netty的direct memory大小设置

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最近遇到一个 netty 的 OutOfDirectMemoryError 报错，是在分配 direct memory 时内存不足导致的，看了下报错提示，要分配的内存大小为 16M，剩余的空间不足。这里 max direct memory 大约有 7G，于是就有一个疑问，这个值是怎么设置的？

代码分析

这里使用的 netty 版本是 4.1.14.Final，如下是报错时的调用栈信息，主要关注下 PlatformDependent 这个类。

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Caused by: io.netty.util.internal.OutOfDirectMemoryError: failed to allocate 16777216 byte(s) of direct memory (used: 7532970287, max: 7549747200)
	at io.netty.util.internal.PlatformDependent.incrementMemoryCounter(PlatformDependent.java:618)
	at io.netty.util.internal.PlatformDependent.allocateDirectNoCleaner(PlatformDependent.java:572)
	at io.netty.buffer.PoolArena$DirectArena.allocateDirect(PoolArena.java:764)
	at io.netty.buffer.PoolArena$DirectArena.newChunk(PoolArena.java:740)
	at io.netty.buffer.PoolArena.allocateNormal(PoolArena.java:244)
	at io.netty.buffer.PoolArena.allocate(PoolArena.java:226)
	at io.netty.buffer.PoolArena.allocate(PoolArena.java:146)
	at io.netty.buffer.PooledByteBufAllocator.newDirectBuffer(PooledByteBufAllocator.java:324)
	at io.netty.buffer.AbstractByteBufAllocator.directBuffer(AbstractByteBufAllocator.java:181)
	at io.netty.buffer.AbstractByteBufAllocator.buffer(AbstractByteBufAllocator.java:117)

找到 PlatformDependent 的第 572 行，位于 allocateDirectNoCleaner 函数内，它的功能是根据指定的容量（capacity）分配一个新的 ByteBuffer

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/**
 * Allocate a new {@link ByteBuffer} with the given {@code capacity}. {@link ByteBuffer}s allocated with
 * this method <strong>MUST</strong> be deallocated via {@link #freeDirectNoCleaner(ByteBuffer)}.
 */
public static ByteBuffer allocateDirectNoCleaner(int capacity) {
    assert USE_DIRECT_BUFFER_NO_CLEANER;

    // 第572行
    // 增加已使用内存计数，若内存不足，直接抛出异常
    incrementMemoryCounter(capacity);
    try {
        // 分配内存
        return PlatformDependent0.allocateDirectNoCleaner(capacity);
    } catch (Throwable e) {
        // 分配失败，减小已使用内存计数
        decrementMemoryCounter(capacity);
        throwException(e);
        return null;
    }
}

查看第 572 行对应的 incrementMemoryCounter 实现，它的功能是增加已使用内存的计数

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private static void incrementMemoryCounter(int capacity) {
    if (DIRECT_MEMORY_COUNTER != null) {
        for (;;) {
            // 获取当前已使用内存计数
            long usedMemory = DIRECT_MEMORY_COUNTER.get();
          
            // 计算新的已使用内存计数
            long newUsedMemory = usedMemory + capacity;
          
            // 超过了最大的容量限制，抛出异常
            if (newUsedMemory > DIRECT_MEMORY_LIMIT) {
                throw new OutOfDirectMemoryError("failed to allocate " + capacity
                        + " byte(s) of direct memory (used: " + usedMemory + ", max: " + DIRECT_MEMORY_LIMIT + ')');
            }
          
            // CAS更新计数值
            if (DIRECT_MEMORY_COUNTER.compareAndSet(usedMemory, newUsedMemory)) {
                break;
            }
        }
    }
}

从代码逻辑可见，它是通过 CAS 更新已使用内存计数。在更新前先判断是否超过了 DIRECT_MEMORY_LIMIT 最大容量限制，若已超过则直接抛出异常，也就是说此时并未真正地分配内存。

这里就有个问题，DIRECT_MEMORY_LIMIT 是怎么设置的？

搜索代码发现，它是在 PlatformDependent 的静态代码块中设置的，代码如下：

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// 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).
// 默认未设置，所以maxDirectMemory值为-1
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) {
        // 取值逻辑在这里
        maxDirectMemory = maxDirectMemory0();
        if (maxDirectMemory <= 0) {
            DIRECT_MEMORY_COUNTER = null;
        } else {
            DIRECT_MEMORY_COUNTER = new AtomicLong();
        }
    } else {
        DIRECT_MEMORY_COUNTER = new AtomicLong();
    }
}
DIRECT_MEMORY_LIMIT = maxDirectMemory;
logger.debug("-Dio.netty.maxDirectMemory: {} bytes", maxDirectMemory);

