Garbage Collection and Swap

[John Richard Moser <nigelenki@comcast.net>]

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I've been researching garbage collection.

Let me first get it out in the open that I *despise* the concept of
garbage collection.  I worried when I first heard of a magical automatic
free()/delete[] that it would do one or more of two things:

~ - Create excessive overhead
~ - Free up ram I'm still using

I've learned that the first may or may not be true.  This means that
they've won on some accounts with that one.

The second may or may not occur as well.  It worries me that it may in
certain cases, due to the way GC works.  Most garbage collectors will
look in the heap, maybe the stack, maybe other places, for what looks
like pointers to allocated ram.  If they don't find any, they free the ram.

So, the first extremely obvious thing you'll notice is that GC may at
times miss things, especially if you use the XOR linked list trick or
use your own makeshift swap file (write pointers to disk, read back in).

After a while, a big one hit me.

THE GARBAGE COLLECTOR WANDERS AROUND IN THE ENTIRE HEAP, AND IN SOME
CASES IN OTHER PARTS OF RAM, LOOKING FOR WHAT LOOKS LIKE POINTERS TO
YOUR ALLOCATED DATA.

Read that.  It's in all caps, so you should read it.  It has meaning.

How about, everything is using Bohem GC.  Bohem wanders around in the
heap concurrently.  So all of your applications are wandering around
through their vm space everywhere, continuously.

You get low on ram.  Let's say an app is using 500M of ram (Mozilla).
What's going to happen?  Obvious.  It's going to yank shit out of swap.

If we all linked against a GC, what kinds of swap hell do you think we'd
encounter?
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