On Fri, Sep 12, 2025 at 03:46:36PM +0200, David Hildenbrand wrote: > On 12.09.25 15:37, Johannes Weiner wrote: > > On Fri, Sep 12, 2025 at 02:25:31PM +0200, David Hildenbrand wrote: > > > On 12.09.25 14:19, Kiryl Shutsemau wrote: > > > > On Thu, Sep 11, 2025 at 09:27:55PM -0600, Nico Pache wrote: > > > > > The following series provides khugepaged with the capability to collapse > > > > > anonymous memory regions to mTHPs. > > > > > > > > > > To achieve this we generalize the khugepaged functions to no longer depend > > > > > on PMD_ORDER. Then during the PMD scan, we use a bitmap to track individual > > > > > pages that are occupied (!none/zero). After the PMD scan is done, we do > > > > > binary recursion on the bitmap to find the optimal mTHP sizes for the PMD > > > > > range. The restriction on max_ptes_none is removed during the scan, to make > > > > > sure we account for the whole PMD range. When no mTHP size is enabled, the > > > > > legacy behavior of khugepaged is maintained. max_ptes_none will be scaled > > > > > by the attempted collapse order to determine how full a mTHP must be to be > > > > > eligible for the collapse to occur. If a mTHP collapse is attempted, but > > > > > contains swapped out, or shared pages, we don't perform the collapse. It is > > > > > now also possible to collapse to mTHPs without requiring the PMD THP size > > > > > to be enabled. > > > > > > > > > > When enabling (m)THP sizes, if max_ptes_none >= HPAGE_PMD_NR/2 (255 on > > > > > 4K page size), it will be automatically capped to HPAGE_PMD_NR/2 - 1 for > > > > > mTHP collapses to prevent collapse "creep" behavior. This prevents > > > > > constantly promoting mTHPs to the next available size, which would occur > > > > > because a collapse introduces more non-zero pages that would satisfy the > > > > > promotion condition on subsequent scans. > > > > > > > > Hm. Maybe instead of capping at HPAGE_PMD_NR/2 - 1 we can count > > > > all-zeros 4k as none_or_zero? It mirrors the logic of shrinker. > > > > > > > > > > I am all for not adding any more ugliness on top of all the ugliness we > > > added in the past. > > > > > > I will soon propose deprecating that parameter in favor of something > > > that makes a bit more sense. > > > > > > In essence, we'll likely have an "eagerness" parameter that ranges from > > > 0 to 10. 10 is essentially "always collapse" and 0 "never collapse if > > > not all is populated". > > > > > > In between we will have more flexibility on how to set these values. > > > > > > Likely 9 will be around 50% to not even motivate the user to set > > > something that does not make sense (creep). > > > > One observation we've had from production experiments is that the > > optimal number here isn't static. If you have plenty of memory, then > > even very sparse THPs are beneficial. > > Exactly. > > And willy suggested something like "eagerness" similar to "swapinness" that > gives us more flexibility when implementing it, including dynamically > adjusting the values in the future. > Ideally we would be able to also apply this to the page faulting paths. In many cases, there's no good reason to create a THP on the first fault... > > > > An extreme example: if all your THPs have 2/512 pages populated, > > that's still cutting TLB pressure in half! > > IIRC, you create more pressure on the huge entries, where you might have > less TLB entries :) But yes, there can be cases where it is beneficial, if > there is absolutely no memory pressure. > Correct, but it depends on the microarchitecture. For modern x86_64 AMD, it happens that the L1 TLB entries are shared between 4K/2M/1G. This was not (is not?) the case for Intel, where e.g back on kabylake, you had separate entries for 4K/2MB/1GB. Maybe in the Great Glorious Future (how many of those do we have?!) it would be a good idea to take this kinds of things into account. Just because we can map a THP, doesn't mean we should. Shower thought: it might be in these cases especially where the FreeBSD reservation system comes in handy - best effort allocating a THP, but not actually mapping it as such until you really _know_ it is hot - and until then, memory reclaim can just break your THP down if it really needs to. -- Pedro
