Increasing TLA size causes more frequent YC gc

Question

hi,
i'm running the latest JRockit with 4GB heap size, 500MB nursery and TLA of 8MB gencon strategy.
What I've noticed is that although garbage collection (young collection) pauses became shorter, their frquency increased siginificantly (almost every second). Previously while running default JRockit settings my pause times were ~200ms every 5s. I assumed that increasing TLA should at least make YC not so frequent.
How should I use TLA sizing from practical point of view (i've read documentation x-times) What would be reasonable YC frequency and pause times? Is there any document where I could find achievable by JRockit pause times and YC, OC frequency? I understand that this will depend on a type of application, allocated objects etc.. but still I'd appreciate any hints. By the way most of my allocated objects are of the same size.

Answer 1

It is quite difficult to give any exact numbers on what reasonable YC frequencies or pause times are. As you say, it depends a lot on what the application is doing. However, it is of course possible to try to tune JRockit to behave more closely to the behavior that you would like.
From what you write I am not sure what type of behavior you would like. Are you looking for short pause times because your application suffers when there are long pauses, or are you looking for less GC-time to get more throughput (more execution time)? The difference is that you can either get more frequent but shorter pauses or less frequent but longer pauses.
A short background on TLA-sizing:
A TLA is a chunk of memory where a thread can do fast object allocation. Only objects that are small enough to fit in the TLA can be allocated there. This means that increasing the TLA size allows you to allocate larger objects in the TLAs. On the other hand, if you are using a nursery, all TLAs come from the nursery. This means that you can get fewer TLAs from the same sized nursery if you use a larger TLA size. Also, threads that don't allocate much get the same TLAs as other threads, which potentially wastes memory if they can not fill the TLAs.
In your case you get 500/8 = 62 TLAs from your nursery as opposed to 500/0.256 = 1953 TLAs if you would have used the default value (256k) for TLA size.
So, a large TLA size may mean that you can allocate larger object in TLAs (and thus in the nursery as opposed to in old space) but it may also increase the YC frequency.
You mention that all of you objects are about the same size. What size is that? I think it would be wise to make sure that the TLA size is large enough to fit those object in, but the TLA size should not be too large.
My feeling is that 8 MB is quite a large size for TLAs. There is a way to specify a range for the TLA size. You can set a minimum value and a preferred size. This allows for more dynamic (and hopefully more efficient) TLA sizing.
-XXtlaZize:min=2k,preferred=256k
I would suggest setting the min value to make sure that your objects fit in TLAs and then experiment with the preferred size a bit.