Messages in this thread | | | Date | Tue, 28 Nov 2023 09:47:02 +0100 | Subject | Re: [RESEND PATCH v7 00/10] Small-sized THP for anonymous memory | From | David Hildenbrand <> |
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On 28.11.23 05:05, Matthew Wilcox wrote: > On Fri, Nov 24, 2023 at 06:34:10PM +0100, David Hildenbrand wrote: >> On 24.11.23 16:53, Matthew Wilcox wrote: >>>> * we already have PMD-sized "large anon folios" in THP >>> >>> Right, those are already accounted as THP, and that's what users expect. >>> If we're allocating 1024 x 64kB chunks of memory, the user won't be able >>> to distinguish that from 32 x 2MB chunks of memory, and yet the >>> performance profile for some applications will be very different. >> >> Very right, and because there will be a difference between 1024 x 64kB, 2048 >> x 32 kB and so forth, we need new memory stats either way. >> >> Ryan had some ideas on that, but currently, that's considered future work, >> just like it likely is for the pagecache as well and needs much more >> thoughts. >> >> Initially, the admin will have to enable all that for anon either way. It >> all boils down to one memory statistic for anon memory (AnonHugePages) . >> that's messed-up already. > > So we have FileHugePages which is very carefully only PMD-sized large > folios. If people start making AnonHugePages count non-PMD-sized > large folios, that's going to be inconsistent.
Right, and that's why we decided to leave these counters alone for now and rather document that they only apply to PMD-sized THP for historical reasons.
We'll want new stats either way. Hopefully we'll make it more future-proof this time.
> >>> am objecting to the use of the term "small THP" on the grounds of >>> confusion and linguistic nonsense. >> >> Maybe that's the reason why FreeBSD calls them "medium-sized superpages", >> because "Medium-sized" seems to be more appropriate to express something "in >> between". > > I don't mind "medium" in the name. > >> So far I thought the reason was because they focused on 64k only. >> >> Never trust a German guy on naming suggestions. John has so far been my >> naming expert, so I'm hoping he can help. >> >> "Sub-pmd-sized THP" is just mouthful. But then, again, this is would just be >> a temporary name, and in the future THP will just naturally come in multiple >> sizes (and others here seem to agree on that). > > I do not. If we'd come to this fifteen years ago, maybe, but people now > have an understanding that THPs are necessarily PMD sized.
Well, I still find people being confused about THP vs. hugetlb, so likely some confusion is unavoidable. :)
In your other mail you write "Perhaps the problem is that people have turned "THP" into a thing in its own right."
I think that's exactly the case, and I see how that can be confusing when spelling out THP and reading "small-huge: does it cancel out?".
-- Cheers,
David / dhildenb
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