Can I get a notification before fullGC?

Andreas Raab andreas.raab at
Thu Oct 12 02:30:33 UTC 2006

Yoshiki Ohshima wrote:
>   Is there any way to get some sort of notification right *before* a
> fullGC happens so that some guys release their cache?

There is no such ability and it would make little sense to add one since 
  the VM doesn't know if you'll ever be able to trigger a manual GC 
before it crashes. In other words: When you run out of memory you need 
that memory back, and *fast*.

>   Is there something I can do for this?  Thank you so much!

What makes sense for a problem like you are describing is to have an 
array that is cleared by the VM when running a full GC. The VM would 
simply nil out everything and if there are no other (strong) references 
to the objects in this array they'd get garbage collected alongside 
everything else. This would give you the precise effect that the 
previous (broken) weak array behavior had.

>   Andreas already had a fix for it:
> and I folded
> it to the eToys image (which should go into other images), and it
> seems to be working as expected.  However, a little thing (which is
> not critical as his patch limits the upper limit of cache entries) is
> that the cache is kept even when a fullGC happens.

Personally, I find this behavior advantageous. The glyph cache can be 
chosen to accommodate the working set for most use cases (I find the 
current one too large by far but see below) and I wouldn't really want 
to recreate the glyphs after each full GC.

BTW, if you're looking at TTFs again, the one thing you might want to 
consider is to fix glyph color caching. Currently, the worst thing you 
can do is to change colors. Say, when using syntax highlighting with 
TTFs. Each time the color changes two bad things happen: First, we 
recompute the glyph from scratch which is expensive, and second we fill 
up the glyph cache with the same glyphs in different colors and 
therefore throwing out previous "good" entries which means more 
computation when that glyph needs to be displayed. I'll bet that fixing 
glyph color caching would allow you dramatically reduce the glyph cache 
size and simply ignore the whole notification on full GC issue.

   - Andreas

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