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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：
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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();
}
}
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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
分类: 填坑心得, JVM
