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分类: Java

2017-02-07 16:49:15

概述

广义的堆外内存

说到堆外内存,那大家肯定想到堆内内存,这也是我们大家接触最多的,我们在jvm参数里通常设置-Xmx来指定我们的堆的最大值,不过这还不是我们理解的Java堆,-Xmx的值是新生代和老生代的和的最大值,我们在jvm参数里通常还会加一个参数-XX:MaxPermSize来指定持久代的最大值,那么我们认识的Java堆的最大值其实是-Xmx和-XX:MaxPermSize的总和,在分代算法下,新生代,老生代和持久代是连续的虚拟地址,因为它们是一起分配的,那么剩下的都可以认为是堆外内存(广义的)了,这些包括了jvm本身在运行过程中分配的内存,codecache,jni里分配的内存,DirectByteBuffer分配的内存等等

狭义的堆外内存

而作为java开发者,我们常说的堆外内存溢出了,其实是狭义的堆外内存,这个主要是指java.nio.DirectByteBuffer在创建的时候分配内存,我们这篇文章里也主要是讲狭义的堆外内存,因为它和我们平时碰到的问题比较密切

JDK/JVM里DirectByteBuffer的实现

DirectByteBuffer通常用在通信过程中做缓冲池,在mina,netty等nio框架中屡见不鲜,先来看看JDK里的实现:

 DirectByteBuffer(int cap) { // package-private super(-1, 0, cap, cap);
        boolean pa = VM.isDirectMemoryPageAligned(); int ps = Bits.pageSize(); long size = Math.max(1L, (long)cap + (pa ? ps : 0));
        Bits.reserveMemory(size, cap); long base = 0; try { base = unsafe.allocateMemory(size);
        } catch (OutOfMemoryError x) {
            Bits.unreserveMemory(size, cap); throw x;
        } unsafe.setMemory(base, size, (byte) 0); if (pa && (base % ps != 0)) { // Round up to page boundary address = base + ps - (base & (ps - 1));
        } else {
            address = base;
        }
        cleaner = Cleaner.create(this, new Deallocator(base, size, cap));
        att = null;



    } 

通过上面的构造函数我们知道,真正的内存分配是使用的Bits.reserveMemory方法

 static void reserveMemory(long size, int cap) { synchronized (Bits.class) { if (!memoryLimitSet && VM.isBooted()) {
                maxMemory = VM.maxDirectMemory();
                memoryLimitSet = true;
            } // -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;
            }
        }

        System.gc(); try {
            Thread.sleep(100);
        } catch (InterruptedException x) { // Restore interrupt status Thread.currentThread().interrupt();
        } synchronized (Bits.class) { if (totalCapacity + cap > maxMemory) throw new OutOfMemoryError("Direct buffer memory");
            reservedMemory += size;
            totalCapacity += cap; count++;
        }

    } 

通过上面的代码我们知道可以通过-XX:MaxDirectMemorySize来指定最大的堆外内存,那么我们首先引入两个问题

  • 堆外内存默认是多大
  • 为什么要主动调用System.gc()

堆外内存默认是多大

如果我们没有通过-XX:MaxDirectMemorySize来指定最大的堆外内存,那么默认的最大堆外内存是多少呢,我们还是通过代码来分析

上面的代码里我们看到调用了sun.misc.VM.maxDirectMemory()

 private static long directMemory = 64 * 1024 * 1024;

    // Returns the maximum amount of allocatable direct buffer memory.
    // The directMemory variable is initialized during system initialization
    // in the saveAndRemoveProperties method.
    //
    public static long maxDirectMemory() { return directMemory;
    } 

看到上面的代码之后是不是误以为默认的最大值是64M?其实不是的,说到这个值得从java.lang.System这个类的初始化说起

 /**
     * Initialize the system class.  Called after thread initialization.
     */ private static void initializeSystemClass() { // VM might invoke JNU_NewStringPlatform() to set those encoding // sensitive properties (user.home, user.name, boot.class.path, etc.) // during "props" initialization, in which it may need access, via // System.getProperty(), to the related system encoding property that // have been initialized (put into "props") at early stage of the // initialization. So make sure the "props" is available at the // very beginning of the initialization and all system properties to // be put into it directly. props = new Properties();
        initProperties(props); // initialized by the VM // There are certain system configurations that may be controlled by // VM options such as the maximum amount of direct memory and // Integer cache size used to support the object identity semantics // of autoboxing.  Typically, the library will obtain these values // from the properties set by the VM.  If the properties are for // internal implementation use only, these properties should be // removed from the system properties. // // See java.lang.Integer.IntegerCache and the // sun.misc.VM.saveAndRemoveProperties method for example. // // Save a private copy of the system properties object that // can only be accessed by the internal implementation.  Remove // certain system properties that are not intended for public access. sun.misc.VM.saveAndRemoveProperties(props);

         ......

        sun.misc.VM.booted();
    } 

上面这个方法在jvm启动的时候对System这个类做初始化的时候执行的,因此执行时间非常早,我们看到里面调用了sun.misc.VM.saveAndRemoveProperties(props):


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