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。
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