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Well, I couldn't hold it and had to run some tests.

I tested all three collectors on XanderCat 12.3 vs Virus and Diamond vs DrussGT

-XX:+UseSerialGC

DvsD: both bots dropped turns like crazy and DrussGT almost ever won with 64% APS
XvsV: XanderCat dropped constantly turns over all battle rounds (sightly more on the first two rounds) and lost quite a few rounds against Virus

-XX:+UseParallelGC -XX:+UseParallelOldGC -XX:MaxGCPauseMillis=1

DvsD: both bots dropped even more turns than with the SerialGC - DrussGT still won with 64+% APS
XvsV: XanderCat dropped a lot turnes in the first round and some in the second - all other rounds had no skipped turns (because of the crummy first round he took the wrong movement and lost a bunch of APS vs Virus)

-XX:+UseParallelGC -XX:+UseParallelOldGC -XX:MaxGCPauseMillis=30

DvsD: less skipped turns for both but still to much - DrussGT still on 64% APS
XvsV: no changes to 1ms pause

-XX:+UseConcMarkSweepGC

DvsD: DrussGT drops a turn here and there - Diamond still drops a lot turns but with a lower frequency - DrussGT wins with 50-55% APS
XvsV: XanderCat stops dropping turns at all (just a few here and there) and wins against Virus with 99%

I also changed some other ratio settings but could not see any visible changes to the overall behavior. To me the concurrent collector looks quite promising and I think I play a litlte with the CMSIncremental.. options to see if there is still room for some improvements.

Very interesting. Thanks again for all the testing. Even if the issue can be fixed by GC settings, maybe I should look to see if I can make my robot more environmentally friendly and stop producing so much garbage? My first thought turns to the way I pick data out of my KD trees (because I sloppily move all data points from a MaxHeap into a List in order to stick to my existing interface, and that List gets created and thrown away on every tree read), but I'm not sure that would help on the first round issue.

Wow! Great research here. I wonder if you enable incremental mode for the concurrent mark sweep GC using -XX:+UseConcMarkSweepGC -XX:+CMSIncrementalMode how that would work. Also, the concurrent collector uses additional cores to do the work, so might not work so well unless there are cores free. Of course, designing a bot so that it is resistant to skipped turns, and uses little CPU and memory, is probably the most important aspect of all.

Uh, I wouldn't change anything in this direction. It was just a quick test (<10 seasons) and I doubt it solves anything. To me it just shows a dependency and might worth further test. I'm still convinced that another skipped turn handling (like Voidious suggested) would be far better. Maybe raising the cpu variable for rumble clients would be a workaround - but this would need that everyone develop his bots within the normal cpu range and not abuse this workaround. Probably not a good idea but like many others I get more and more annoyed by the skipped turns because it complicates even very simple tasks.

I did a little reading on the garbage collector looking for information on how I could reduce my garbage collection footprint. Short of stripping things out of my robot permanently, I did come up with a few things I could potentially do to reduce the amount of garbage collection. I'm going to try to implement some of it, but it's just guesswork as to whether or not it will actually improve things or not.