Tuning GC for Java audio application

Question

I've noticed that when playing audio in java, MarkSweepCompact stage in gc is too long and results in short periods of silence, which is unacceptable. So I need to use a low pause gc. I've tried Parallel and CMS, they seem to work better because I suppose the pause is shorter and they don't do full collection as often as the default one.

So far I've tested my program with the following options for ParallelGC:

-XX:+UseParallelGC 
-XX:MaxGCPauseMillis=70

and for ConcurrentMarkSweep:

-XX:+UseConcMarkSweepGC
-XX:+CMSIncrementalMode
-XX:+CMSIncrementalPacing

I also tried G1GC, but it's still experimental in java 6. Options for both modes:

-Xms15m
-Xmx40m
-XX:+UnlockExperimentalVMOptions
-XX:+CMSClassUnloadingEnabled
-XX:+TieredCompilation
-XX:+AggressiveOpts
-XX:+UseAdaptiveSizePolicy
-Dsun.java2d.noddraw=false
-Dswing.aatext=true
-XX:MaxPermSize=25m
-XX:MaxHeapFreeRatio=10
-XX:MinHeapFreeRatio=10

Which GC is better in this situation? Can any of these settings be optimized for best CPU performance and minimal memory usage as well?

EDIT To recognize the pause I record time to write audio data to the output line, usually it's between 92 to 120 ms (I'm writing 16384 bytes = ~92ms), ad when Full GC is run, it's 200+ ms:

65.424: [Full GC (System) [PSYoungGen: 872K->0K(2432K)] [PSOldGen: 12475K->12905K(16960K)] 13348K->12905K(19392K) [PSPermGen: 15051K->15051K(22272K)], 0.2145081 secs] [Times: user=0.20 sys=0.00, real=0.21 secs] 
Was writing 16384 bytes, time to write 263 ms

EDIT2 Allocation pattern for my app is the following: it loads bunch of objects at startup, then it starts playing and I guess most of the objects after that are allocated by the gui, because staring/pausing the audio doesn't change the GC graph much. This is what visualgc shows with parallel gc:

The graph starts at startup and I start playback. Labeled are

1) sound delay and full gc, I think it increased Old size:

101.646: [Full GC [PSYoungGen: 64K->0K(6848K)] [PSOldGen: 15792K->12773K(19328K)] 15856K->12773K(26176K) [PSPermGen: 15042K->14898K(23808K)], 0.2411479 secs] [Times: user=0.19 sys=0.00, real=0.24 secs]

2) I open the app window and pause playback. Nothing really changes, a bit later it increases eden size.

3) I open the windows and start playback again.

So I need to increase allocated Old Gen size? How do I do that? I'm running with -XX:NewRatio=10 and -XX:NewSize=10m

Thank you.

Answer 1

The log you provide is too small to provide real analysis but it says that it spent 200ms doing v little as the old gen is basically full. This means your heap is too small or you have a memory leak. There is not much you can do to tune the GC algorithm in this situation. Therefore the rest of this reply is about how you can get more information out of the app and/or how to tune GC once you have eliminated the memory leak or have a bigger heap.

To a large extent, low pause means doing all you can to keep the collections as young collections only.

You really need to log exactly when you start and finish writing and then correlate that with the STW pauses that occur in the JVM during that period otherwise you really have no idea what might be causing the issue or how severe the issue really is.

Things I'd do immediately;

1. change your logging so that you output a single line easily parseable by a script (perhaps starttime,endtime,duration)
2. add the PrintGCApplicationStoppedTime and PrintGCApplicationConcurrentTime switches so that you get a record of every STW pause and not just GC events
3. use the latest JVM (i.e. 6u23) as there have been a lot of improvements to hotspot over the last year or two so there point using an older one
4. You don't say if you're memory constrained but I'd definitely increase the heap size if you can, 40M is pretty small so you don't have much space to play with
5. Run the app with visualgc connected, it's gives a more comprehensive view on what is going on IMO as you have all the different views up at one time

The key thing is determine where you're running out of space and why. The answer to that likely lies in what the allocation pattern of your app is like, is it generating a load of short lived objects such that you're burning through your tiny eden really quickly? is the tenuring threshold too high such that you're ping ponging objects through the survivor spaces before they get tenured anyway and thus forcing frequent tenured gcs (slow)?

A few other things to bear in mind...

• iCMS (incremental) was intended for use on 1 or 2 core machines, does that describe your machine? how many cores do you have? you may just want to drop that option
• CMS does have a single threaded phase (init mark), this might be hurting you
• CMS typically prefers a bigger heap than other collectors, yours is pretty small

Edit after visualgc graph added to question Since you're memory constrained then you need to make best use of the space you have, the only way to do this is going to be through repeated benchmarking... ideally with a repeatable test.

• you can use -Xmn to specify set the size of the young generation, the remainder will be given to tenured.
• you might want to tune your use of the survivor spaces so that you let them get fuller before they're swapped and to let objects live there for longer before they get tenured
  • -XX:TargetSurvivorRatio=90 sets it so a survivor space needs to be 90% full before it is copied, obviously there is a trade off here between the cost of copying and using the space
  • use -XX:+PrintTenuringDistribution to show the size of each space and how things are, you can also see this in visualgc
  • use -XX:+MaxTenuringThreshold to specify how many times an object can survive a young collection (be copied from 1 survivor space to another) before it is tenured, e.g. if you know you only get short lived garbage or stuff that lives forever then setting this to 1 is sensible
• you need to understand what is triggering the tenured collections and might consider taking action to make it trigger later
  • for CMS this may involve tweaking -XX:CMSInitiatingOccupancyFraction=<value>, e.g. set to 80 and it will trigger CMS at 80% occupancy of tenured (NB: this is be a bad thing to get wrong so you may prefer to let hotspot manage this; set it too small and it collects too often killing performance, set it too big and it may trigger too late causing an unscheduled full collection with correspondingly long pause time
• if it really is old collections that are hurting you and you need low pause then use CMS and ParNew

Finally get a profiler and work out where the garbage is coming from, you might find it is easier to control the rate at which garbage is generated then pouring effort into the black hole that can be GC tuning!

Answer 2

This implies that too many objects objects are being promoted out of eden space as the main GC shouldn't have to be dealing with much. You could increase the ratio of space given to the new generations with -XX:NewRatio. Try 10 and move up.
Better still, investigate how you can reduce the scope over which objects in your program remain referenced.