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Expand Up @@ -132,6 +132,8 @@ manager.memoryClass

我使用的手机内存是 16 g，调用返回的是 256Mb，

> manager.memoryClass 对应 build.prop 中 dalvik.vm.heapgrowthlimit
#### 申请更大的堆内存

```kotlin
Expand All @@ -141,6 +143,14 @@ manager.largeMemoryClass

可分配的最大对内存上限，**需要在 manifest 文件中设置 android:largeHeap="true" 方可启用**

> manager.largeMemoryClass 对应 build.prop 中 dalvik.vm.heapsize
#### Runtime.maxMemory

获取进程最大可获取的内存上限，等于上面这两个值之一



#### /system/build.prop

该目录是Android内存配置相关的文件，里面保存了系统的内存的限制等数据，执行 adb 命令可看到 Android 配置的内存相关信息：
Expand All @@ -162,3 +172,235 @@ dalvik.vm.heapsize=256m #所有情况下（包括设置android:largeHeap="true"

未设置android:largeHeap="true"的时候，只要申请的内存超过了heapgrowthlimit就会触发oom，而当设置android:largeHeap="true"的时候，只有内存超过了heapsize才会触发oom。heapsize已经是该应用能申请的最大内存（这里不包括native申请的内存）。

### OOM 演示

#### 堆内存分配失败

堆内存分配失败对应的是 /art/runtime/gc/heap.cc ，如下代码

```c++
oid Heap::ThrowOutOfMemoryError(Thread* self, size_t byte_count, AllocatorType allocator_type) {
// If we're in a stack overflow, do not create a new exception. It would require running the
// constructor, which will of course still be in a stack overflow.
if (self->IsHandlingStackOverflow()) {
self->SetException(
Runtime::Current()->GetPreAllocatedOutOfMemoryErrorWhenHandlingStackOverflow());
return;
}
//....
std::ostringstream oss;
size_t total_bytes_free = GetFreeMemory();
oss << "Failed to allocate a " << byte_count << " byte allocation with " << total_bytes_free
<< " free bytes and " << PrettySize(GetFreeMemoryUntilOOME()) << " until OOM,"
<< " target footprint " << target_footprint_.load(std::memory_order_relaxed)
<< ", growth limit "
<< growth_limit_;

self->ThrowOutOfMemoryError(oss.str().c_str());
}
```
通过上面的分析，我们也知道系统对每个应用都做了最大内存的约束，超过这个值就会 OOM ，下面通过一段代码来演示一下这种类型的 OOM
```kotlin
fun testOOM() {
val manager = requireContext().getSystemService(Context.ACTIVITY_SERVICE) as ActivityManager
Timber.e("app maxMemory ${manager.memoryClass} Mb")
Timber.e("large app maxMemory ${manager.largeMemoryClass} Mb")
Timber.e("current app maxMemory ${Runtime.getRuntime().maxMemory() / 1024 / 1024} Mb")
var count = 0
val list = mutableListOf<ByteArray>()
while (true) {
Timber.e("count $count total ${count * 20}")
list.add(ByteArray(1024 * 1024 * 20))
count++
}
}
```

上面代码中每次申请 20mb，测试分为两种情况，

1. 未开启 largeHeap:

```text
E app maxMemory 256 Mb
E large app maxMemory 512 Mb
E current app maxMemory 256 Mb
E count 0 total 0
E count 1 total 20
E count 2 total 40
E count 3 total 60
E count 4 total 80
E count 5 total 100
E count 6 total 120
E count 7 total 140
E count 8 total 160
E count 9 total 180
E count 10 total 200
E count 11 total 220
E count 12 total 240
java.lang.OutOfMemoryError: Failed to allocate a 20971536 byte allocation with 12386992 free bytes and 11MB until OOM, target footprint 268435456, growth limit 268435456
......
```
可以看到一共分配了 12次，在第十二次的时候抛出了异常，显示 分配 20 mb 失败，空闲只有 11 mb，
2. 开启 largeHeap
```
app maxMemory 256 Mb
large app maxMemory 512 Mb
current app maxMemory 512 Mb
E count 0 total 0
E count 1 total 20
E count 2 total 40
E count 3 total 60
E count 4 total 80
E count 5 total 100
E count 6 total 120
E count 7 total 140
E count 8 total 160
E count 9 total 180
E count 10 total 200
E count 11 total 220
E count 12 total 240
E count 13 total 260
E count 14 total 280
E count 15 total 300
E count 16 total 320
E count 17 total 340
E count 18 total 360
E count 19 total 380
E count 20 total 400
E count 21 total 420
E count 22 total 440
E count 23 total 460
E count 24 total 480
E count 25 total 500
FATAL EXCEPTION: main
Process: com.dzl.duanzil, PID: 31874
java.lang.OutOfMemoryError: Failed to allocate a 20971536 byte allocation with 8127816 free bytes and 7937KB until OOM, target footprint 536870912, growth limit 536870912
```
可以看到分配了25 次，可使用的内存也增加到了 512 mb
#### 创建线程失败
线程创建会消耗大量的内存资源，创建的过程涉及 java 层 和 native 层，本质上是在 native 层完成的，对应的是 /art/runtime/thread.cc ，如下代码
```c++
void Thread::CreateNativeThread(JNIEnv* env, jobject java_peer, size_t stack_size, bool is_daemon) {
//........
env->SetLongField(java_peer, WellKnownClasses::java_lang_Thread_nativePeer, 0);
{
std::string msg(child_jni_env_ext.get() == nullptr ?
StringPrintf("Could not allocate JNI Env: %s", error_msg.c_str()) :
StringPrintf("pthread_create (%s stack) failed: %s",
PrettySize(stack_size).c_str(), strerror(pthread_create_result)));
ScopedObjectAccess soa(env);
soa.Self()->ThrowOutOfMemoryError(msg.c_str());
}
}
```

这里借用网上的一张照片来看一下创建线程的流程

<img src="https://raw.githubusercontent.com/LvKang-insist/PicGo/main/img/202305221618788.jpg" alt="2841684743512_.pic" style="zoom:50%;" />

根据上图可以看到主要有两部分，分别是创建 JNI Env 和 创建线程

##### 创建 JNI Env 失败

1. FD 溢出导致 JNIEnv 创建失败

