为什么XX:MaxDirectMemorySize不能限制Unsafe.allocateMemory？

Question

下面的代码将分配大量的直接内存，但不会导致java.lang.OutOfMemoryError:直接缓冲区内存：

//JVM args: -Xms10m -Xmx10m -XX:MaxDirectMemorySize=10m
    public class DirectMemoryOOM {
        public static void main(String[] args) throws NoSuchFieldException, IllegalAccessException {
            Field f = Unsafe.class.getDeclaredFields()[0];
            f.setAccessible(true);
            Unsafe us = (Unsafe) f.get(null);
            long size = 1024 * 1024 * 1024;
            while (true) {
                long p = us.allocateMemory(size);
                for (int i = 0; i < size; i++) {
                    us.putByte(p + i, Byte.MAX_VALUE);
                }
            }
        }
    }

但是下面的代码将得到java.lang.OutOfMemoryError:直接缓冲区内存。我从Java unsafe memory allocation limit中看到了答案，但是ByteBuffer.allocateDirect是使用Unsafe.allocateMemory()实现的

//JVM args: -Xms10m -Xmx10m -XX:MaxDirectMemorySize=10m
public class DirectMemoryOOM {
    public static void main(String[] args) throws NoSuchFieldException, IllegalAccessException {
        int size = 1024 * 1024;
        System.out.println(sun.misc.VM.maxDirectMemory());
        while (true) {
            ByteBuffer.allocateDirect(size);
        }
    }
}

为什么第一个限制会失败呢？

Answer 1

正如最初的答案所述：Unsafe.allocateMemory()是os::malloc的包装器，它不关心VM施加的任何内存限制。

ByteBuffer.allocateDirect()将调用此方法，但在此之前，它将调用Bits.reserveMemory() (在我的Java7：DirectByteBuffer.java:123版本中)，它检查进程的内存使用情况，并抛出您提到的异常。

Answer 2

错误来自Bits.reserveMemory，在调用allocateDirect时，它在unsafe.allocateMemory(size)之前被调用。

reserveMemory方法处理此验证：

synchronized (Bits.class) {
    if (totalCapacity + cap > maxMemory)
        throw new OutOfMemoryError("Direct buffer memory");
    reservedMemory += size;
    totalCapacity += cap;
    count++;
}

如果所需的分配高于从

maxMemory = VM.maxDirectMemory();

直接调用allocateMemory将继续本机方法，并且不会验证最大容量(这解释了为什么在第一个代码段中没有得到错误)，这是--XX:MaxDirectMemorySize的主要目标，如reserveMemory中的注释所解释的那样

// -XX:MaxDirectMemorySize limits the total capacity rather than the
// actual memory usage, which will differ when buffers are page
// aligned.
if (cap <= maxMemory - totalCapacity) {
     reservedMemory += size;
     totalCapacity += cap;
     count++;
     return;
}

值得一提的是，您的第一个片段实现不是一个好的实践。Bits.java中的注释指定在分配直接内存时始终调用reserveMemory：

// These methods should be called whenever direct memory is allocated or
// freed.  They allow the user to control the amount of direct memory
// which a process may access.  All sizes are specified in bytes.
static void reserveMemory(long size, int cap) {