stay Jdk1.8 after ,jvm The memory model has been adjusted , That is, the implementation of the method area is from permanent generation (Perm Space) It becomes a meta space (MetaSpace). Metaspace is not in the virtual machine , Instead, it directly uses the system memory . For convenience , Hereinafter, virtual machine memory is called heap memory , Against it , Direct memory is called off heap memory .

Why use off heap memory

Before talking about why we should use off heap memory , We have to talk about heap memory . In heap memory is Java Virtual machine memory , Managed by virtual machines , Developers don't have to worry about the allocation and recycling of memory space . But there are advantages and disadvantages , The disadvantage of heap memory is ：

1. Garbage recycling has costs , The more objects in the heap ,GC The greater the cost .
2. Use heap memory for file 、 Online IO when ,JVM Will use the off heap memory for an additional transfer , That is, one more memory copy .

And off heap memory directly uses machine memory , Managed by the operating system . To a certain extent, it reduces the impact of garbage collection on Applications .

Actual case

The blogger himself has never encountered the scene of having to use off heap memory in actual work . however 《 Ant message middleware (MsgBroker) stay YGC Exploration on Optimization 》 There is a relatively complete case in this article ( Reference material 2).

The business scenario in this article is the use of ant's message oriented middleware . To ensure the reliability of the message , Persisting messages to a database , In order to reduce the accuracy of message delivery db Reading pressure , The message is cached . In this way, the virtual machine will allocate memory for cached messages . When the message in the cache is not delivered successfully for various reasons , These news will be maintained all the time , And more and more . In heap space, objects change from young generation to old generation , It takes up more and more space . The final problem is that of the younger generation gc Time is too long ( exceed 100ms).

So here comes the question , Why does the increase of objects in old age lead to ygc The time is getting longer ？ According to the answer in the text ：
stay ygc Most of the time is older-gen scanning, This stage is mainly used to scan the references held by the elderly generation to the objects of the younger generation . In a scenario where messages are cached and a large number of messages cannot be delivered , A large number of young objects have been transformed into old objects , And holds references to younger generation objects . under these circumstances , The scanning time is relatively long .

The final solution is to use off heap memory , Reduce message pairs JVM Memory usage , And use the Netty Network layer framework , Achieved the ideal YGC Time .

Out of heap memory usage

Off heap memory can be used in two ways .

Unsafe Class action

sun.misc.Unsafe Provides a set of methods to allocate out of heap memory , Redistribute and release .

Unsafe yes java Back door for developers , It is used to directly operate the system memory and is not affected by jvm Have jurisdiction over , The implementation is similar to c++ Style operation . But generally, it is not recommended to use , As the class name shows , It's not safe , Easy to cause memory leaks .

Unsafe Most methods of class are native Method , Directly call methods in other languages ( For the most part c++) To operate , Many details cannot be traced , Can only roughly understand . Unsafe Class in jdk9 Then moved to jdk.unsupported Module .

• public native long allocateMemory(long size): Allocate memory space
• public native long reallocateMemory(long address, long size): Reallocate a piece of memory , The data from address Copy to the new memory block in the cache pointed to .
• public native void freeMemory(long address): Free memory

Unsafe The constructor of a class is private , Provides getUnsafe Method is used to get its instance . However, this method is not open to ordinary developers , Exceptions will be reported when using ：java.lang.SecurityException: Unsafe. You can use this class through the reflection mechanism . As shown below ：

public class unsafeDemo {
    public static void main(String[] args) {
        Unsafe unsafe = null;
        long memoryAddress = 0;
        try {
            //  obtain Unsafe Private singleton object of type 
            Field field = Unsafe.class.getDeclaredField("theUnsafe");
            field.setAccessible(true);
            unsafe = (Unsafe) field.get(null);
            memoryAddress = unsafe.allocateMemory(1024);
            //  take int Type integer is stored in the specified address 
            unsafe.putInt(memoryAddress, 5);
            //  Get an integer according to the address 
            int a = unsafe.getInt(memoryAddress);
            System.out.println(a);
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            assert unsafe != null;
            unsafe.freeMemory(memoryAddress);
        }
    }
}

NIO Class action

JDK1.4 Introduced NIO, have access to Native Function library directly allocates out of heap memory , And then through a Java In the pile DirectByteBuffer Object operates as a reference to this memory .