首先取 io.netty.maxDirectMemory 属性值，根据它的不同取值有如下含义：

  • < 0，不使用清理器（cleaner），从 java 继承 max direct memory 设置
  • == 0，使用清理器（cleaner），netty 不会强制最大内存，而是使用 jdk 设置
  • > 0，不使用清理器（cleaner），表示 netty 的最大 direct memory 限制

它的默认值是 -1，根据代码逻辑会执行到 maxDirectMemory = maxDirectMemory0() 这行，该方法的实现如下：

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

private static long maxDirectMemory0() {
    long maxDirectMemory = 0;
    ClassLoader systemClassLoader = null;
    try {
        // 1. 通过反射调用sun.misc.VM.maxDirectMemory()
        systemClassLoader = getSystemClassLoader();
        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 {
        // 2. 通过MBean获取-XX:MaxDirectMemorySize配置，因为Android没有这些类，所以使用反射获取
        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) {
        // 3. 仍未获取到，则从Runtime获取
        maxDirectMemory = Runtime.getRuntime().maxMemory();
        logger.debug("maxDirectMemory: {} bytes (maybe)", maxDirectMemory);
    } else {
        logger.debug("maxDirectMemory: {} bytes", maxDirectMemory);
    }

    return maxDirectMemory;
}

它的逻辑为：

  1. 通过反射调用 sun.misc.VM.maxDirectMemory()，若取到则返回
  2. 否则，获取 -XX:MaxDirectMemorySize 配置，若取到则返回
  3. 否则，调用 Runtime.getRuntime().maxMemory() 获取

结论

根据以上的分析，要设置 direct memory 的最大容量，既可以通过 netty 的 io.netty.maxDirectMemory 属性配置，也可以通过 jvm 的 -XX:MaxDirectMemorySize 参数设置，其中前者的优先级更高。

默认情况下，上面两项均未配置，则是通过 sun.misc.VM.maxDirectMemory() 获取 direct memory 的最大容量。

测试

通过如下程序验证上面的结论

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public class DirectMemoryLimit {
    public static void main(String[] args) throws Exception {

        Long directMemoryLimit = getDirectMemoryLimit();

        System.out.println("directMemoryLimit: " + directMemoryLimit + " byte");
    }

    private static Long getDirectMemoryLimit() throws ClassNotFoundException, NoSuchFieldException, IllegalAccessException {
        Class<?> clazz = Class.forName("io.netty.util.internal.PlatformDependent");
        if (clazz == null) {
            return null;
        }

        Field directMemoryLimit = clazz.getDeclaredField("DIRECT_MEMORY_LIMIT");
        if (directMemoryLimit == null) {
            return null;
        }

        directMemoryLimit.setAccessible(true);

        // 取DIRECT_MEMORY_LIMIT的值
        return directMemoryLimit.getLong(clazz);
    }
}

通过jvm设置

设置 jvm 参数 -XX:MaxDirectMemorySize=20m ，执行并查看结果

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directMemoryLimit: 20971520 byte

其中 20M byte = 20 * 1024 * 1024 byte = 20971520 byte

通过netty设置

通过设置 io.netty.maxDirectMemory 属性，覆盖 -XX:MaxDirectMemorySize 配置的大小。

增加 jvm 参数 -Dio.netty.maxDirectMemory=1024，执行并查看结果

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directMemoryLimit: 1024 byte

去掉设置项

去掉上面的两个设置，再次执行并查看结果

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directMemoryLimit: 1908932608 byte

增加如下代码

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System.out.println("maxMemory: " + Runtime.getRuntime().maxMemory());

执行并查看输出结果

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maxMemory: 1908932608

可见默认情况下，通过 sun.misc.VM.maxDirectMemory() 获取并设置的 DIRECT_MEMORY_LIMIT 取值与 Runtime.getRuntime().maxMemory() 一致

