Long GC pauses in application

Question

I am currently running an application which requires a maximum heap size of 16GB.

Currently I use the following flags to handle garbage collection.

-XX\:+UseParNewGC, -XX\:+UseConcMarkSweepGC, -XX:CMSInitiatingOccupancyFraction=50, -XX\:+DisableExplicitGC, -XX\:+PrintGCDateStamps, -XX\:+PrintGCDetails, -Xloggc\:/home/user/logs/gc.log

However, I have noticed that during some garbage collections, the application locks up for a few seconds and then carries on - This is completely unacceptable as it's a game server.

An exert from my garbage collection logs can be found here.

Any advice on what I should change in order to reduce these long pauses would be greatly appreciated.

Answer 1

Any advice on what I should change in order to reduce these long pauses would be greatly appreciated.

The chances are that the CMS GC cannot keep up with the amount of garbage your system is generating. But the work that the GC has to perform is actually more closely related to the amount of NON-garbage that your system is retaining.

So ...

• Try to reduce the actual memory usage of your application; e.g. by not caching so much stuff, or reducing the size of your "world".
• Try to reduce the rate at which your application generates garbage.
• Upgrade to a machine with more cores so that there are more cores available to run the parallel GC threads when necessary.

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To Mysticial:

Yes in hindsight, it might have been better to implement the server in C++. However, we don't know anything about "the game". If it involves a complicated world model with complicated heterogeneous data structures, then implementing it in C++ could mean that that you replace the "GC pause" problem with the problem that the server crashes all the time due to problems with the way it manages its data structures.

Answer 2

Looking at your logs, I don't see any long pauses. But young GC is very frequent. Promotion rate is very low though (most garbage cleared by young GC as it should). At same time your old space utilization is low.

BTW are we talking about minecraft server?

To reduce frequency of young GC you should increase its size. I would suggest start with -XX:NewSize=8G -XX:MaxNewSize=8G

For such large young space, you should also reduce survivor space size -XX:SurvivorRatio=512

GC tuning is a path of trial and errors, so you may need some more iterations and tweaking.

You can find couple of useful articles at mu blog

• HotSpot JVM GC options cheatsheet
• Understanding young GC pauses in HotSpot JVM