Allocate a block of off heap memory as follows ：

ByteBuffer bf = ByteBuffer.allocateDirect(10 * 1024);

Check the source code to see allocateDirect Function definition of ：

/**
 * Allocates a new direct byte buffer.
 *
 * <p> The new buffer's position will be zero, its limit will be its
 * capacity, its mark will be undefined, and each of its elements will be
 * initialized to zero.  Whether or not it has a
 * {@link #hasArray backing array} is unspecified.
 *
 * @param  capacity
 *         The new buffer's capacity, in bytes
 *
 * @return  The new byte buffer
 *
 * @throws  IllegalArgumentException
 *          If the <tt>capacity</tt> is a negative integer
 */
public static ByteBuffer allocateDirect(int capacity) {
    return new DirectByteBuffer(capacity);
}

You can see , This method returns a DirectByteBuffer object . Further inspection DirectByteBuffer The creation process of ：

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;
}

In the process , Real memory allocation is used Bits.reserveMemory Method .Bits.reserveMemory() The main logic is as follows ：

• Bits Class has a global variable totalCapacity, Used to identify DirectByteBuffer The total size of . Every time I apply for memory , Check whether the size limit is exceeded first ( It can be done by -XX:MaxDirectMemorySize Set up ). If the limit has been exceeded , It will call System.gc(), Looking forward to actively reclaiming a little off heap memory . Then sleep for a while , have a look totalCapacity Is it available . If the memory is still insufficient , Throw out OOM abnormal .

Creating DirectByteBuffer The end of the object , adopt Cleaner.create(this, new Deallocator(base, size, cap)) Created a Cleaner object . The object's function is ： When DirectByteBuffer When the object is recycled , Release its corresponding off heap memory .

Recovery of out of heap memory

Out of heap memory is based on GC The recycling of

As mentioned above , Out of heap memory passes through in heap DirectByteBuffer Object to reference .DirectByteBuffer The object itself is very small , But it represents a large amount of memory allocated , It's called “ Iceberg ” object . When DirectByteBuffer The object is gc when , The off heap memory it refers to will also be recycled .

in other words , The recycling of off heap memory is DirectByteBuffer Triggered when recycled , and DirectByteBuffer The recycling of is in the heap GC What happened .

Recall the pile GC The mechanism of ： When the new generation is full , It will trigger YongGC, If the object is not invalidated at this time , Will not be recycled ; After several times YongGC After that, the objects of the new generation that still survive are moved to the elderly generation . When the old generation is full Full GC.

If DirectByteBuffer The object survived several times YongGC Later, they were migrated to the elderly generation , Even if it fails, it can stay in the elderly generation . be relative to YongGC, In the old days Full GC The frequency of is relatively low . It doesn't happen in the elderly Full GC when , Invalid DirectByteBuffer Objects always occupy a large amount of out of heap memory without releasing .

Of course , There is another situation that can also trigger DirectByteBuffer Recycling . That's what I mentioned above , When out of heap memory is applied and space is insufficient , Will call System.gc() To tell the virtual machine what to do GC 了 . But this way is not reliable , Because it will only be triggered when there is insufficient memory space outside the heap . and , If set -DisableExplicitGC It's forbidden system.gc(), Then it can't be recycled .

Active recycling of off heap memory

Active recycling of off heap memory refers to the use of DirectByteBuffer Of clean Object for memory recovery . As shown below ：

public class ByteBufferTest {
    public static void main(String[] args) throws Exception {
        long size = Runtime.getRuntime().maxMemory();
        System.out.println(" The default maximum out of heap memory is ：" + size / 1024.0 / 1024.0 + "mb");
        ByteBuffer bf = ByteBuffer.allocateDirect(1024 * 1024 * 1024);
        Thread.sleep(2000);
        System.out.println("cleaner start");
        // clean(bf);
        ByteBuffer bf2 = ByteBuffer.allocateDirect(1024 * 1024 * 1024);
        Thread.sleep(2000);
        clean(bf2);
    }

    private static void clean(final ByteBuffer byteBuffer) throws Exception {
        if (byteBuffer.isDirect()) {
            Field cleanerField = byteBuffer.getClass().getDeclaredField("cleaner");
            cleanerField.setAccessible(true);
            Cleaner cleaner = (Cleaner) cleanerField.get(byteBuffer);
            cleaner.clean();
        }
//        //  The second method is to get cleaner
//        if (byteBuffer.isDirect()) {
//            Cleaner cleaner = ((DirectBuffer)byteBuffer).cleaner();
//            cleaner.clean();
//        }
    }
}

In no way -XX:MaxDirectMemorySize Parameter to specify the maximum out of heap memory , The default out of heap memory is similar to heap memory , adopt Runtime.getRuntime().maxMemory() Can be obtained .

Then I applied 1024m Out of heap memory , And pass cleaner I did an out of heap memory recycling . Then apply again 1024m Out of heap memory . The operation effect is as follows ：

 The default maximum out of heap memory is ：1753.0mb
cleaner start

If you comment out clean(bf) This business , There's no problem running . As mentioned above , When there is insufficient space to apply for out of heap memory , The system will call System.gc() To trigger Full GC, In order to achieve the purpose of reclaiming memory outside the heap .

If you comment out clean(bf) meanwhile , Appoint -XX:+DisableExplicitGC Parameter to prohibit explicit triggering gc, When applying for the second off heap memory , Will report OOM error , because System.gc() The call to is invalid . The running results are as follows ：

Reference material

[1]. https://www.jianshu.com/p/17e72bb01bf1
[2]. https://juejin.im/post/5a9cb49df265da239706561a?utm_source=gold_browser_extension
[3]. https://www.jianshu.com/p/ae3847326e69