[Haskell-cafe] GHC FreeBSD memory model (was: 8.4.3 release)

[Viktor Dukhovni]

On Thu, Jul 05, 2018 at 09:32:31AM -0400, Ben Gamari wrote:

> We use a much different memory allocation strategy on FreeBSD which
> (AFAIK) does not allow one to reserve address space without also
> committing.

Here's a demo of mapping in a 1TB heap on FreeBSD 11.1, on a machine
with "just" 64GB of RAM and 160GB swap.  I would expect anon memory
mappings to behave similarly on older systems.

    $ cat foo.c
    #include <stdio.h>
    #include <stdlib.h>
    #include <unistd.h>
    #include <sys/mman.h>

    int main(int argc, char **argv)
    {
	size_t heaplen = 1ULL << 40; /* 1TB */
	unsigned char *heap;

	heap = mmap(NULL, heaplen, PROT_READ|PROT_WRITE, MAP_ANON, -1, 0);
	heap[0] = 'A';
	heap[heaplen-1] = 'z';
	printf ("%p(%zd) %c %c\n", heap, heaplen, heap[0], heap[heaplen-1]);
	sleep(2);
	return 0;
    }

    $ ./foo & sleep 1; ps -o vsz= -o rss= $! ; wait
    [1] 75193
    0x800e00000(1099511627776) A z
    1073750140 2076
    [1]+  Done                    ./foo

    $ printf "0x%8x\n" $(( 1073750140 * 1024 ))
    0x1000081f000

The process gets a contiguous 1TB of anonymous demand-paged zero-filled
memory, and a virtual size of 1TB + 0x81f000 bytes.   The resident
set size is a modest 2MB.

-- 
	Viktor.