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.
Hmm, it's possible this could be fixed fairly easily in that case. The original reason for disabling the two-step allocator is #12695, where GHC failed to build due to MAP_NORESERVE being undefined. I had assumed that this meant that reservation-only mappings weren't defined however now that I look again at the mmap(2) manpage it looks like I am probably wrong. MAP_NORESERVE merely flags to the system that swap space should not be reserved for the mapping. It looks like this functionality was simply never implemented in FreeBSD and the flag eventually removed in [1]. However, your example suggests that the kernel may be doing the right thing by default. It sounds to me like we could likely remove the assertion enforcing the availability of MAP_NORESERVE and enable two-step allocation on FreeBSD. My only question is how one performs reservation-and-commit mappings on FreeBSD. This is important since without committing any access to the mapping may crash the program in the case of OOM. Does FreeBSD simply not provide a way to safely map known-good memory? Regardless, thanks for bringing this up! Cheers, -Ben [1] https://reviews.freebsd.org/D848