序

本文主要研究一下jvm的-XX:MaxDirectMemorySize

-XX:MaxDirectMemorySize

-XX:MaxDirectMemorySize=size用于设置New I/O(java.nio) direct-buffer allocations的最大大小，size的单位可以使用k/K、m/M、g/G；如果没有设置该参数则默认值为0，意味着JVM自己自动给NIO direct-buffer allocations选择最大大小

System.initPhase1

java.base/java/lang/System.java

public final class System {/* Register the natives via the static initializer.** VM will invoke the initializeSystemClass method to complete* the initialization for this class separated from clinit.* Note that to use properties set by the VM, see the constraints* described in the initializeSystemClass method.*/private static native void registerNatives();static {registerNatives();}/** Don't let anyone instantiate this class */private System() {}/*** Initialize the system class.  Called after thread initialization.*/private static void initPhase1() {// VM might invoke JNU_NewStringPlatform() to set those encoding// sensitive properties (user.home, user.name, boot.class.path, etc.)// during "props" initialization.// The charset is initialized in System.c and does not depend on the Properties.Map<String, String> tempProps = SystemProps.initProperties();VersionProps.init(tempProps);// 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// masked from the system properties.//// Save a private copy of the system properties object that// can only be accessed by the internal implementation.VM.saveProperties(tempProps);props = createProperties(tempProps);StaticProperty.javaHome();          // Load StaticProperty to cache the property valueslineSeparator = props.getProperty("line.separator");FileInputStream fdIn = new FileInputStream(FileDescriptor.in);FileOutputStream fdOut = new FileOutputStream(FileDescriptor.out);FileOutputStream fdErr = new FileOutputStream(FileDescriptor.err);setIn0(new BufferedInputStream(fdIn));setOut0(newPrintStream(fdOut, props.getProperty("sun.stdout.encoding")));setErr0(newPrintStream(fdErr, props.getProperty("sun.stderr.encoding")));// Setup Java signal handlers for HUP, TERM, and INT (where available).Terminator.setup();// Initialize any miscellaneous operating system settings that need to be// set for the class libraries. Currently this is no-op everywhere except// for Windows where the process-wide error mode is set before the java.io// classes are used.VM.initializeOSEnvironment();// The main thread is not added to its thread group in the same// way as other threads; we must do it ourselves here.Thread current = Thread.currentThread();current.getThreadGroup().add(current);// register shared secretssetJavaLangAccess();// Subsystems that are invoked during initialization can invoke// VM.isBooted() in order to avoid doing things that should// wait until the VM is fully initialized. The initialization level// is incremented from 0 to 1 here to indicate the first phase of// initialization has completed.// IMPORTANT: Ensure that this remains the last initialization action!VM.initLevel(1);}//......
}
复制代码

System的initPhase1方法会调用VM.saveProperties(tempProps)方法来保存一份系统配置供内部实现使用；其中tempProps为SystemProps.initProperties()

jvm.cpp

hotspot/share/prims/jvm.cpp

  // Convert the -XX:MaxDirectMemorySize= command line flag// to the sun.nio.MaxDirectMemorySize property.// Do this after setting user properties to prevent people// from setting the value with a -D option, as requested.// Leave empty if not suppliedif (!FLAG_IS_DEFAULT(MaxDirectMemorySize)) {char as_chars[256];jio_snprintf(as_chars, sizeof(as_chars), JULONG_FORMAT, MaxDirectMemorySize);Handle key_str = java_lang_String::create_from_platform_dependent_str("sun.nio.MaxDirectMemorySize", CHECK_NULL);Handle value_str  = java_lang_String::create_from_platform_dependent_str(as_chars, CHECK_NULL);result_h->obj_at_put(ndx * 2,  key_str());result_h->obj_at_put(ndx * 2 + 1, value_str());ndx++;}
复制代码

jvm.cpp里头有一段代码用于把-XX:MaxDirectMemorySize命令参数转换为key为sun.nio.MaxDirectMemorySize的属性

