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.