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

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Message-ID: <ZWVnBwe8dUO5CgTp@casper.infradead.org>
Date:   Tue, 28 Nov 2023 04:05:27 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     Ryan Roberts <ryan.roberts@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Yin Fengwei <fengwei.yin@...el.com>,
        Yu Zhao <yuzhao@...gle.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Yang Shi <shy828301@...il.com>,
        "Huang, Ying" <ying.huang@...el.com>, Zi Yan <ziy@...dia.com>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Itaru Kitayama <itaru.kitayama@...il.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        John Hubbard <jhubbard@...dia.com>,
        David Rientjes <rientjes@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Hugh Dickins <hughd@...gle.com>,
        Kefeng Wang <wangkefeng.wang@...wei.com>, linux-mm@...ck.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [RESEND PATCH v7 00/10] Small-sized THP for anonymous memory

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.

> > 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.