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Date: Thu, 8 Dec 2005 17:06:16 -0800
From: Steve Kargl <sgk@troutmask.apl.washington.edu>
To: freebsd-hackers@freebsd.org
Message-ID: <20051209010616.GA59667@troutmask.apl.washington.edu>
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X-Mailman-Approved-At: Fri, 09 Dec 2005 01:25:11 +0000
Subject: sysctl, HW_PHYSMEM, and crippled gcc
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Anyone have any insight into fixing gcc to make better
use of system memory on systems with more than 4 GB.
It appears that libiberty/physmem.c tries to use sysctl()
to determine the amount of physical memory in a system.

  { /* This works on *bsd and darwin.  */
    unsigned int physmem;
    size_t len = sizeof physmem;
    static int mib[2] = { CTL_HW, HW_PHYSMEM };

    if (sysctl (mib, ARRAY_SIZE (mib), &physmem, &len, NULL, 0) == 0
        && len == sizeof (physmem))
      return (double) physmem;
  }

This works if you have less than 4GB because of the unsigned
int physmem.  I have 12 GB, which of course, when expanded
to the number of bytes doesn't fit into a unsigned int physmem.

What is the ramification?  Well, gcc uses this estimate of
memory to size internal parameters.

troutmask:sgk[259] gcc -v h.c
Using built-in specs.
Configured with: FreeBSD/amd64 system compiler
Thread model: posix
gcc version 3.4.4 [FreeBSD] 20050518
GGC heuristics: --param ggc-min-expand=30 --param ggc-min-heapsize=4096

In particular, ggc-min-heapsize=4096 is ridiculously small for a
system with 12 GB of memory.

-- 
Steve