[plt-scheme] asking mzscheme and DrScheme to GC less to get speedups at the expense of memory usage?

From: Matthew Flatt (mflatt at cs.utah.edu)
Date: Wed Dec 30 12:36:14 EST 2009

There's currently no such control, but

 * Do you know how much time is being spent by the GC? When you run in
   DrScheme, does it spend a lot of time with the green recycle icon
   on or flashing very quickly?

 * You can partially simulate a request to use more memory by
   allocating a big byte string and holding onto it. A GC is triggered
   based on current memory use versus the memory use after the most
   recent garbage collection. So, if you hold onto a bunch of data,
   more data will be used before another GC. (Be sure to allocate a big
   byte string or character string, and not a big vector, because you
   don't want the GC to have to traverse the big object.)

At Wed, 30 Dec 2009 12:20:18 -0500, Lee Spector wrote:
> I have some memory-hungry and compute-intensive programs that I'm running in 
> DrScheme under Mac OS X and in mzscheme under Linux. Under DrScheme I set the 
> memory limit to something fairly high, while under Linux, if I understand 
> correctly, there is no limit.
> In both cases, however, less memory is actually being used than I would 
> expect. I suppose this means that a lot of the memory turns quickly to garbage 
> and is being quickly collected, which is nice in some respects, but in the 
> current case I want maximum execution speed and would be happy for the thing 
> to eat several GB more memory if that would help things to run faster.
> The reason I suspect this might be possible is that I used to experience 
> similar things in various Lisps, and I could get substantial speedups by 
> telling them to GC less. For example, in CMUCL the default GC parameters would 
> always cause my programs (which are often genetic programming systems with 
> large populations) to thrash in GC and run very slowly overall, but if I 
> launched CMUCL with a command-line argument that told it not to GC until a 
> particular (high) threshold of allocation was reached then it would run much 
> faster. This makes a big difference for runs that can take hours or days.
> Is there a way to do something similar with mzscheme and/or DrScheme? Or would 
> it not help for some reason? I'm currently conducting a run that I would 
> expect to eat lots of RAM and it's using a measly 1.9% of my system memory. 
> I'd be happy for it to use 99% if that would improve the runtime.
> I've browsed the reference and found a number of sections related to GC but 
> nothing addressing this issue specifically.
> Thanks for any help you can provide,
>  -Lee
> --
> Lee Spector, Professor of Computer Science
> School of Cognitive Science, Hampshire College
> 893 West Street, Amherst, MA 01002-3359
> lspector at hampshire.edu, http://hampshire.edu/lspector/
> Phone: 413-559-5352, Fax: 413-559-5438
> Check out Genetic Programming and Evolvable Machines:
> http://www.springer.com/10710 - http://gpemjournal.blogspot.com/
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