linux-kernel - Re: [PATCH] mm: sort freelist by rank number

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Date:   Fri, 7 Aug 2020 10:08:41 +0300
From:   Pekka Enberg <penberg@...il.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     pullip.cho@...sung.com, Andrew Morton <akpm@...ux-foundation.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>, hyesoo.yu@...sung.com,
        janghyuck.kim@...sung.com
Subject: Re: [PATCH] mm: sort freelist by rank number

Hi Cho and David,

On Mon, Aug 3, 2020 at 10:57 AM David Hildenbrand <david@...hat.com> wrote:
>
> On 03.08.20 08:10, pullip.cho@...sung.com wrote:
> > From: Cho KyongHo <pullip.cho@...sung.com>
> >
> > LPDDR5 introduces rank switch delay. If three successive DRAM accesses
> > happens and the first and the second ones access one rank and the last
> > access happens on the other rank, the latency of the last access will
> > be longer than the second one.
> > To address this panelty, we can sort the freelist so that a specific
> > rank is allocated prior to another rank. We expect the page allocator
> > can allocate the pages from the same rank successively with this
> > change. It will hopefully improves the proportion of the consecutive
> > memory accesses to the same rank.
>
> This certainly needs performance numbers to justify ... and I am sorry,
> "hopefully improves" is not a valid justification :)
>
> I can imagine that this works well initially, when there hasn't been a
> lot of memory fragmentation going on. But quickly after your system is
> under stress, I doubt this will be very useful. Proof me wrong. ;)
>
> ... I dislike this manual setting of "dram_rank_granule". Yet another mm
> feature that can only be enabled by a magic command line parameter where
> users have to guess the right values.
>
> (side note, there have been similar research approaches to improve
> energy consumption by switching off ranks when not needed).

I was thinking of the exact same thing. PALLOC [1] comes to mind, but
perhaps there are more recent ones?

I also dislike the manual knob, but is there a way for the OS to
detect this by itself? My (perhaps outdated) understanding was that
the DRAM address mapping scheme, for example, is not exposed to the
OS.

I think having more knowledge of DRAM controller details in the OS
would be potentially beneficial for better page allocation policy, so
maybe try come up with something more generic, even if the fallback to
providing this information is a kernel command line option.

[1] http://cs-people.bu.edu/rmancuso/files/papers/palloc-rtas2014.pdf

- Pekka