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public static void saveAndRemoveProperties(Properties var0) {
    if (booted) {
        throw new IllegalStateException("System initialization has completed");
    } else {
        savedProps.putAll(var0);
        String var1 = (String)var0.remove("sun.nio.MaxDirectMemorySize");
        if (var1 != null) {
            if (var1.equals("-1")) {
                // sun.nio.MaxDirectMemorySize的配置值不是-1时
                directMemory = Runtime.getRuntime().maxMemory();
            } else {
                long var2 = Long.parseLong(var1);
                if (var2 > -1L) {
                    directMemory = var2;
                }
            }
        }

        var1 = (String)var0.remove("sun.nio.PageAlignDirectMemory");
        if ("true".equals(var1)) {
            pageAlignDirectMemory = true;
        }

        var1 = var0.getProperty("sun.lang.ClassLoader.allowArraySyntax");
        allowArraySyntax = var1 == null ? defaultAllowArraySyntax : Boolean.parseBoolean(var1);
        var0.remove("java.lang.Integer.IntegerCache.high");
        var0.remove("sun.zip.disableMemoryMapping");
        var0.remove("sun.java.launcher.diag");
        var0.remove("sun.cds.enableSharedLookupCache");
    }
}

Runtime.getRuntime().maxMemory()

这是一个 native 方法，从注释来看，它用于获取 java 虚拟机的最大可用内存。因为新生代里的 survivor 区采用的是复制算法，其可用空间只有一个 survivor 区大小，所以 java 堆总的可用空间大小为：老年代大小 + 新生代大小 - 一个 survivor 区大小

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/**
 * Returns the maximum amount of memory that the Java virtual machine will
 * attempt to use.  If there is no inherent limit then the value {@link
 * java.lang.Long#MAX_VALUE} will be returned.
 *
 * @return  the maximum amount of memory that the virtual machine will
 *          attempt to use, measured in bytes
 * @since 1.4
 */
public native long maxMemory();

到 jdk 源码找到该方法的 native 实现（jdk/src/share/native/java/lang/Runtime.c）

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JNIEXPORT jlong JNICALL
Java_java_lang_Runtime_maxMemory(JNIEnv *env, jobject this)
{
    return JVM_MaxMemory();
}

它只是一个入口，具体实现在 hotspot 源码中（hotspot/src/share/vm/prims/jvm.cpp）

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JVM_ENTRY_NO_ENV(jlong, JVM_MaxMemory(void))
  JVMWrapper("JVM_MaxMemory");
  // 计算 MaxMemory
  size_t n = Universe::heap()->max_capacity();
  return convert_size_t_to_jlong(n);
JVM_END

到 Universe.hpp 里找到 heap() 的实现。它返回了一个 CollectedHeap 类型的静态属性 _collectedHeap，该静态属性是在 initialize_heap() 里初始化的。根据 GC 策略的不同，_collectedHeap 被初始化为不同的实现。

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// The particular choice of collected heap.
static CollectedHeap* heap() { return _collectedHeap; }

// 初始化代码
jint Universe::initialize_heap() {
  if (UseParallelGC) {
#if INCLUDE_ALL_GCS
    Universe::_collectedHeap = new ParallelScavengeHeap();
#else  // INCLUDE_ALL_GCS
    fatal("UseParallelGC not supported in this VM.");
#endif // INCLUDE_ALL_GCS

  } else if (UseG1GC) {
#if INCLUDE_ALL_GCS
    G1CollectorPolicyExt* g1p = new G1CollectorPolicyExt();
    g1p->initialize_all();
    G1CollectedHeap* g1h = new G1CollectedHeap(g1p);
    Universe::_collectedHeap = g1h;
#else  // INCLUDE_ALL_GCS
    fatal("UseG1GC not supported in java kernel vm.");
#endif // INCLUDE_ALL_GCS

  } else {
    GenCollectorPolicy *gc_policy;

    if (UseSerialGC) {
      gc_policy = new MarkSweepPolicy();
    } else if (UseConcMarkSweepGC) {
#if INCLUDE_ALL_GCS
      if (UseAdaptiveSizePolicy) {
        gc_policy = new ASConcurrentMarkSweepPolicy();
      } else {
        gc_policy = new ConcurrentMarkSweepPolicy();
      }
#else  // INCLUDE_ALL_GCS
    fatal("UseConcMarkSweepGC not supported in this VM.");
#endif // INCLUDE_ALL_GCS
    } else { // default old generation
      gc_policy = new MarkSweepPolicy();
    }
    gc_policy->initialize_all();

    // 使用CMS GC策略时的实现
    Universe::_collectedHeap = new GenCollectedHeap(gc_policy);
  }

打开 GenCollectedHeap，找到 max_capacity() 的实现。它是将各个分代的最大容量相加。

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size_t GenCollectedHeap::max_capacity() const {
  size_t res = 0;
  for (int i = 0; i < _n_gens; i++) {
    res += _gens[i]->max_capacity();
  }
  return res;
}

剩下的源码还没搞清楚，有空再补。。。

吃吃吃

参考

https://stackoverflow.com/questions/52980629/runtime-getruntime-maxmemory-calculate-method

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