On 28 Mar 2023 11:12:54 +0100 Ryan Roberts <ryan.roberts@xxxxxxx> > On 27/03/2023 16:48, Vlastimil Babka wrote: > > > > Hm if it's 4 entries on arm64 and presumably 8 on AMD, maybe we can only > > care about how actively accessed are the individual "subpages" above that > > size, to avoid dealing with this uncertainty whether HW tracks them. At such > > smallish sizes we shouldn't induce massive overhead? > > I'm not sure I've fully understood this point. For arm64's HPA, there is no > "uncertainty [about] whether HW tracks them"; HW will always track access/dirty > individually for each base page. The problem is the inverse; if SW (or HW) sets > those bits differently in each page, then TLB coalescing performance may > decrease. Or are you actually suggesting that SW should always set the bits the > same for a 4 or 8 page run, and forgo the extra granularity? That inverse side looks like a bleak cloud above anon order-5/6 pages for instance. > >> I'm hearing that there are workloads where being able to use the contiguous bit > >> really does make a difference, so I would like to explore solutions that can > >> work when we only have access/dirty at the folio level. > > > > And on the higher orders where we have explicit control via bits, we could > > split the explicitly contiguous mappings once in a while to determine if the > > sub-folios are still accessed? Although maybe with 16x4kB pages limit it may > > still be not worth the trouble? > > I have a bigger-picture question; why is it useful to split these large folios? > I think there are 2 potential reasons (but would like to be educated): > > 1. If a set of sub-pages that were pre-faulted as part of a large folio have > _never_ been accessed and we are under memory pressure, I guess we would like to > split the folio and free those pages? > > 2. If a set of subpages within a folio are cold (but were written in the past) > and a separate set of subpages within the same folio are hot and we are under > memory pressure, we would like to swap out the cold pages? > > If the first reason is important, I guess we would want to initially map > non-contig, then only remap as contig once every subpage has been touched at > least once. > > For the second reason, my intuition says that a conceptual single access and > dirty bit per folio should be sufficient, and folios could be split from > time-to-time to see if one half is cold? It makes no sense to detect hot pages at the cost of order-5 compound page particularly given the bleakness above.
