Back to index...

	/*
	* Copyright (c) 2000, 2021, Oracle and/or its affiliates. All rights reserved.
	* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
	*
	* This code is free software; you can redistribute it and/or modify it
	* under the terms of the GNU General Public License version 2 only, as
	* published by the Free Software Foundation. Oracle designates this
	* particular file as subject to the "Classpath" exception as provided
	* by Oracle in the LICENSE file that accompanied this code.
	*
	* This code is distributed in the hope that it will be useful, but WITHOUT
	* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
	* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
	* version 2 for more details (a copy is included in the LICENSE file that
	* accompanied this code).
	*
	* You should have received a copy of the GNU General Public License version
	* 2 along with this work; if not, write to the Free Software Foundation,
	* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
	*
	* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
	* or visit www.oracle.com if you need additional information or have any
	* questions.
	*/

	package java.nio;

	import jdk.internal.access.JavaLangRefAccess;
	import jdk.internal.access.SharedSecrets;
	import jdk.internal.misc.Unsafe;
	import jdk.internal.misc.VM;
	import jdk.internal.misc.VM.BufferPool;

	import java.util.concurrent.atomic.AtomicLong;

	/**
	* Access to bits, native and otherwise.
	*/

	class Bits { // package-private

	private Bits() { }


	// -- Swapping --

	static short swap(short x) {
	return Short.reverseBytes(x);
	}

	static char swap(char x) {
	return Character.reverseBytes(x);
	}

	static int swap(int x) {
	return Integer.reverseBytes(x);
	}

	static long swap(long x) {
	return Long.reverseBytes(x);
	}


	// -- Unsafe access --

	private static final Unsafe UNSAFE = Unsafe.getUnsafe();

	// -- Processor and memory-system properties --

	private static int PAGE_SIZE = -1;

	static int pageSize() {
	if (PAGE_SIZE == -1)
	PAGE_SIZE = UNSAFE.pageSize();
	return PAGE_SIZE;
	}

	static long pageCount(long size) {
	return (size + (long)pageSize() - 1L) / pageSize();
	}

	private static boolean UNALIGNED = UNSAFE.unalignedAccess();

	static boolean unaligned() {
	return UNALIGNED;
	}


	// -- Direct memory management --

	// A user-settable upper limit on the maximum amount of allocatable
	// direct buffer memory. This value may be changed during VM
	// initialization if it is launched with "-XX:MaxDirectMemorySize=<size>".
	private static volatile long MAX_MEMORY = VM.maxDirectMemory();
	private static final AtomicLong RESERVED_MEMORY = new AtomicLong();
	private static final AtomicLong TOTAL_CAPACITY = new AtomicLong();
	private static final AtomicLong COUNT = new AtomicLong();
	private static volatile boolean MEMORY_LIMIT_SET;

	// max. number of sleeps during try-reserving with exponentially
	// increasing delay before throwing OutOfMemoryError:
	// 1, 2, 4, 8, 16, 32, 64, 128, 256 (total 511 ms ~ 0.5 s)
	// which means that OOME will be thrown after 0.5 s of trying
	private static final int MAX_SLEEPS = 9;

	// These methods should be called whenever direct memory is allocated or
	// freed. They allow the user to control the amount of direct memory
	// which a process may access. All sizes are specified in bytes.
	static void reserveMemory(long size, long cap) {

	if (!MEMORY_LIMIT_SET && VM.initLevel() >= 1) {
	MAX_MEMORY = VM.maxDirectMemory();
	MEMORY_LIMIT_SET = true;
	}

	// optimist!
	if (tryReserveMemory(size, cap)) {
	return;
	}

	final JavaLangRefAccess jlra = SharedSecrets.getJavaLangRefAccess();
	boolean interrupted = false;
	try {

	// Retry allocation until success or there are no more
	// references (including Cleaners that might free direct
	// buffer memory) to process and allocation still fails.
	boolean refprocActive;
	do {
	try {
	refprocActive = jlra.waitForReferenceProcessing();
	} catch (InterruptedException e) {
	// Defer interrupts and keep trying.
	interrupted = true;
	refprocActive = true;
	}
	if (tryReserveMemory(size, cap)) {
	return;
	}
	} while (refprocActive);

	// trigger VM's Reference processing
	System.gc();

	// A retry loop with exponential back-off delays.
	// Sometimes it would suffice to give up once reference
	// processing is complete. But if there are many threads
	// competing for memory, this gives more opportunities for
	// any given thread to make progress. In particular, this
	// seems to be enough for a stress test like
	// DirectBufferAllocTest to (usually) succeed, while
	// without it that test likely fails. Since failure here
	// ends in OOME, there's no need to hurry.
	long sleepTime = 1;
	int sleeps = 0;
	while (true) {
	if (tryReserveMemory(size, cap)) {
	return;
	}
	if (sleeps >= MAX_SLEEPS) {
	break;
	}
	try {
	if (!jlra.waitForReferenceProcessing()) {
	Thread.sleep(sleepTime);
	sleepTime <<= 1;
	sleeps++;
	}
	} catch (InterruptedException e) {
	interrupted = true;
	}
	}

	// no luck
	throw new OutOfMemoryError
	("Cannot reserve "
	+ size + " bytes of direct buffer memory (allocated: "
	+ RESERVED_MEMORY.get() + ", limit: " + MAX_MEMORY +")");

	} finally {
	if (interrupted) {
	// don't swallow interrupts
	Thread.currentThread().interrupt();
	}
	}
	}

	private static boolean tryReserveMemory(long size, long cap) {

	// -XX:MaxDirectMemorySize limits the total capacity rather than the
	// actual memory usage, which will differ when buffers are page
	// aligned.
	long totalCap;
	while (cap <= MAX_MEMORY - (totalCap = TOTAL_CAPACITY.get())) {
	if (TOTAL_CAPACITY.compareAndSet(totalCap, totalCap + cap)) {
	RESERVED_MEMORY.addAndGet(size);
	COUNT.incrementAndGet();
	return true;
	}
	}

	return false;
	}


	static void unreserveMemory(long size, long cap) {
	long cnt = COUNT.decrementAndGet();
	long reservedMem = RESERVED_MEMORY.addAndGet(-size);
	long totalCap = TOTAL_CAPACITY.addAndGet(-cap);
	assert cnt >= 0 && reservedMem >= 0 && totalCap >= 0;
	}

	static final BufferPool BUFFER_POOL = new BufferPool() {
	@Override
	public String getName() {
	return "direct";
	}
	@Override
	public long getCount() {
	return Bits.COUNT.get();
	}
	@Override
	public long getTotalCapacity() {
	return Bits.TOTAL_CAPACITY.get();
	}
	@Override
	public long getMemoryUsed() {
	return Bits.RESERVED_MEMORY.get();
	}
	};

	// These numbers represent the point at which we have empirically
	// determined that the average cost of a JNI call exceeds the expense
	// of an element by element copy. These numbers may change over time.
	static final int JNI_COPY_TO_ARRAY_THRESHOLD = 6;
	static final int JNI_COPY_FROM_ARRAY_THRESHOLD = 6;
	}

Back to index...