Messages in this thread | | | Date | Thu, 23 Nov 2023 17:50:52 +0100 | Subject | Re: [RESEND PATCH v7 00/10] Small-sized THP for anonymous memory | From | David Hildenbrand <> |
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On 23.11.23 17:18, Matthew Wilcox wrote: > On Thu, Nov 23, 2023 at 05:05:37PM +0100, David Hildenbrand wrote: >> On 23.11.23 16:59, Matthew Wilcox wrote: >>> On Wed, Nov 22, 2023 at 04:29:40PM +0000, Ryan Roberts wrote: >>>> Note: I'm resending this at Andrew's suggestion due to having originally sent >>>> it during LPC. I'm hoping its in a position where the feedback is minor enough >>>> that I can rework in time for v6.8, but so far haven't had any. >>>> >>>> Hi All, >>>> >>>> This is v7 of a series to implement small-sized THP for anonymous memory >>>> (previously called "large anonymous folios"). The objective of this is to >>> >>> I'm still against small-sized THP. We've now got people asking whether >>> the THP counters should be updated when dealing with large folios that >>> are smaller than PMD sized. It's sowing confusion, and we should go >>> back to large anon folios as a name. >>> >> >> I disagree. >> >> https://lore.kernel.org/all/65dbdf2a-9281-a3c3-b7e3-a79c5b60b357@redhat.com/ > > And yet: > https://lore.kernel.org/linux-mm/20231106193315.GB3661273@cmpxchg.org/ > > "This is a small THP so we don't account it as a THP, we only account > normal THPs as THPs" is a bizarre position to take. > > Not to mention that saying a foo is a small huge baz is just bizarre. > Am I a small giant? Or just a large human?
I like that analogy. Yet, "small giant" sounds "bigger" in some way IMHO ;)
I'll note that "small-sized THP" is just a temporary feature name, it won't be exposed as such to the user in sysfs etc. In a couple of years, it will be forgotten.
To me it makes sense: it's a hugepage (not a page) but smaller compared to what we previously had. But again, there won't be a "small_thp" toggle anywhere.
Long-term it's simply going to be a THP. Quoting from my writeup:
"Nowadays, when somebody says that they are using hugetlb huge pages, the first question frequently is "which huge page size?". The same will happen with transparent huge pages I believe.".
Regarding the accounting: as I said a couple of times, "AnonHugePages" should have been called "AnonPmdMapped" or similar; that's what it really is: as soon as a THP is PTE-mapped, it's not accounted there. But we can't fix that I guess, unless we add some "world switch" for any workloads that would care about a different accounting.
So we're really only concerned about: * AnonHugePages * ShmemHugePages * FileHugePages
The question is if we really want to continue extending/adjusting the old meminfo interfaces and talk about how to perform accounting there.
Because, as we learned, we might get a new file-based sysfs based interface, because Greg seems to be against exposing new values in the old single-file-based one.
In a new one, we have all freedom to expose what we actually want nowadays, and can just document that the old interface was designed with the assumption that there is only a single THP size.
... like hugetlb, where we also only expose the "default hugetlb size" parameters for legacy reasons:
HugePages_Total: 0 HugePages_Free: 0 HugePages_Rsvd: 0 HugePages_Surp: 0 Hugepagesize: 2048 kB
-- Cheers,
David / dhildenb
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