0 NSObject
objc[29984]: [0x10480d0a0] 0x1013249a0 NSObject
objc[29984]: [0x10480d0a8] 0x101324500 NSObject
objc[29984]: [0x10480d0b0] 0x101323ff0 NSObject
objc[29984]: [0x10480d0b8] 0x101323290 NSObject
objc[29984]: [0x10480d0c0] 0x101322d90 NSObject
objc[29984]: [0x10480d0c8] 0x101322630 NSObject
objc[29984]: [0x10480d0d0] 0x101322660 NSObject
objc[29984]: [0x10480d0d8] 0x10130f190 NSObject
objc[29984]: [0x10480d0e0] 0x10130f1c0 NSObject
objc[29984]: [0x10480d0e8] 0x10130e510 NSObject
objc[29984]: [0x10480d0f0] 0x10130e7d0 NSObject
objc[29984]: [0x10480d0f8] 0x10130d7b0 NSObject
objc[29984]: [0x10480d100] 0x10130d080 NSObject
objc[29984]: [0x10480d108] 0x10130d140 NSObject
objc[29984]: [0x10480d110] 0x10130be70 NSObject
objc[29984]: [0x10480d118] 0x10130ade0 NSObject
objc[29984]: [0x10480d120] 0x10130a1b0 NSObject
objc[29984]: [0x10480d128] 0x101305530 NSObject
objc[29984]: [0x10480d130] 0x101308ca0 NSObject
objc[29984]: [0x10480d138] 0x101308e60 NSObject
objc[29984]: [0x10480d140] 0x101304900 NSObject
objc[29984]: [0x10480d148] 0x1013387a0 NSObject
objc[29984]: [0x10480d150] 0x1013387b0 NSObject
objc[29984]: [0x10480d158] 0x1013387c0 NSObject
objc[29984]: [0x10480d160] 0x1013387d0 NSObject
objc[29984]: [0x10480d168] 0x1013387e0 NSObject
objc[29984]: [0x10480d170] 0x1013387f0 NSObject
objc[29984]: [0x10480d178] 0x101338800 NSObject
objc[29984]: [0x10480d180] 0x101338810 NSObject
objc[29984]: [0x10480d188] 0x101338820 NSObject
objc[29984]: [0x10480d190] 0x101338830 NSObject
objc[29984]: [0x10480d198] 0x101338840 NSObject
objc[29984]: [0x10480d1a0] 0x101338850 NSObject
objc[29984]: [0x10480d1a8] 0x101338860 NSObject
objc[29984]: [0x10480d1b0] 0x101338870 NSObject
objc[29984]: [0x10480d1b8] 0x101338880 NSObject
objc[29984]: [0x10480d1c0] 0x101338890 NSObject
objc[29984]: [0x10480d1c8] 0x101338bb0 NSObject
objc[29984]: [0x10480d1d0] 0x101338bc0 NSObject
objc[29984]: [0x10480d1d8] 0x101338bd0 NSObject
objc[29984]: [0x10480d1e0] 0x101338be0 NSObject
objc[29984]: [0x10480d1e8] 0x101338bf0 NSObject
objc[29984]: [0x10480d1f0] 0x101338c00 NSObject
objc[29984]: [0x10480d1f8] 0x101338c10 NSObject
objc[29984]: [0x10480d200] 0x101338c20 NSObject
objc[29984]: [0x10480d208] 0x101338c30 NSObject
objc[29984]: [0x10480d210] 0x101338c40 NSObject
objc[29984]: [0x10480d218] 0x101338c50 NSObject
objc[29984]: [0x10480d220] 0x101338c60 NSObject
objc[29984]: [0x10480d228] 0x101338c70 NSObject
objc[29984]: [0x10480d230] 0x101338c80 NSObject
objc[29984]: [0x10480d238] 0x101338c90 NSObject
objc[29984]: [0x10480d240] 0x101338ca0 NSObject
objc[29984]: [0x10480d248] 0x101338cb0 NSObject
objc[29984]: [0x10480d250] 0x101338cc0 NSObject
objc[29984]: [0x10480d258] 0x101338cd0 NSObject
objc[29984]: [0x10480d260] 0x101338ce0 NSObject
objc[29984]: [0x10480d268] 0x101338cf0 NSObject
objc[29984]: [0x10480d270] 0x101338d00 NSObject
objc[29984]: [0x10480d278] 0x101338d10 NSObject
objc[29984]: [0x10480d280] 0x101338d20 NSObject
objc[29984]: [0x10480d288] 0x101338d30 NSObject
objc[29984]: [0x10480d290] 0x101338d40 NSObject
