默认情况下，并不是等堆内存耗尽，才会报 OutOfMemoryError，而是如果 JVM 觉得 GC 效率不高，也会报这个错误。
那么怎么评价 GC 效率不高呢？来看下源码： 呢？来看下源码gcOverheadChecker.cpp：
void GCOverheadChecker::check_gc_overhead_limit(GCOverheadTester* time_overhead, GCOverheadTester* space_overhead, bool is_full_gc, GCCause::Cause gc_cause, SoftRefPolicy* soft_ref_policy) { // 忽略显式gc命令，比如System.gc()，或者通过JVMTI命令的gc，或者通过jcmd命令的gc if (GCCause::is_user_requested_gc(gc_cause) || GCCause::is_serviceability_requested_gc(gc_cause)) { return; } bool print_gc_overhead_limit_would_be_exceeded = false; if (is_full_gc) { //如果gc时间过长，并且gc回收的空间还是不多 //gc时间占用98%以上为gc时间过长，可以通过 -XX:GCTimeLimit= 配置，参考gc_globals.hpp: GCTimeLimit //回收空间小于2%为gc回收空间不多，可以通过 -XX:GCHeapFreeLimit= 配置，参考gc_globals.hpp: GCHeapFreeLimit if (time_overhead->is_exceeded() && space_overhead->is_exceeded()) { _gc_overhead_limit_count++; //如果UseGCOverheadLimit这个状态位为开启 //默认情况下，是开启的，可以通过启动参数-XX:-UseGCOverheadLimit关闭，参考：gc_globals.hpp: UseGCOverheadLimit if (UseGCOverheadLimit) { //如果超过规定次数，这个次数默认不可配置，必须开启develop编译jdk才能配置，参考gc_globals.hpp: GCOverheadLimitThreshold if (_gc_overhead_limit_count >= GCOverheadLimitThreshold){ //设置状态位，准备抛出OOM set_gc_overhead_limit_exceeded(true); //清空计数 reset_gc_overhead_limit_count(); } else { //如果还没到达次数，但是也快到达的时候，清空所有的软引用 bool near_limit = gc_overhead_limit_near(); if (near_limit) { soft_ref_policy->set_should_clear_all_soft_refs(true); log_trace(gc, ergo)("Nearing GC overhead limit, will be clearing all SoftReference"); } } } //需要打印日志，提示GC效率不高 print_gc_overhead_limit_would_be_exceeded = true; } else { // Did not exceed overhead limits reset_gc_overhead_limit_count(); } } if (UseGCOverheadLimit) { if (gc_overhead_limit_exceeded()) { log_trace(gc, ergo)("GC is exceeding overhead limit of " UINTX_FORMAT "%%", GCTimeLimit); reset_gc_overhead_limit_count(); } else if (print_gc_overhead_limit_would_be_exceeded) { assert(_gc_overhead_limit_count > 0, "Should not be printing"); log_trace(gc, ergo)("GC would exceed overhead limit of " UINTX_FORMAT "%% %d consecutive time(s)", GCTimeLimit, _gc_overhead_limit_count); } } }
默认配置：gc_globals.hpp
product(bool, UseGCOverheadLimit, true, \ "Use policy to limit of proportion of time spent in GC " \ "before an OutOfMemory error is thrown") \ \ product(uintx, GCTimeLimit, 98, \ "Limit of the proportion of time spent in GC before " \ "an OutOfMemoryError is thrown (used with GCHeapFreeLimit)") \ range(0, 100) \ \ product(uintx, GCHeapFreeLimit, 2, \ "Minimum percentage of free space after a full GC before an " \ "OutOfMemoryError is thrown (used with GCTimeLimit)") \ range(0, 100) \ \ develop(uintx, GCOverheadLimitThreshold, 5, \ "Number of consecutive collections before gc time limit fires") \ range(1, max_uintx)
可以总结出：默认情况下，启用了 UseGCOverheadLimit，连续 5 次，碰到 GC 时间占比超过 98%，GC 回收的内存不足 2% 时，会抛出这个异常。