VM.saveProperties

java.base/jdk/internal/misc/VM.java

public class VM {// the init level when the VM is fully initializedprivate static final int JAVA_LANG_SYSTEM_INITED     = 1;private static final int MODULE_SYSTEM_INITED        = 2;private static final int SYSTEM_LOADER_INITIALIZING  = 3;private static final int SYSTEM_BOOTED               = 4;private static final int SYSTEM_SHUTDOWN             = 5;// 0, 1, 2, ...private static volatile int initLevel;private static final Object lock = new Object();//......// A user-settable upper limit on the maximum amount of allocatable direct// buffer memory.  This value may be changed during VM initialization if// "java" is launched with "-XX:MaxDirectMemorySize=<size>".//// The initial value of this field is arbitrary; during JRE initialization// it will be reset to the value specified on the command line, if any,// otherwise to Runtime.getRuntime().maxMemory().//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;}//......// Save a private copy of the system properties and remove// the system properties that are not intended for public access.//// This method can only be invoked during system initialization.public static void saveProperties(Map<String, String> props) {if (initLevel() != 0)throw new IllegalStateException("Wrong init level");// only main thread is running at this time, so savedProps and// its content will be correctly published to threads started laterif (savedProps == null) {savedProps = props;}// Set the maximum amount of direct memory.  This value is controlled// by the vm option -XX:MaxDirectMemorySize=<size>.// The maximum amount of allocatable direct buffer memory (in bytes)// from the system property sun.nio.MaxDirectMemorySize set by the VM.// If not set or set to -1, the max memory will be used// The system property will be removed.String s = props.get("sun.nio.MaxDirectMemorySize");if (s == null || s.isEmpty() || s.equals("-1")) {// -XX:MaxDirectMemorySize not given, take defaultdirectMemory = Runtime.getRuntime().maxMemory();} else {long l = Long.parseLong(s);if (l > -1)directMemory = l;}// Check if direct buffers should be page aligneds = props.get("sun.nio.PageAlignDirectMemory");if ("true".equals(s))pageAlignDirectMemory = true;}//......
}
复制代码

VM的saveProperties方法读取sun.nio.MaxDirectMemorySize属性，如果为null或者是空或者是-1，那么则设置为Runtime.getRuntime().maxMemory()；如果有设置MaxDirectMemorySize且值大于-1，那么使用该值作为directMemory的值；而VM的maxDirectMemory方法则返回的是directMemory的值

获取maxDirectMemory的值

实例

    public BufferPoolMXBean getDirectBufferPoolMBean(){return ManagementFactory.getPlatformMXBeans(BufferPoolMXBean.class).stream().filter(e -> e.getName().equals("direct")).findFirst().orElseThrow();}public JavaNioAccess.BufferPool getNioBufferPool(){return SharedSecrets.getJavaNioAccess().getDirectBufferPool();}/*** -XX:MaxDirectMemorySize=60M*/@Testpublic void testGetMaxDirectMemory(){ByteBuffer.allocateDirect(25*1024*1024);System.out.println(Runtime.getRuntime().maxMemory() / 1024.0 / 1024.0);System.out.println(VM.maxDirectMemory() / 1024.0 / 1024.0);System.out.println(getDirectBufferPoolMBean().getTotalCapacity() / 1024.0 / 1024.0);System.out.println(getNioBufferPool().getTotalCapacity() / 1024.0 / 1024.0);}
复制代码

输出

输出结果如下：

4096.0
60.0
25.0
25.0
复制代码

• 由于java9模块化之后，VM从原来的sun.misc.VM变更到java.base模块下的jdk.internal.misc.VM；上面代码默认是unamed module，要使用jdk.internal.misc.VM就需要使用--add-exports java.base/jdk.internal.misc=ALL-UNNAMED将其导出到UNNAMED，这样才可以运行
• 同理java9模块化之后，SharedSecrets从原来的sun.misc.SharedSecrets变更到java.base模块下的jdk.internal.access.SharedSecrets；要使用--add-exports java.base/jdk.internal.access=ALL-UNNAMED将其导出到UNNAMED，这样才可以运行
• 从输出结果可以看出，Runtime.getRuntime().maxMemory()输出的值是正确的，而BufferPoolMXBean及JavaNioAccess.BufferPool的getTotalCapacity返回的都是directBuffer大小，而非max值

使用API查看directBuffer使用情况

实例

    /*** -XX:MaxDirectMemorySize=60M*/@Testpublic void testGetDirectMemoryUsage(){ByteBuffer.allocateDirect(30*1024*1024);System.out.println(getDirectBufferPoolMBean().getMemoryUsed() / 1024.0 / 1024.0);System.out.println(getNioBufferPool().getMemoryUsed() / 1024.0 / 1024.0);}
复制代码

