如何在某些代码执行之间禁用 GC 或获取 GC 暂停时间？

Question

I read a article before that FGC influenced timing and made application produce wrong result.
The code example is like:

long start = System.currentTimeInMillis();
doSomething();
// what if here comes a long time FGC pause
long end = System.currentTimeInMillis();
if (end - start > TIME_OUT) {
    xxxx
}

if GC happens between start and end assignment,the time will be more than the actual value it is and affect the result.

There are some ways to fix.
One way is to disable GC between them,I know it's impossible to disable GC a while now.I want to know if there any way to remove safepoint between some code(or to refactor the code to achieve it?).

The other is to get GC pause time, seems (maybe) not harder than the first way.

Back to the question:
If I have a time sensitive logic, how can I avoid it to be affected by GC?

Answer 1

You can do few things here (in increasing order of difficulty and you can combine some of these):

1. Run your tests with as big heap as you can muster (it will work only if you can avoid any GC; as soon as you hit GC, it will be a long pause).

2. Use better timing than wall clock. See Java Microbenchmark Harness

3. Use many runs to average impact of any GC out by removing the top outliers. JMH from point above helps here.

4. Ask your JVM how long it spent GC using MX Beans . YMMV depending on JVM.

5. Log all GC pauses into GC log and use these in calculation

6. Modify your code to do no allocations in the critical section (see this or this article for some tips)

7. Azul Systems's JVM called ZING has pauseless GC . This will cost money.