[TCLCORE] Re: Hintermayer's string allocation patch

[Jeffrey H. <jef...@aj...>]

> From: Jim Ingham [mailto:ji...@ap...]
	...
> On Wednesday, August 30, 2000, at 04:49 PM, Jeffrey Hobbs wrote:
> 
> > Yes and no.  Sure, the memory is just reserved, but the mem allocator 
> > will still bomb if you ask for more space than it could promise.  The 
> > point of the mem efficiency part is that we don't get too greedy about 
> > needing so much more space than we need.  That way we can push the 
> > boundaries of how much mem we will actually be able to use without 
> > the mem allocator crapping out because we're overly greedy. 
> >  
> 
> Yes, but the point of the last note was that what "more" is is pretty 
> huge.  For instance, I think in general the allocator doesn't check 
> whether a given allocation fits in the available swap space but 
> rather fills it as you actually write out pages to swap.  This is the 
> right thing to do, since that way you can for instance allocate a 
> large matrix, and if you only fill it sparsely, you aren't limited by 
> the size of the matrix, only by the pages you use.  MacOS X, does it 
> this way, at least.  I imagine other Unixes work in much the same way...

It's always a bummer about those theories, because Unix really does limit
you to known available mem space (note that 1610612735 == 0x5fffffff):

nerja [~/tmp/mem] 183 % gcc -o testmem mem.c
nerja [~/tmp/mem] 184 % testmem 1610612735
Oops, could not allocate 1610612735 bytes
nerja [~/tmp/mem] 185 % testmem 810612735
Oops, could not allocate 810612735 bytes
nerja [~/tmp/mem] 186 % testmem 410612735
Oops, could not allocate 410612735 bytes
nerja [~/tmp/mem] 187 % testmem 210612735
I allocated 210612735d bytes
^C
nerja [~/tmp/mem] 188 % cat mem.c
int
main (int argc, char **argv)
{
  int bigsize = 0x5fffffff;
  char * foo;
  if (argc == 2) {
      bigsize = atoi(argv[1]);
  }
  foo = (char *) malloc(bigsize);
  if (!foo) {
    printf("Oops, could not allocate %d bytes\n", bigsize);
    return 1;
  } else {
    printf("I allocated %ud bytes\n", bigsize);
  }

  foo[500] = 'c';
  foo[0xffffff] = 'd';
  while (1) ;
  return 0;
}

With the last, you get 480K resident on a 200+MB size program from top.
This is a Solaris/x86 machine.  You could jack up your swap space quite
a bit, but that'll get you into trouble for other reasons if someone
actually got around to using it.

Jeffrey Hobbs                     Tcl Ambassador
ho...@Aj...          Ajuba Solutions (nee Scriptics)

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