On Mon, 30 Jan 2023 21:18:32 +0000, Oliver Upton <oliver.upton@xxxxxxxxx> wrote: > > 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. Only if it is not using anonymous memory. That's the important distinction. > > 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. Absolutely. The overcommited case would suffer from the upfront allocation (these systems are usually very densely packed). Thanks, M. -- Without deviation from the norm, progress is not possible.