输出

输出结果如下：

30.0
30.0
复制代码

可以看到BufferPoolMXBean及JavaNioAccess.BufferPool的getMemoryUsed可以返回directBuffer大小

OOM

java.lang.OutOfMemoryError: Direct buffer memoryat java.base/java.nio.Bits.reserveMemory(Bits.java:175)at java.base/java.nio.DirectByteBuffer.<init>(DirectByteBuffer.java:118)at java.base/java.nio.ByteBuffer.allocateDirect(ByteBuffer.java:317)
复制代码

如果上面的ByteBuffer.allocateDirect改为分配超过60M，则运行抛出OutOfMemoryError

使用NMT查看directBuffer使用情况

jcmd 3088 VM.native_memory scale=MB
3088:Native Memory Tracking:Total: reserved=5641MB, committed=399MB
-                 Java Heap (reserved=4096MB, committed=258MB)(mmap: reserved=4096MB, committed=258MB)-                     Class (reserved=1032MB, committed=6MB)(classes #1609)(  instance classes #1460, array classes #149)(mmap: reserved=1032MB, committed=5MB)(  Metadata:   )(    reserved=8MB, committed=5MB)(    used=3MB)(    free=2MB)(    waste=0MB =0.00%)(  Class space:)(    reserved=1024MB, committed=1MB)(    used=0MB)(    free=0MB)(    waste=0MB =0.00%)-                    Thread (reserved=18MB, committed=18MB)(thread #18)(stack: reserved=18MB, committed=18MB)-                      Code (reserved=242MB, committed=7MB)(mmap: reserved=242MB, committed=7MB)-                        GC (reserved=203MB, committed=60MB)(malloc=18MB #2443)(mmap: reserved=185MB, committed=43MB)-                  Internal (reserved=1MB, committed=1MB)(malloc=1MB #1257)-                     Other (reserved=30MB, committed=30MB)(malloc=30MB #2)-                    Symbol (reserved=1MB, committed=1MB)(malloc=1MB #13745)-        Shared class space (reserved=17MB, committed=17MB)(mmap: reserved=17MB, committed=17MB)
复制代码

从Other部分可以看到其值跟ByteBuffer.allocateDirect使用的值一致，改变ByteBuffer.allocateDirect的值再重新查看，可以发现Other部分跟着改变；因而初步断定Other部分应该是可以反映direct memory的使用大小

小结

• -XX:MaxDirectMemorySize=size用于设置New I/O(java.nio) direct-buffer allocations的最大大小，size的单位可以使用k/K、m/M、g/G；如果没有设置该参数则默认值为0，意味着JVM自己自动给NIO direct-buffer allocations选择最大大小；从代码java.base/jdk/internal/misc/VM.java中可以看到默认是取的Runtime.getRuntime().maxMemory()
• 使用jdk.internal.misc.VM.maxDirectMemory()可以获取maxDirectMemory的值；由于java9模块化之后，VM从原来的sun.misc.VM变更到java.base模块下的jdk.internal.misc.VM；上面代码默认是unamed module，要使用jdk.internal.misc.VM就需要使用--add-exports java.base/jdk.internal.misc=ALL-UNNAMED将其导出到UNNAMED，这样才可以运行
• BufferPoolMXBean及JavaNioAccess.BufferPool(通过SharedSecrets获取)的getMemoryUsed可以获取direct memory的大小；其中java9模块化之后，SharedSecrets从原来的sun.misc.SharedSecrets变更到java.base模块下的jdk.internal.access.SharedSecrets；要使用--add-exports java.base/jdk.internal.access=ALL-UNNAMED将其导出到UNNAMED，这样才可以运行

另外使用NMT也可以查看direct memory的使用情况，其包含在了Other部分

doc

• Default HotSpot Maximum Direct Memory Size
• JVM源码分析之堆外内存完全解读
• Java堆外内存增长问题排查Case
• DirectByteBuffer堆外内存溢出问题排查
• Default for XX:MaxDirectMemorySize
• java.lang.OutOfMemoryError: Direct buffer memory (with tcnative) #6813
• VM.java MaxDirectMemorySize
• Summary of understanding "-XX:MaxDirectMemorySize" setting.
• native-mem-tracking.md
• Java Platform, Standard Edition Tools Reference