聊聊netty的maxDirectMemory

序

本文主要研究一下netty的maxDirectMemory

PlatformDependent

netty-common-4.1.33.Final-sources.jar!/io/netty/util/internal/PlatformDependent.java

public final class PlatformDependent {

    private static final InternalLogger logger = InternalLoggerFactory.getInstance(PlatformDependent.class);

    private static final Pattern MAX_DIRECT_MEMORY_SIZE_ARG_PATTERN = Pattern.compile(
            "//s*-XX:MaxDirectMemorySize//s*=//s*([0-9]+)//s*([kKmMgG]?)//s*$");

    private static final boolean IS_WINDOWS = isWindows0();
    private static final boolean IS_OSX = isOsx0();

    private static final boolean MAYBE_SUPER_USER;

    private static final boolean CAN_ENABLE_TCP_NODELAY_BY_DEFAULT = !isAndroid();

    private static final Throwable UNSAFE_UNAVAILABILITY_CAUSE = unsafeUnavailabilityCause0();
    private static final boolean DIRECT_BUFFER_PREFERRED;
    private static final long MAX_DIRECT_MEMORY = maxDirectMemory0();

    //......

    static {
        if (javaVersion() >= 7) {
            RANDOM_PROVIDER = new ThreadLocalRandomProvider() {
                @Override
                public Random current() {
                    return java.util.concurrent.ThreadLocalRandom.current();
                }
            };
        } else {
            RANDOM_PROVIDER = new ThreadLocalRandomProvider() {
                @Override
                public Random current() {
                    return ThreadLocalRandom.current();
                }
            };
        }

        // Here is how the system property is used:
        //
        // * <  0  - Don't use cleaner, and inherit max direct memory from java. In this case the
        //           "practical max direct memory" would be 2 * max memory as defined by the JDK.
        // * == 0  - Use cleaner, Netty will not enforce max memory, and instead will defer to JDK.
        // * >  0  - Don't use cleaner. This will limit Netty's total direct memory
        //           (note: that JDK's direct memory limit is independent of this).
        long maxDirectMemory = SystemPropertyUtil.getLong("io.netty.maxDirectMemory", -1);

        if (maxDirectMemory == 0 || !hasUnsafe() || !PlatformDependent0.hasDirectBufferNoCleanerConstructor()) {
            USE_DIRECT_BUFFER_NO_CLEANER = false;
            DIRECT_MEMORY_COUNTER = null;
        } else {
            USE_DIRECT_BUFFER_NO_CLEANER = true;
            if (maxDirectMemory < 0) {
                maxDirectMemory = MAX_DIRECT_MEMORY;
                if (maxDirectMemory <= 0) {
                    DIRECT_MEMORY_COUNTER = null;
                } else {
                    DIRECT_MEMORY_COUNTER = new AtomicLong();
                }
            } else {
                DIRECT_MEMORY_COUNTER = new AtomicLong();
            }
        }
        logger.debug("-Dio.netty.maxDirectMemory: {} bytes", maxDirectMemory);
        DIRECT_MEMORY_LIMIT = maxDirectMemory >= 1 ? maxDirectMemory : MAX_DIRECT_MEMORY;

        int tryAllocateUninitializedArray =
                SystemPropertyUtil.getInt("io.netty.uninitializedArrayAllocationThreshold", 1024);
        UNINITIALIZED_ARRAY_ALLOCATION_THRESHOLD = javaVersion() >= 9 && PlatformDependent0.hasAllocateArrayMethod() ?
                tryAllocateUninitializedArray : -1;
        logger.debug("-Dio.netty.uninitializedArrayAllocationThreshold: {}", UNINITIALIZED_ARRAY_ALLOCATION_THRESHOLD);

        MAYBE_SUPER_USER = maybeSuperUser0();

        if (!isAndroid()) {
            // only direct to method if we are not running on android.
            // See https://github.com/netty/netty/issues/2604
            if (javaVersion() >= 9) {
                CLEANER = CleanerJava9.isSupported() ? new CleanerJava9() : NOOP;
            } else {
                CLEANER = CleanerJava6.isSupported() ? new CleanerJava6() : NOOP;
            }
        } else {
            CLEANER = NOOP;
        }

        // We should always prefer direct buffers by default if we can use a Cleaner to release direct buffers.
        DIRECT_BUFFER_PREFERRED = CLEANER != NOOP
                                  && !SystemPropertyUtil.getBoolean("io.netty.noPreferDirect", false);
        if (logger.isDebugEnabled()) {
            logger.debug("-Dio.netty.noPreferDirect: {}", !DIRECT_BUFFER_PREFERRED);
        }

        /*
         * We do not want to log this message if unsafe is explicitly disabled. Do not remove the explicit no unsafe
         * guard.
         */
        if (CLEANER == NOOP && !PlatformDependent0.isExplicitNoUnsafe()) {
            logger.info(
                    "Your platform does not provide complete low-level API for accessing direct buffers reliably. " +
                    "Unless explicitly requested, heap buffer will always be preferred to avoid potential system " +
                    "instability.");
        }
    }

    private static long maxDirectMemory0() {
        long maxDirectMemory = 0;

        ClassLoader systemClassLoader = null;
        try {
            systemClassLoader = getSystemClassLoader();

