Java Runtime: повседневные обязанности JVM

Андрей Паньгин ведущий разработчик проекта Одноклассники

JVM Iceberg

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JVM Iceberg

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Launcher • java.exe – just a simple C program

1. Locate JRE

2. Select version

3. Parse arguments

4. JNI_CreateJavaVM

5. JNIEnv::FindClass

6. JNIEnv::GetStaticMethodID (main)

7. JNIEnv::CallStaticVoidMethod

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Other launchers • Differ only by Main Class

– javac com.sun.tools.javac.Main

– javadoc com.sun.tools.javadoc.Main

– javah com.sun.tools.javah.Main

– jconsole sun.tools.jconsole.JConsole

– jmap sun.tools.jmap.JMap

– …

• Even -version calls Java

– sun.misc.Version.print()

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Class loading • Class loading != Class initialization

• ClassLoader prepares byte[] definition

• Real loading is done inside VM

– ClassLoader.defineClass0

– Parses, verifies and creates internal VM structures

• Unreferenced classes may be unloaded

– -XX:+CMSClassUnloadingEnabled

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Class Metadata • Constant Pool, Interfaces, Methods, Fields

• Exceptions, Annotations, vtables, itables

• Java 8: PermGen Metaspace

– java.lang.OutOfMemoryError: PermGen space

• Field reordering

– -XX:+CompactFields, -XX:FieldsAllocationStyle=1

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Class data sharing • Snapshot of commonly used classes

– jre/lib/classlist

• Mapped into memory directly from disk

– classes.jsa

• Shared between multiple JVM instances

• Improves start-up time, reduces footprint

– -XX:+UseSharedSpaces

– -XX:+DumpSharedSpaces

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Bytecode verification • Java 6 Split Verifier

– Inferencing verifier (javac) + Type checking (run-time)

– StackMapTable attribute in .class

• Skip verification

– -XX:-BytecodeVerificationRemote

– -XX:-BytecodeVerificationLocal

– Inherit sun.reflect.MagicAccessorImpl

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Class initialization • When?

• 12-step procedure described in JLS §12.4

• Basically, the invocation of <clinit>

(static initializer)

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Bytecodes • Stack machine

• 203 Java bytecodes

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Stack manipulation iconst, fconst, bipush, pop, dup, swap

Type conversion i2l, f2i, l2d

Arithmetic / logic iadd, isub, imul, ixor, ishr

Local variables iload, aload, istore, fstore

Arrays iaload, aaload, iastore, aastore, arraylength

Fields getfield, putfield, getstatic, putstatic

Branches ifeq, ifgt, goto, tableswitch, lookupswitch

Method calls invokevirtual, invokeinterface, invokestatic, return

Allocation new, anewarray, multianewarray

Other monitorenter, monitorexit, instanceof, athrow

JVM specific bytecodes • fast_igetfield, fast_iputfield

– -XX:+RewriteBytecodes

• fast_iload2, fast_aaccess_0

– -XX:+RewriteFrequentPairs

• fast_binaryswitch

• return_register_finalizer

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Bytecode interpreter • C++ and Assembler (template) interpreter

• Generated at VM start-up

• Run-time code profiling

– -XX:CompileThreshold=10000

• Run-time CP resolution and bytecode rewriting

• Optimizations

– Dispatch table, top-of-stack caching

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Stack • Java (expression) vs. Native (execution)

– -XX:ThreadStackSize=320

• Guard pages

– -XX:StackShadowPages=20

– -XX:StackYellowPages=2

– -XX:StackRedPages=1

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Frame

Empty

Shadow

Frame

Frame

Stack frame

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old SP old FP

Method

Monitors

Locals

Expression stack

Return addr

SP

Previous frame

Expression stack

FP

Frame types • Interpreted, Compiled, Native

– Different layout

• Inlining

– 1 frame for multiple nested methods

• Deoptimization

– When optimistic assumptions fail

– (exceptions, class hierarchy changes etc.)

