On Mon 04-01-21 15:00:31, Dave Hansen wrote: > On 1/4/21 12:11 PM, David Hildenbrand wrote: > >> Yeah, it certainly can't be the default, but it *is* useful for > >> thing where we know that there are no cache benefits to zeroing > >> close to where the memory is allocated. > >> > >> The trick is opting into it somehow, either in a process or a VMA. > >> > > The patch set is mostly trying to optimize starting a new process. So > > process/vma doesn‘t really work. > > Let's say you have a system-wide tunable that says: pre-zero pages and > keep 10GB of them around. Then, you opt-in a process to being allowed > to dip into that pool with a process-wide flag or an madvise() call. > You could even have the flag be inherited across execve() if you wanted > to have helper apps be able to set the policy and access the pool like > how numactl works. While possible, it sounds quite heavy weight to me. Page allocator would have to somehow maintain those pre-zeroed pages. This pool will also become a very scarce resource very soon because everybody just want to run faster. So this would open many more interesting questions. A global knob with all or nothing sounds like an easier to use and maintain solution to me. > Dan makes a very good point about using filesystems for this, though. > It wouldn't be rocket science to set up a special tmpfs mount just for > VM memory and pre-zero it from userspace. For qemu, you'd need to teach > the management layer to hand out zeroed files via mem-path=. Agreed. That would be an interesting option. > Heck, if > you taught MADV_FREE how to handle tmpfs, you could even pre-zero *and* > get the memory back quickly if those files ended up over-sized somehow. We can probably allow MADV_FREE on shmem but that would require an exclusively mapped page. Shared case is really tricky because of silent data corruption in uncoordinated userspace. -- Michal Hocko SUSE Labs