```kotlin
E/art: ashmem_create_region failed for 'indirect ref table': Too many open files java.lang.OutOfMemoryError:Could not allocate JNI Env at java.lang.Thread.nativeCreate(Native Method) at java.lang.Thread.start(Thread.java:730)
```

2. 虚拟内存不足导致 JNIEnv 创建失败

```kotlin
E OOM_TEST: create thread : 1104
W com.demo: Throwing OutOfMemoryError "Could not allocate JNI Env: Failed anonymous mmap(0x0, 8192, 0x3, 0x22, -1, 0): Operation not permitted. See process maps in the log." (VmSize 2865432 kB)
E InputEventSender: Exception dispatching finished signal.
E MessageQueue-JNI: Exception in MessageQueue callback: handleReceiveCallback
MessageQueue-JNI: java.lang.OutOfMemoryError: Could not allocate JNI Env: Failed anonymous mmap(0x0, 8192, 0x3, 0x22, -1, 0): Operation not permitted. See process maps in the log.
E MessageQueue-JNI: at java.lang.Thread.nativeCreate(Native Method)
E MessageQueue-JNI: at java.lang.Thread.start(Thread.java:887)

E AndroidRuntime: FATAL EXCEPTION: main
E AndroidRuntime: Process: com.demo, PID: 3533
E AndroidRuntime: java.lang.OutOfMemoryError: Could not allocate JNI Env: Failed anonymous mmap(0x0, 8192, 0x3, 0x22, -1, 0): Operation not permitted. See process maps in the log.
E AndroidRuntime: at java.lang.Thread.nativeCreate(Native Method)
E AndroidRuntime: at java.lang.Thread.start(Thread.java:887)
```

##### 创建线程失败

1. 虚拟机内存不足导致失败

native 通过 FixStackSize 设置线程大小

```c++
static size_t FixStackSize(size_t stack_size) {
if (stack_size == 0) {
stack_size = Runtime::Current()->GetDefaultStackSize();
}
stack_size += 1 * MB;
if (kMemoryToolIsAvailable) {
stack_size = std::max(2 * MB, stack_size);
} if (stack_size < PTHREAD_STACK_MIN) {
stack_size = PTHREAD_STACK_MIN;
}
if (Runtime::Current()->ExplicitStackOverflowChecks()) {
stack_size += GetStackOverflowReservedBytes(kRuntimeISA);
} else {
stack_size += Thread::kStackOverflowImplicitCheckSize +
GetStackOverflowReservedBytes(kRuntimeISA);
}
stack_size = RoundUp(stack_size, kPageSize);
return stack_size;
}
```

```kotlin
W/libc: pthread_create failed: couldn't allocate 1073152-bytes mapped space: Out of memory
W/tch.crowdsourc: Throwing OutOfMemoryError with VmSize 4191668 kB "pthread_create (1040KB stack) failed: Try again"
java.lang.OutOfMemoryError: pthread_create (1040KB stack) failed: Try again
at java.lang.Thread.nativeCreate(Native Method)
at java.lang.Thread.start(Thread.java:753)
```
2. 线程数量超过限制
用一段简单的代码来测试一下
```kotlin
fun testOOM() {
var count = 0
while (true) {
val thread = Thread(Runnable {
Thread.sleep(1000000000)
})
thread.start()
count++
Timber.e("current thread count $count")
}
}
```
通过打印日志发现，一共创建了 2473 个线程，当然这些线程都是没有任务的线程，报错信息如下所示
```kotlin
pthread_create failed: couldn't allocate 1085440-bytes mapped space: Out of memory
Throwing OutOfMemoryError "pthread_create (1040KB stack) failed: Try again" (VmSize 4192344 kB)

FATAL EXCEPTION: main
Process: com.dzl.duanzil, PID: 18085
java.lang.OutOfMemoryError: pthread_create (1040KB stack) failed: Try again
at java.lang.Thread.nativeCreate(Native Method)
```




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