From: John (john_at_ucc.gu.uwa.edu.au)
Date: 2002-10-07 16:26:01
Ruud writes:
>Then this question: what is against moving used blocks to fill up the free
>blocks? The only reason I see is that it will cost time. But I could handle
>this problem telling the compiler to discard this feature or to execute this
>feature only when memory is needed and not enough successive free blocks are
>found.
The program probably has pointers to the block. If you move it and
don't update *all* of the pointers, things are going to fail
spectacularly. It's not just pointers to the start of the block that
you have to worry about - the block might be a record, and you might
have pointers to fields inside it.
To do this, you need the cooperation of the compiler, as there's no way
you can tell what is a pointer and what isn't by just looking at memory.
I'm not aware of any C or Pascal compilers that allow this (with the
sort-of exception of the Mac, which requires the programmer to use
handles and manually double-indirect everything). It's usually done
only with languages that were designed for garbage collection, such as
Java.
Also, moves are very expensive. You should try to do as few as
possible. They're only necessary when memory becomes fragmented, so
perhaps only move blocks when the allocator can't find a large enough
free block.
It's a fairly popular research area. Go googling for "garbage
collection", and you'll find so much to read you'll never get the
compiler finished...
In my opinion, all modern languages should require garbage collection.
Pascal, C, and C++ should be forgotten as quickly as possible. Watch
how quickly I change my mind when I try implementing it :-)
John
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