java.lang.OutOfMemoryError：GC overhead limit exceeded填坑心得

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我遇到这样的问题，本地部署时抛出异常java.lang.OutOfMemoryError：GC overhead limit exceeded导致服务起不来，查看日志发现加载了太多资源到内存，本地的性能也不好，gc时间消耗的较多。解决这种问题两种方法是，增加参数，-XX:-UseGCOverheadLimit，关闭这个特性，同时增加heap大小，-Xmx1024m。坑填了，but why？

OOM大家都知道，就是JVM内存溢出了，那GC overhead limit exceed呢？

GC overhead limt exceed检查是Hotspot VM 1.6定义的一个策略，通过统计GC时间来预测是否要OOM了，提前抛出异常，防止OOM发生。Sun 官方对此的定义是：“并行/并发回收器在GC回收时间过长时会抛出OutOfMemroyError。过长的定义是，超过98%的时间用来做GC并且回收了不到2%的堆内存。用来避免内存过小造成应用不能正常工作。“

听起来没啥用...预测OOM有啥用？起初开来这玩意只能用来Catch住释放内存资源，避免应用挂掉。后来发现一般情况下这个策略不能拯救你的应用，但是可以在应用挂掉之前做最后的挣扎，比如数据保存或者保存现场（Heap Dump）。

而且有些时候这个策略还会带来问题，比如加载某个大的内存数据时频繁OOM。

假如你也生产环境中遇到了这个问题，在不知道原因时不要简单的猜测和规避。可以通过-verbose:gc -XX:+PrintGCDetails看下到底什么原因造成了异常。通常原因都是因为old区占用过多导致频繁Full GC，最终导致GC overhead limit exceed。如果gc log不够可以借助于JProfile等工具查看内存的占用，old区是否有内存泄露。分析内存泄露还有一个方法-XX:+HeapDumpOnOutOfMemoryError，这样OOM时会自动做Heap Dump，可以拿MAT来排查了。还要留意young区，如果有过多短暂对象分配，可能也会抛这个异常。

日志的信息不难理解，就是每次gc时打条日志，记录GC的类型，前后大小和时间。举个例子。

33.125: [GC [DefNew: 16000K->16000K(16192K), 0.0000574 secs][Tenured: 2973K->2704K(16384K), 0.1012650 secs] 18973K->2704K(32576K), 0.1015066 secs]

100.667:[Full GC [Tenured: 0K->210K(10240K), 0.0149142 secs] 4603K->210K(19456K), [Perm : 2999K->2999K(21248K)], 0.0150007 secs] 

GC和Full GC代表gc的停顿类型，Full GC代表stop-the-world。箭头两边是gc前后的区空间大小，分别是young区、tenured区和perm区，括号里是该区的总大小。冒号前面是gc发生的时间，单位是秒，从jvm启动开始计算。DefNew代表Serial收集器，为Default New Generation的缩写，类似的还有PSYoungGen，代表Parallel Scavenge收集器。这样可以通过分析日志找到导致GC overhead limit exceeded的原因，通过调节相应的参数解决问题。

文中涉及到的名词解释，

Eden Space：堆内存池，大多数对象在这里分配内存空间。

Survivor Space：堆内存池，存储在Eden Space的gc中存活下来的对象。

Tenured Generation：堆内存池，存储Survivor Space中存活过几次gc的对象。

Permanent Generation：非堆空间，存储的是class和method对象。

Code Cache：非堆空间，JVM用来存储编译和存储native code。

最后附上GC overhead limit exceed HotSpot的实现：

  bool print_gc_overhead_limit_would_be_exceeded = false;
  if (is_full_gc) {
    if (gc_cost() > gc_cost_limit &&
      free_in_old_gen < (size_t) mem_free_old_limit &&
      free_in_eden < (size_t) mem_free_eden_limit) {
      // Collections, on average, are taking too much time, and
      //      gc_cost() > gc_cost_limit
      // we have too little space available after a full gc.
      //      total_free_limit < mem_free_limit
      // where
      //   total_free_limit is the free space available in
      //     both generations
      //   total_mem is the total space available for allocation
      //     in both generations (survivor spaces are not included
      //     just as they are not included in eden_limit).
      //   mem_free_limit is a fraction of total_mem judged to be an
      //     acceptable amount that is still unused.
      // The heap can ask for the value of this variable when deciding
      // whether to thrown an OutOfMemory error.
      // Note that the gc time limit test only works for the collections
      // of the young gen + tenured gen and not for collections of the
      // permanent gen.  That is because the calculation of the space
      // freed by the collection is the free space in the young gen +
      // tenured gen.
      // At this point the GC overhead limit is being exceeded.
      inc_gc_overhead_limit_count();
      if (UseGCOverheadLimit) {
        if (gc_overhead_limit_count() >=
            AdaptiveSizePolicyGCTimeLimitThreshold){
          // All conditions have been met for throwing an out-of-memory
          set_gc_overhead_limit_exceeded(true);
          // Avoid consecutive OOM due to the gc time limit by resetting
          // the counter.
          reset_gc_overhead_limit_count();
        } else {
          // The required consecutive collections which exceed the
          // GC time limit may or may not have been reached. We
          // are approaching that condition and so as not to
          // throw an out-of-memory before all SoftRef's have been
          // cleared, set _should_clear_all_soft_refs in CollectorPolicy.
          // The clearing will be done on the next GC.
          bool near_limit = gc_overhead_limit_near();
          if (near_limit) {
            collector_policy->set_should_clear_all_soft_refs(true);
            if (PrintGCDetails && Verbose) {
              gclog_or_tty->print_cr("  Nearing GC overhead limit, "
                "will be clearing all SoftReference");
            }
          }
        }
      }
      // Set this even when the overhead limit will not
      // cause an out-of-memory.  Diagnostic message indicating
      // that the overhead limit is being exceeded is sometimes
      // printed.
      print_gc_overhead_limit_would_be_exceeded = true;

    } else {
      // Did not exceed overhead limits
      reset_gc_overhead_limit_count();
    }
  }

参照&延伸阅读：

http://javaeesupportpatterns.blogspot.com/2012/01/gc-overhead-limit-exceeded-understand.html

http://www.oracle.com/technetwork/java/javase/gc-tuning-6-140523.html

http://reins.altervista.org/java/gc1.4.2_example.html

http://stackoverflow.com/questions/2129044/java-heap-terminology-young-old-and-permanent-generations

http://book.51cto.com/art/201306/399236.htm

https://blogs.oracle.com/jonthecollector/entry/presenting_the_permanent_generation