java full gc taking too long

Question

I have a Java client which consumes a large amount of data from a server. If the client does not keep up with the data stream at a fast enough rate, the server disconnects the socket connection. My client gets disconnected a few times per day. I ran jconsole to see the memory usage, and the heap space graph looks like a fairly well defined sawtooth pattern, oscillating between about 0.5GB and 1.8GB (2GB of heap space is allocated). But every time I get disconnected is during a full GC (but not on every full GC). I see the full GC takes a bit over 1 second on average. Depending on the time of day, full GC happens as often as every 5 minutes when busy, or up to 30 minutes can go by in between full GCs during the slow periods.

I suspect if I can reduce the full GC time, the client will be able to better keep up with the incoming data, but I do not have much experience with GC tuning. Does anyone have some insight on if this might be a good idea, and how to do it? Or is there an alternative idea which may work as well?

** UPDATE ** I used -XX:+UseConcMarkSweepGC and it improved, but I still got disconnected during the very busy moments. So I increased the heap allocation to 3GB to help weather through the busy moments and it seems to be chugging along pretty well now, but it's only been 1 day without a disconnection. Maybe if I get some time I will go through and try to reduce the amount of garbage created which I'm confident will help as well. Thanks for all the suggestions.

Answer 1

Full GC could take very long to complete, and is not that easy to tune.

One way to (easily) tune it is to increase the heap space - generally speaking, double the heap space can double the interval between two GCs, but will double the time consumed by a GC. If the program you are running has very clear usage patterns, maybe you can consider increase the heap space to make the interval so large that you can guarantee to have some idle time to try to make the system perform a GC. On the other hand, following this logic, if the heap is small a full garbage collection will finish in a instant, but that seems like inviting more troubles than helping.

Also, -XX:+UseConcMarkSweepGC might help since it will try to perform the GC operations concurrently (not stopping your program; see here).

Here's a very nice talk by Til Gene (CTO of Azul systems, maker of high performance JVM, and published several GC algos), about GC in JVM in general.