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Since Java 7 is going to use the new G1 garbage collection by default is Java going to be able to handle an order of magnitude larger heap without supposed "devastating" GC pause times? Has anybody actually implemented G1 in production, what were your experiences?
To be fair the only time I have seen really long GC pauses is on very large heaps, much more than a workstation would have. To clarify my question; will G1 open the gateway to heaps in the hundreds of GB? TB?
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G1 copies objects from one or more regions of the heap to a single region on the heap, and in the process both compacts and frees up memory. This evacuation is performed in parallel on multi-processors, to decrease pause times and increase throughput.
Explicitly Set the GC Algorithm For example on Java 8, the default GC is Parallel GC, while on Java 11 the default is G1 GC. This means that on upgrading your Java version, this will be changed on your behalf, for better or worse, unless you've explicitly set the GC.
One difference is that G1 is a compacting collector. Also, G1 offers more predictable garbage collection pauses than the CMS collector, and allows users to specify desired pause targets. As with CMS, G1 is designed for applications that require shorter GC pauses.
The default garbage collector in Java 11 is the G1 garbage collector (G1GC). The aim of G1GC is to strike a balance between latency and throughput. The G1 garbage collector attempts to achieve high throughput by meeting pause time goals with high probability.
I've been testing it out with a heavy application: 60-70GB allocated to heap, with 20-50GB in use at any time. With these sorts of applications, it's an understatement to say that your mileage may vary. I'm running JDK 1.6_22 on Linux. The minor versions are important-- before about 1.6_20, there were bugs in G1 that caused random NullPointerExceptions.
I've found that it is very good at keeping within the pause target you give it most of the time. The default appears to be a 100ms (0.1 second) pause, and I've been telling it to do half that (-XX:MaxGCPauseMillis=50). However, once it gets really low on memory, it panics and does a full stop-the-world garbage collection. With 65GB, that takes between 30 seconds and 2 minutes. (The number of CPUs probably doesn't make a difference; it's probably limited by the bus speed.)
Compared with CMS (which is not the default server GC, but it should be for web servers and other real-time applications), typical pauses are much more predictable and can be made much shorter. So far I'm having better luck with CMS for the huge pauses, but that may be random; I'm seeing them only a few times every 24 hours. I'm not sure which one will be more appropriate in my production environment at the moment, but probably G1. If Oracle keeps tuning it, I suspect G1 will ultimately be the clear winner.
If you're not having a problem with the existing garbage collectors, there's no reason to consider G1 right now. If you are running a low-latency application, such as a GUI application, G1 is probably the right choice, with MaxGCPauseMillis set really low. If you're running a batch-mode application, G1 doesn't buy you anything.
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