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Michael Zucchi

 B.E. (Comp. Sys. Eng.)

  also known as zed
  & handle of notzed

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Wednesday, 26 October 2011, 12:47

GC, finalisers

So I was doing some memory profiling the other day (using netbeans excellent excellent profiler - boy I could've used this 10 years ago) to try to track down some resource leakages and I noticed that xuggle was really exercising the system heavily.

So it seems I might look at moving to use jjmpeg in my client's application fairly soon. There are some other reasons as well: i.e. not being able to run in a 64-bit JVM on microsoft windows is starting to become a problem, and the bundled ffmpeg is just a bit out of date.

Since I haven't implemented memory handling completely in jjmpeg I went about looking how to do it 'properly'. I was just going to try to use finalisers, but then I came across this article on

java finalisers

java finalisers which said it probably wasn't a good idea.

I was going to have a short look this morning but suddenly it was 4 hours later and although I had something which works i'm not sure yet that I like it. It seems the cleanest way to implement the suggestions of using weak references, and mixing the auto-generated and hand-crafted code I want, so I will probably end up running with it. The public api didn't need to change.

Previously, the binding worked with an object class hierarchy something like this

 AVNative [
   ByteBuffer p (points to allocated/mapped native memory)
 ]
   +- AVFormatContextAbstract [
    Generated field accessors and native methods
    Most methods are object methods
   ]
    +- AVFormatContext [
      Public factory methods/constructors
      Hand-coded specific methods
      Hand-coded helper native methods
      Hand-coded finalise/dispose methods
    ]

The new structure:

WeakReference<AVObject>
+- AVNative [
   ByteBuffer p pointing to native memory
   internal dispose() method
   weak reference queue/cleanup as from article above
   Weak reference is AVObject
 ]
 +- AVFormatContextNativeAbstract [
   Generated field accessors and native methods
   All methods and field accessors are static
   ]
   +- AVFormatContextNative [
     Hand-coded helper native methods
     Implements native resource dispose
   ]

Together with

AVObject [
  AVNative n (the pointer to the native wrapper object)
  public dispose method
  ]
  +- AVFormatContextAbstract [
      Generated public access methods which use AVFormatContextNative(Abstract) methods.
    ]
    +- AVFormaContext [
      Public factory methods/constructors
      Hand-coded specific methods
      ]

So yeah - a bit more complicated, and it requires 2 objects for each instance (and often 3 including the C side instance it's wrapping), as well as the overhead of the weakreference instance data and the list entry for tracking the references. The extra layer of indirection also adds another method invocation/stack frame to every method call.

On the other hand, it lets the client code use dispose() when it wants to, or if it forgets then dispose will automatically be called eventually. And makes it obvious in the code where dispose needs to sit.

As usual it's a question of trade-offs. If the article is correct then presumably these trade-offs are worth it.

In this case the whole point of using jjmpeg is to avoid numerous allocations every frame anyway: I can allocate working and output buffers once and just use them directly. In this case the actual number of objects is quite small and doesn't happen very often, so I suspect that either mechanism would work about as well as the other.

Well this distraction has blown my morning away; I'd better leave it for now so I can clock up some work hours after lunch.

Update I figured i'd gone too far down this route to do anything other than keep it. I've checked this in now as well as a bunch of other stuff described on the project page. Update 2: Oracle keeps breaking links, but i've updated the pointer. I'm looking at this again (September 2012) because of some issues in jjmpeg.

Tagged hacking, java, jjmpeg.
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