            // When using IBM J9 / Eclipse OpenJ9 we should not use VM.maxDirectMemory() as it not reflects the
            // correct value.
            // See:
            //  - https://github.com/netty/netty/issues/7654
            String vmName = SystemPropertyUtil.get("java.vm.name", "").toLowerCase();
            if (!vmName.startsWith("ibm j9") &&
                    // https://github.com/eclipse/openj9/blob/openj9-0.8.0/runtime/include/vendor_version.h#L53
                    !vmName.startsWith("eclipse openj9")) {
                // Try to get from sun.misc.VM.maxDirectMemory() which should be most accurate.
                Class<?> vmClass = Class.forName("sun.misc.VM", true, systemClassLoader);
                Method m = vmClass.getDeclaredMethod("maxDirectMemory");
                maxDirectMemory = ((Number) m.invoke(null)).longValue();
            }
        } catch (Throwable ignored) {
            // Ignore
        }

        if (maxDirectMemory > 0) {
            return maxDirectMemory;
        }

        try {
            // Now try to get the JVM option (-XX:MaxDirectMemorySize) and parse it.
            // Note that we are using reflection because Android doesn't have these classes.
            Class<?> mgmtFactoryClass = Class.forName(
                    "java.lang.management.ManagementFactory", true, systemClassLoader);
            Class<?> runtimeClass = Class.forName(
                    "java.lang.management.RuntimeMXBean", true, systemClassLoader);

            Object runtime = mgmtFactoryClass.getDeclaredMethod("getRuntimeMXBean").invoke(null);

            @SuppressWarnings("unchecked")
            List<String> vmArgs = (List<String>) runtimeClass.getDeclaredMethod("getInputArguments").invoke(runtime);
            for (int i = vmArgs.size() - 1; i >= 0; i --) {
                Matcher m = MAX_DIRECT_MEMORY_SIZE_ARG_PATTERN.matcher(vmArgs.get(i));
                if (!m.matches()) {
                    continue;
                }

                maxDirectMemory = Long.parseLong(m.group(1));
                switch (m.group(2).charAt(0)) {
                    case 'k': case 'K':
                        maxDirectMemory *= 1024;
                        break;
                    case 'm': case 'M':
                        maxDirectMemory *= 1024 * 1024;
                        break;
                    case 'g': case 'G':
                        maxDirectMemory *= 1024 * 1024 * 1024;
                        break;
                }
                break;
            }
        } catch (Throwable ignored) {
            // Ignore
        }

        if (maxDirectMemory <= 0) {
            maxDirectMemory = Runtime.getRuntime().maxMemory();
            logger.debug("maxDirectMemory: {} bytes (maybe)", maxDirectMemory);
        } else {
            logger.debug("maxDirectMemory: {} bytes", maxDirectMemory);
        }

        return maxDirectMemory;
    }

    /**
     * Returns the maximum memory reserved for direct buffer allocation.
     */
    public static long maxDirectMemory() {
        return DIRECT_MEMORY_LIMIT;
    }

    //......
}

VM.maxDirectMemory是读取-XX:MaxDirectMemorySize配置，如果有设置且大于0则使用该值，如果没有设置该参数则默认值为0，则默认是取的Runtime.getRuntime().maxMemory()
io.netty.maxDirectMemory

ByteBuffer.allocateDirect

java.base/java/nio/ByteBuffer.java

public abstract class ByteBuffer
    extends Buffer
    implements Comparable<ByteBuffer>
{

    //......

    /**
     * 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, each of its elements will be
     * initialized to zero, and its byte order will be
     * {@link ByteOrder#BIG_ENDIAN BIG_ENDIAN}.  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 {@code capacity} is a negative integer
     */
    public static ByteBuffer allocateDirect(int capacity) {
        return new DirectByteBuffer(capacity);
    }

    //......
}

ByteBuffer.allocateDirect方法实际是创建了DirectByteBuffer

DirectByteBuffer

java.base/java/nio/DirectByteBuffer.java

class DirectByteBuffer extends MappedByteBuffer implements DirectBuffer {
    //......

    // Primary constructor
    //
    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;

    }

    //......
}

DirectByteBuffer的构造器里头会调用Bits.reserveMemory，出现OutOfMemoryError，则调用Bits.unreserveMemory(size, cap)，然后抛出OutOfMemoryError

Bits.reserveMemory

java.base/java/nio/Bits.java

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

class Bits {                            // package-private

    private Bits() { }

    // -- 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, int 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("Direct buffer memory");

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

    private static boolean tryReserveMemory(long size, int 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;
    }

    //......
}

• Bits.reserveMemory方法会先调用tryReserveMemory尝试分配direct memory，不成功则继续往下执行do while(refprocActive)
• refprocActive这段循环是不断尝试allocation直到分配成功，或者直到没有引用来处理且分配失败
• 如果refprocActive循环没有分配成功，则调用System.gc()，然后进入最后一段循环尝试分配；最后这段循环如果分配成功则返回，分配不成功且sleeps大于等于MAX_SLEEPS，则跳出循环，最后抛出OutOfMemoryError("Direct buffer memory")异常

小结

VM.maxDirectMemory是读取-XX:MaxDirectMemorySize配置，如果有设置且大于0则使用该值，如果没有设置该参数则默认值为0，则默认是取的Runtime.getRuntime().maxMemory()
io.netty.maxDirectMemory

doc

• 聊聊jvm的-XX:MaxDirectMemorySize
• In Netty 4, do I need to set option -XX:MaxDirectMemorySize?
• Netty之Java堆外内存扫盲贴
• 直播一次问题排查过程
• Change default value of io.netty.maxDirectMemory ? #6349
• LEAK: ByteBuf.release() was not called before it's garbage-collected #422