linux-kernel - Re: [RESEND PATCH] mm: align larger anonymous mappings on THP boundaries

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Date: Tue, 7 May 2024 10:17:32 -0700
From: Yang Shi <shy828301@...il.com>
To: Ryan Roberts <ryan.roberts@....com>
Cc: Kefeng Wang <wangkefeng.wang@...wei.com>, David Hildenbrand <david@...hat.com>, 
	Matthew Wilcox <willy@...radead.org>, Yang Shi <yang@...amperecomputing.com>, riel@...riel.com, 
	cl@...ux.com, akpm@...ux-foundation.org, linux-kernel@...r.kernel.org, 
	linux-mm@...ck.org, Ze Zuo <zuoze1@...wei.com>
Subject: Re: [RESEND PATCH] mm: align larger anonymous mappings on THP boundaries

On Tue, May 7, 2024 at 8:53 AM Ryan Roberts <ryan.roberts@....com> wrote:
>
> On 07/05/2024 14:53, Kefeng Wang wrote:
> >
> >
> > On 2024/5/7 19:13, David Hildenbrand wrote:
> >>
> >>> https://github.com/intel/lmbench/blob/master/src/lat_mem_rd.c#L95
> >>>
> >>>> suggest. If you want to try something semi-randomly; it might be useful to rule
> >>>> out the arm64 contpte feature. I don't see how that would be interacting
> >>>> here if
> >>>> mTHP is disabled (is it?). But its new for 6.9 and arm64 only. Disable with
> >>>> ARM64_CONTPTE (needs EXPERT) at compile time.
> >>> I don't enabled mTHP, so it should be not related about ARM64_CONTPTE,
> >>> but will have a try.
> >
> > After ARM64_CONTPTE disabled, memory read latency is similar with ARM64_CONTPTE
> > enabled(default 6.9-rc7), still larger than align anon reverted.
>
> OK thanks for trying.
>
> Looking at the source for lmbench, its malloc'ing (512M + 8K) up front and using
> that for all sizes. That will presumably be considered "large" by malloc and
> will be allocated using mmap. So with the patch, it will be 2M aligned. Without
> it, it probably won't. I'm still struggling to understand why not aligning it in
> virtual space would make it more performant though...

Yeah, I'm confused too.

I just ran the same command on 6.6.13 (w/o the thp alignment patch and
mTHP stuff) and 6.9-rc4 (w/ the thp alignment patch and all mTHP
stuff) on my arm64 machine, but I didn't see such a pattern.

The result has a little bit fluctuation, for example, 6.6.13 has
better result with 4M/6M/8M, but 6.9-rc4 has better result for
12M/16M/32M/48M/64M, and the difference may be quite noticeable. But
anyway I didn't see such a regression pattern.

The benchmark is supposed to measure cache and memory latency, its
result strongly relies on the cache and memory subsystem, for example,
hw prefetcher, etc.

>
> Is it possible to provide the smaps output for at least that 512M+8K block for
> both cases? It might give a bit of a clue.
>
> Do you have traditional (PMD-sized) THP enabled? If its enabled and unaligned
> then the front of the buffer wouldn't be mapped with THP, but if it is aligned,
> it will. That could affect it.
>
> >
> >>
> >> cont-pte can get active if we're just lucky when allocating pages in the right
> >> order, correct Ryan?
> >>
>