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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 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 values lineSeparator = 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 secrets setJavaLangAccess(); // 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 supplied if (!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 initialized private 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= ". // // 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 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 later if (savedProps == null) { savedProps = props; } // Set the maximum amount of direct memory. This value is controlled // by the vm option -XX:MaxDirectMemorySize= . // 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 default directMemory = Runtime.getRuntime().maxMemory(); } else { long l = Long.parseLong(s); if (l > -1) directMemory = l; } // Check if direct buffers should be page aligned s = 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 */ @Test public 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：
4096.060.025.025.0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 */ @Test public 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：
30.030.0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 memory at java.base/java.nio.Bits.reserveMemory(Bits.java:175) at java.base/java.nio.DirectByteBuffer.(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=MB3088: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