On Fri, Jan 27, 2023 at 07:45:15AM -0800, Ricardo Koller wrote: > Hi Marc, > > On Thu, Jan 26, 2023 at 12:10 PM Marc Zyngier <maz@xxxxxxxxxx> wrote: [...] > > > > The one thing that would convince me to make it an option is the > > amount of memory this thing consumes. 512+ pages is a huge amount, and > > I'm not overly happy about that. Why can't this be a userspace visible > > option, selectable on a per VM (or memslot) basis? > > > > It should be possible. I am exploring a couple of ideas that could > help when the hugepages are not 1G (e.g., 2M). However, they add > complexity and I'm not sure they help much. > > (will be using PAGE_SIZE=4K to make things simpler) > > This feature pre-allocates 513 pages before splitting every 1G range. > For example, it converts 1G block PTEs into trees made of 513 pages. > When not using this feature, the same 513 pages would be allocated, > but lazily over a longer period of time. > > Eager-splitting pre-allocates those pages in order to split huge-pages > into fully populated trees. Which is needed in order to use FEAT_BBM > and skipping the expensive TLBI broadcasts. 513 is just the number of > pages needed to break a 1G huge-page. > > We could optimize for smaller huge-pages, like 2M by splitting 1 > huge-page at a time: only preallocate one 4K page at a time. The > trick is how to know that we are splitting 2M huge-pages. We could > either get the vma pagesize or use hints from userspace. I'm not sure > that this is worth it though. The user will most likely want to split > big ranges of memory (>1G), so optimizing for smaller huge-pages only > converts the left into the right: > > alloc 1 page | | alloc 512 pages > split 2M huge-page | | split 2M huge-page > alloc 1 page | | split 2M huge-page > split 2M huge-page | => | split 2M huge-page > ... > alloc 1 page | | split 2M huge-page > split 2M huge-page | | split 2M huge-page > > Still thinking of what else to do. I think that Marc's suggestion of having userspace configure this is sound. After all, userspace _should_ know the granularity of the backing source it chose for guest memory. We could also interpret a cache size of 0 to signal that userspace wants to disable eager page split for a VM altogether. It is entirely possible that the user will want a differing QoS between slice-of-hardware and overcommitted VMs. -- Thanks, Oliver