objc[29984]: [0x10480d298] 0x101338d50 NSObject
objc[29984]: [0x10480d2a0] 0x101338d60 NSObject
objc[29984]: [0x10480d2a8] 0x101338d70 NSObject
objc[29984]: [0x10480d2b0] 0x101338d80 NSObject
objc[29984]: [0x10480d2b8] 0x101338d90 NSObject
objc[29984]: [0x10480d2c0] 0x101338da0 NSObject
objc[29984]: [0x10480d2c8] 0x101338db0 NSObject
objc[29984]: [0x10480d2d0] 0x101338dc0 NSObject
objc[29984]: [0x10480d2d8] 0x101338dd0 NSObject
objc[29984]: [0x10480d2e0] 0x101338de0 NSObject
objc[29984]: [0x10480d2e8] 0x101338df0 NSObject
objc[29984]: [0x10480d2f0] 0x101338e00 NSObject
objc[29984]: [0x10480d2f8] 0x101338e10 NSObject
objc[29984]: [0x10480d300] 0x101338e20 NSObject
objc[29984]: [0x10480d308] 0x101338e30 NSObject
objc[29984]: [0x10480d310] 0x101338e40 NSObject
objc[29984]: [0x10480d318] 0x101338e50 NSObject
objc[29984]: [0x10480d320] 0x101338e60 NSObject
objc[29984]: [0x10480d328] 0x101338e70 NSObject
objc[29984]: [0x10480d330] 0x101338e80 NSObject
objc[29984]: [0x10480d338] 0x101338e90 NSObject
objc[29984]: [0x10480d340] 0x101338ea0 NSObject
objc[29984]: [0x10480d348] 0x101338eb0 NSObject
objc[29984]: [0x10480d350] 0x101338ec0 NSObject
objc[29984]: [0x10480d358] 0x101338ed0 NSObject
objc[29984]: [0x10480d360] 0x101338ee0 NSObject
objc[29984]: [0x10480d368] 0x101338ef0 NSObject
objc[29984]: [0x10480d370] 0x101338f00 NSObject
objc[29984]: [0x10480d378] 0x101338f10 NSObject
objc[29984]: [0x10480d380] 0x101338f20 NSObject
objc[29984]: [0x10480d388] 0x101338f30 NSObject
objc[29984]: [0x10480d390] 0x101338f40 NSObject
objc[29984]: [0x10480d398] 0x101338f50 NSObject
objc[29984]: [0x10480d3a0] 0x101338f60 NSObject
objc[29984]: [0x10480d3a8] 0x101338f70 NSObject
objc[29984]: [0x10480d3b0] 0x101338f80 NSObject
objc[29984]: [0x10480d3b8] 0x101338f90 NSObject
objc[29984]: [0x10480d3c0] 0x101338fa0 NSObject
objc[29984]: [0x10480d3c8] 0x101338fb0 NSObject
objc[29984]: [0x10480d3d0] 0x101338fc0 NSObject
objc[29984]: [0x10480d3d8] 0x101338fd0 NSObject
objc[29984]: [0x10480d3e0] 0x101338fe0 NSObject
objc[29984]: [0x10480d3e8] 0x101338ff0 NSObject
objc[29984]: [0x10480d3f0] 0x101339000 NSObject
objc[29984]: [0x10480d3f8] 0x101339010 NSObject
objc[29984]: [0x10480d400] 0x101339020 NSObject
objc[29984]: [0x10480d408] 0x101339030 NSObject
objc[29984]: [0x10480d410] 0x101339040 NSObject
objc[29984]: [0x10480d418] 0x101339050 NSObject
objc[29984]: [0x10480d420] 0x101339060 NSObject
objc[29984]: [0x10480d428] 0x101339070 NSObject
objc[29984]: [0x10480d430] 0x101339080 NSObject
objc[29984]: [0x10480d438] 0x101339090 NSObject
objc[29984]: [0x10480d440] 0x1013390a0 NSObject
objc[29984]: [0x10480d448] 0x1013390b0 NSObject
objc[29984]: [0x10480d450] 0x1013390c0 NSObject
objc[29984]: [0x10480d458] 0x1013390d0 NSObject
objc[29984]: [0x10480d460] 0x1013390e0 NSObject
objc[29984]: [0x10480d468] 0x1013390f0 NSObject
objc[29984]: [0x10480d470] 0x101339100 NSObject
objc[29984]: [0x10480d478] 0x101339110 NSObject
objc[29984]: [0x10480d480] 0x101339120 NSObject
