[racket] Garbage collection informaton

I've pushed a change to GC logging to the git repo:

 * The logged message test now includes the start time in CPU
   milliseconds (like `current-process-milliseconds').

 * If you create a log receiver within Racket, you can look for
   messages that have a `gc-info' prefab struct (10 fields) value,
   which provides all the information in the string message plus start
   and end times in real time (like `current-inexact-milliseconds').
   See section 15.4 in the Reference for more information.

At Wed, 28 Sep 2011 19:42:39 -0400, Curtis Dutton wrote:
> I'm in the midst of writing a nice performance/stress testing harness for
> racket.
> 
> To measure performance with a GC language, it is very important to be able
> to log accurate GC occurrences, along with how long they take.
> 
> The debug log seems to really output what I need except maybe a call to
> (current-inexact-milliseconds) at the the start of the gc. Is there any sort
> of way for me to determine exactly what time the GC begins along with other
> sorts of good GC relatied information?
> 
> Is there any lower level hook I could get into?
> 
> Thanks,
>     Curtis
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