Date: Thu, 8 Dec 2005 22:05:48 -0600 From: Dan Nelson <dnelson@allantgroup.com> To: Steve Kargl <sgk@troutmask.apl.washington.edu> Cc: freebsd-hackers@freebsd.org Subject: Re: sysctl, HW_PHYSMEM, and crippled gcc Message-ID: <20051209040548.GD95420@dan.emsphone.com> In-Reply-To: <20051209010616.GA59667@troutmask.apl.washington.edu> References: <20051209010616.GA59667@troutmask.apl.washington.edu>
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In the last episode (Dec 08), Steve Kargl said:
> 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.
physmem is actually an unsigned long, not an unsigned int, so on amd64
that sysctl call should fail anyway (amd64 is LP64, so a long won't fit
into an int).
> 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.
On all my FreeBSD boxes from 128MB to 1GB of RAM, I get the exact same
heuristic values as you, so I'm not sure whether the code works at all.
I seem to remember having the opposite problem on a memory-limited
machine which insisted in allocating a relatively huge percentage of
RAM for a sort, and gnu sort uses the same physmem() call for its
dynamic sizing.
--
Dan Nelson
dnelson@allantgroup.com
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