objc[29984]: [0x10480d488] 0x101339130 NSObject
objc[29984]: [0x10480d490] 0x101339140 NSObject
objc[29984]: [0x10480d498] 0x101339150 NSObject
objc[29984]: [0x10480d4a0] 0x101339160 NSObject
objc[29984]: [0x10480d4a8] 0x101339170 NSObject
objc[29984]: [0x10480d4b0] 0x101339180 NSObject
objc[29984]: [0x10480d4b8] 0x101339190 NSObject
objc[29984]: [0x10480d4c0] 0x1013391a0 NSObject
objc[29984]: [0x10480d4c8] 0x1013391b0 NSObject
objc[29984]: [0x10480d4d0] 0x1013391c0 NSObject
objc[29984]: [0x10480d4d8] 0x1013391d0 NSObject
objc[29984]: [0x10480d4e0] 0x1013391e0 NSObject
objc[29984]: [0x10480d4e8] 0x1013391f0 NSObject
objc[29984]: [0x10480d4f0] 0x101339200 NSObject
objc[29984]: [0x10480d4f8] 0x101339210 NSObject
objc[29984]: [0x10480d500] 0x101339220 NSObject
objc[29984]: [0x10480d508] 0x101339230 NSObject
objc[29984]: [0x10480d510] 0x101339240 NSObject
objc[29984]: [0x10480d518] 0x101339250 NSObject
objc[29984]: [0x10480d520] 0x101339260 NSObject
objc[29984]: [0x10480d528] 0x101339270 NSObject
objc[29984]: [0x10480d530] 0x101339280 NSObject
objc[29984]: [0x10480d538] 0x101339290 NSObject
objc[29984]: [0x10480d540] 0x1013392a0 NSObject
objc[29984]: [0x10480d548] 0x1013392b0 NSObject
objc[29984]: [0x10480d550] 0x1013392c0 NSObject
objc[29984]: [0x10480d558] 0x1013392d0 NSObject
objc[29984]: [0x10480d560] 0x1013392e0 NSObject
objc[29984]: [0x10480d568] 0x1013392f0 NSObject
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objc[29984]: [0x10480dfa0] 0x101335390 NSObject
objc[29984]: [0x10480dfa8] 0x1013353a0 NSObject
objc[29984]: [0x10480dfb0] 0x1013353b0 NSObject
objc[29984]: [0x10480dfb8] 0x1013353c0 NSObject
objc[29984]: [0x10480dfc0] 0x1013353d0 NSObject
objc[29984]: [0x10480dfc8] 0x1013353e0 NSObject
objc[29984]: [0x10480dfd0] 0x1013353f0 NSObject
objc[29984]: [0x10480dfd8] 0x101335400 NSObject
objc[29984]: [0x10480dfe0] 0x101335410 NSObject
objc[29984]: [0x10480dfe8] 0x101335420 NSObject
objc[29984]: [0x10480dff0] 0x101335430 NSObject
objc[29984]: [0x10480dff8] 0x101335440 NSObject
objc[29984]: [0x104809000] ................ PAGE (hot)
objc[29984]: [0x104809038] 0x101335450 NSObject
我们看到这里有产生PAGE (hot) ，我们再以下两行打印
objc[29984]: [0x10480d000] ................ PAGE (full) (cold)
objc[29984]: [0x104809000] ................ PAGE (hot)
当前页满的时候，新建了一个PAGE,并设置hot
这里可能看出504刚好满一页，我们算下，页面的大小 是多少？
504 8 + 56=4088+8(哨兵对象)=4096=10244=4k。
通过这里可以看出PAGESIZE在MAC下是4k。
我们再看下AutoreleasePoolPage中的#define PAGE_MIN_SIZE (1 << PAGE_MIN_SHIFT)，而#define PAGE_MIN_SHIFT 12，1左移12位，也就是2的12次方，就是4096。iOS是16k的在大小
在分页的时候哨兵对象并没有创建，因为我们已经有一个了，不需要再创建。
所以第二个对象就可以创建505个对象了。
那我们的pop是如何操作的，我们来看下objc_autoreleasePoolPop函数
void
objc_autoreleasePoolPop(void *ctxt)
{
AutoreleasePoolPage::pop(ctxt);
}
我们再看一这里的pop函数
static inline void
pop(void *token)
{
AutoreleasePoolPage *page;
id *stop;
if (token == (void*)EMPTY_POOL_PLACEHOLDER) {
// Popping the top-level placeholder pool.
page = hotPage();
if (!page) {
// Pool was never used. Clear the placeholder.
return setHotPage(nil);
}
// Pool was used. Pop its contents normally.
