On Mon, Aug 26, 2019 at 09:40:35AM +0200, Michal Hocko wrote: > On Thu 22-08-19 18:29:34, Kirill A. Shutemov wrote: > > On Thu, Aug 22, 2019 at 02:56:56PM +0200, Vlastimil Babka wrote: > > > On 8/22/19 10:04 AM, Michal Hocko wrote: > > > > On Thu 22-08-19 01:55:25, Yang Shi wrote: > > > >> Available memory is one of the most important metrics for memory > > > >> pressure. > > > > > > > > I would disagree with this statement. It is a rough estimate that tells > > > > how much memory you can allocate before going into a more expensive > > > > reclaim (mostly swapping). Allocating that amount still might result in > > > > direct reclaim induced stalls. I do realize that this is simple metric > > > > that is attractive to use and works in many cases though. > > > > > > > >> Currently, the deferred split THPs are not accounted into > > > >> available memory, but they are reclaimable actually, like reclaimable > > > >> slabs. > > > >> > > > >> And, they seems very common with the common workloads when THP is > > > >> enabled. A simple run with MariaDB test of mmtest with THP enabled as > > > >> always shows it could generate over fifteen thousand deferred split THPs > > > >> (accumulated around 30G in one hour run, 75% of 40G memory for my VM). > > > >> It looks worth accounting in MemAvailable. > > > > > > > > OK, this makes sense. But your above numbers are really worrying. > > > > Accumulating such a large amount of pages that are likely not going to > > > > be used is really bad. They are essentially blocking any higher order > > > > allocations and also push the system towards more memory pressure. > > > > > > > > IIUC deferred splitting is mostly a workaround for nasty locking issues > > > > during splitting, right? This is not really an optimization to cache > > > > THPs for reuse or something like that. What is the reason this is not > > > > done from a worker context? At least THPs which would be freed > > > > completely sound like a good candidate for kworker tear down, no? > > > > > > Agreed that it's a good question. For Kirill :) Maybe with kworker approach we > > > also wouldn't need the cgroup awareness? > > > > I don't remember a particular locking issue, but I cannot say there's > > none :P > > > > It's artifact from decoupling PMD split from compound page split: the same > > page can be mapped multiple times with combination of PMDs and PTEs. Split > > of one PMD doesn't need to trigger split of all PMDs and underlying > > compound page. > > > > Other consideration is the fact that page split can fail and we need to > > have fallback for this case. > > > > Also in most cases THP split would be just waste of time if we would do > > them at the spot. If you don't have memory pressure it's better to wait > > until process termination: less pages on LRU is still beneficial. > > This might be true but the reality shows that a lot of THPs might be > waiting for the memory pressure that is essentially freeable on the > spot. So I am not really convinced that "less pages on LRUs" is really a > plausible justification. Can we free at least those THPs which are > unmapped completely without any pte mappings? Unmapped completely pages will be freed with current code. Deferred split only applies to partly mapped THPs: at least on 4k of the THP is still mapped somewhere. > > Main source of partly mapped THPs comes from exit path. When PMD mapping > > of THP got split across multiple VMAs (for instance due to mprotect()), > > in exit path we unmap PTEs belonging to one VMA just before unmapping the > > rest of the page. It would be total waste of time to split the page in > > this scenario. > > > > The whole deferred split thing still looks as a reasonable compromise > > to me. > > Even when it leads to all other problems mentioned in this and memcg > deferred reclaim series? Yes. You would still need deferred split even if you *try* to split the page on the spot. split_huge_page() can fail (due to pin on the page) and you will need to have a way to try again later. You'll not win anything in complexity by trying split_huge_page() immediately. I would ague you'll create much more complexity. > > We may have some kind of watermark and try to keep the number of deferred > > split THP under it. But it comes with own set of problems: what if all > > these pages are pinned for really long time and effectively not available > > for split. > > Again, why cannot we simply push the freeing where there are no other > mappings? This should be pretty common case, right? Partly mapped THP is not common case at all. To get to this point you will need to create a mapping, fault in THP and then unmap part of it. It requires very active memory management on application side. This kind of applications usually knows if THP is a fit for them. > I am still not sure that waiting for the memory reclaim is a general > win. It wins CPU cycles by not doing the work that is likely unneeded. split_huge_page() is not particularly lightweight operation from locking and atomic ops POV. > Do you have any examples of workloads that measurably benefit from > this lazy approach without any other downsides? In other words how > exactly do we measure cost/benefit model of this heuristic? Example? Sure. Compiling mm/memory.c in my setup generates 8 deferred split. 4 of them triggered from exit path. The rest 4 comes from MADV_DONTNEED. It doesn't make sense to convert any of them to in-place split: for short-lived process any split if waste of time without any benefit. -- Kirill A. Shutemov
