Re: [RESEND PATCH v7 00/10] Small-sized THP for anonymous memory

[David Hildenbrand]

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