// Pool pages remain allocated for re-use as usual.
page = coldPage();
token = page->begin();
} else {
page = pageForPointer(token);
}
stop = (id *)token;
if (*stop != POOL_BOUNDARY) {
if (stop == page->begin() && !page->parent) {
// Start of coldest page may correctly not be POOL_BOUNDARY:
// 1. top-level pool is popped, leaving the cold page in place
// 2. an object is autoreleased with no pool
} else {
// Error. For bincompat purposes this is not
// fatal in executables built with old SDKs.
return badPop(token);
}
}
if (slowpath(PrintPoolHiwat || DebugPoolAllocation || DebugMissingPools)) {
return popPageDebug(token, page, stop);
}
return popPage<false>(token, page, stop);
}
这里获取到page = hotPage();这个hot页，如果不是hot。
if (!page) {
// Pool was never used. Clear the placeholder.
return setHotPage(nil);
}
置空
page = coldPage();
token = page->begin();
不断的移动这个页面，移除popPage<false>(token, page, stop);这里的调用
template<bool allowDebug>
static void
popPage(void *token, AutoreleasePoolPage *page, id *stop)
{
if (allowDebug && PrintPoolHiwat) printHiwat();
page->releaseUntil(stop);
// memory: delete empty children
if (allowDebug && DebugPoolAllocation && page->empty()) {
// special case: delete everything during page-per-pool debugging
AutoreleasePoolPage *parent = page->parent;
page->kill();
setHotPage(parent);
} else if (allowDebug && DebugMissingPools && page->empty() && !page->parent) {
// special case: delete everything for pop(top)
// when debugging missing autorelease pools
page->kill();
setHotPage(nil);
} else if (page->child) {
// hysteresis: keep one empty child if page is more than half full
if (page->lessThanHalfFull()) {
page->child->kill();
}
else if (page->child->child) {
page->child->child->kill();
}
}
}
这里先
AutoreleasePoolPage *parent = page->parent;
拿到parent页面，接着
page->kill();
杀掉当前页，然后
setHotPage(parent);
把父页设置成hot。
而kill就是不断的内存平移
void kill()
{
// Not recursive: we don't want to blow out the stack
// if a thread accumulates a stupendous amount of garbage
AutoreleasePoolPage *page = this;
while (page->child) page = page->child;
AutoreleasePoolPage *deathptr;
do {
deathptr = page;
page = page->parent;
if (page) {
page->unprotect();
page->child = nil;
page->protect();
}
delete deathptr;
} while (deathptr != this);
}
先入后出，不断的递归。
通过哨兵对象是否等于哨兵边界来判断标记完成了。
5 自动释放池的拓展
- 临时变量什么时候释放
- 自动释放池原理
- 自动释放池能否嵌套使用
临时变量什么时候释放就是在作用域的范围
自动释放池原理就是上述所讲
自动释放池可以嵌套使用
我们看下代码
#import <Foundation/Foundation.h>
extern void _objc_autoreleasePoolPrint(void);
int main(int argc, const char * argv[]) {
@autoreleasepool {
NSObject *objc = [[[NSObject alloc] init] autorelease];
@autoreleasepool {
NSObject *objc2 = [[[NSObject alloc] init] autorelease];
NSLog(@"%@", objc2);
_objc_autoreleasePoolPrint();
}
_objc_autoreleasePoolPrint();
}
return 0;
}
我们运行下项目，看下效果，如下
objc[30334]: ##############
objc[30334]: AUTORELEASE POOLS for thread 0x1000ebe00
objc[30334]: 4 releases pending.
objc[30334]: [0x100811000] ................ PAGE (hot) (cold)
objc[30334]: [0x100811038] ################ POOL 0x100811038
objc[30334]: [0x100811040] 0x10060d9b0 NSObject
objc[30334]: [0x100811048] ################ POOL 0x100811048
objc[30334]: [0x100811050] 0x10060cdd0 NSObject
objc[30334]: ##############
objc[30334]: ##############
objc[30334]: AUTORELEASE POOLS for thread 0x1000ebe00
objc[30334]: 2 releases pending.
objc[30334]: [0x100811000] ................ PAGE (hot) (cold)
objc[30334]: [0x100811038] ################ POOL 0x100811038
objc[30334]: [0x100811040] 0x10060d9b0 NSObject
objc[30334]: ##############
KCObjcBuild was compiled with optimization - stepping may behave oddly; variables may not be available.
从这可以看出objc2是加入到objc2的自动释放池中，这里也说明是可以嵌套使用，相互不影响。
但是如果不autorelease是不加入到自动释放池中的（在MAC中）
但是以alloc、new、copy、mutablecopy创建的是不会加入到自动释放池中的
总结
这篇文章主要介绍了自动释放池的结构、创建、压栈等相关知识， 到这里内存管理的相关知识，我们已经基本介绍完了，但这只是个人的一份见解，内存相关的知识还有很多很多，本人也只是对其有那么一点点的研究，如有遗漏，还望海涵。