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Threads • Java threads & VM threads

• States: in_java, in_vm, in_native, blocked

• Thread pointer in register

– Fast Thread.currentThread()

• Priority policy (0 – normal, 1 – aggressive)

– -XX:ThreadPriorityPolicy=N

• TLAB allocation

– -XX:+UseTLAB, -XX:TLABSize=0

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Synchronization • Simple uncontended lock – CAS

• Biased locking

– -XX:+UseBiasedLocking

– -XX:BiasedLockingStartupDelay=4000

– -XX:BiasedLockingBulkRebiasThreshold=20

– -XX:BiasedLockingBulkRevokeThreshold=40

• Contended lock optimizations

– Spin Yield Park

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Java-level locks • java.util.concurrent.locks.LockSupport

• park() / unpark()

• OS-level primitives

– Mutex + Condition variable

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wait / notify • What’s wrong with this code?

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void setCompleted() { synchronized (lock) { completed = true; lock.notifyAll(); } }

void waitForCompletion() { synchronized (lock) { if (!completed) { lock.wait(); } } }

void setCompleted() { synchronized (lock) { completed = true; lock.notifyAll(); } }

wait / notify • What’s wrong with this code?

• -XX:+FilterSpuriousWakeups

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void setCompleted() { synchronized (lock) { completed = true; lock.notifyAll(); } }

void waitForCompletion() { synchronized (lock) { if (!completed) { lock.wait(); } } }

void setCompleted() { synchronized (lock) { completed = true; lock.notifyAll(); } }

Object header

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unused hashCode 0 age 0 01

Unlocked

Displaced header ptr 00

Thin lock

Inflated lock ptr 10

Inflated lock

JavaThread epoch 0 age 1 01

Biased lock Stack

Monitor

Safepoints • When?

– GC phases

– Thread dump

– Deoptimization

– Revoke/rebias BiasedLock

• How?

– Interpreted: switch dispatch table

– Compiled: page polling

– Native: on return and on JNI calls

• -XX:+PrintSafepointStatistics

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Native methods • System.loadLibrary()

• Lazy linking

• Expensive invocation

1. Create stack frame

2. Set up arguments according to native calling convention

3. Pass JNIEnv* and jclass

4. Lock / unlock if synchronized

5. Trace method entry / method exit

6. Check for safepoint

7. Check exceptions

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JNI functions • Executed in VM context

• Check for safepoint

• jobject == index in thread-local JNIHandles array

• Optional verification

– -XX:+CheckJNICalls

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Exceptions • Which code is faster?

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for (;;) { int index = getNextIndex(); if (index >= arr.length) { break; } sum += array[index]; }

try { for (;;) { int index = getNextIndex(); sum += arr[index]; } } catch (IndexOutOfBoundsException e) { // break }

Exceptions • Which code is faster?

• try-catch is free (exception tables)

• throw is expensive

(find handler, unwind stack, release locks, build stack trace)

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for (;;) { int index = getNextIndex(); if (index >= arr.length) { break; } sum += array[index]; }

try { for (;;) { int index = getNextIndex(); sum += arr[index]; } } catch (IndexOutOfBoundsException e) { // break }

Reflection • getDeclaredFields(), getDeclaredMethods()

– VM Internal structures Java representation

• Field getters and setters

– sun.misc.Unsafe

• Method.invoke()

1. Native implementation

2. Dynamic bytecode generation (up to 10x faster)

-Dsun.reflect.inflationThreshold=15

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MethodAccessor example • void Point.move(float x, float y, boolean relative);

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class PointMove_MethodAccessor implements MethodAccessor { public Object invoke(Object target, Object[] args) { float x = ((Float) args[0]).floatValue(); float y = ((Float) args[1]).floatValue(); boolean relative = ((Boolean) args[2]).booleanValue(); try { ((Point) target).move(x, y, relative); } catch (Throwable t) { throw new InvocationTargetException(t); } return null; } }

Troubleshooting • Signal handler (SIGSEGV, SIGILL, SIGFPE…)

• Not all SEGV are fatal

– Polling page

– Implicit NullPointerException, StackOverflowError

• hs_err.log

– Threads, Stack frames, Memory map

– Registers, Top of stack, Instructions

– -XX:+UnlockDiagnosticVMOptions -XX:+PrintAssembly

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Still much to learn

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Thank you! • OpenJDK sources

– http://hg.openjdk.java.net

• Contacts

– andrey.pangin@odnoklassniki.ru

• Open Source @ Odnoklassniki

– https://github.com/odnoklassniki

• Career @ Odnoklassniki

– http://v.ok.ru

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