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

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Date:   Wed, 19 Aug 2020 16:12:32 +0300
From:   Pekka Enberg <penberg@...il.com>
To:     Cho KyongHo <pullip.cho@...sung.com>
Cc:     David Hildenbrand <david@...hat.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 KyongHo and David,

On 07.08.20 09:08, Pekka Enberg wrote:
> > > 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.

On Tue, Aug 18, 2020 at 12:02 PM Cho KyongHo <pullip.cho@...sung.com> wrote:
> I don't find if there is a way to deliver detailed DRAM information
> through ACPI, ATAG or something similar. But I didn't find.

Perhaps that's still the case today then.

In the context of this patch, what I am hoping is that we'd turn the
"dram_rank_granule" parameter -- even if it's still a manually
configured thing -- into an abstraction that we can later extend. IOW,
something similar to what the "energy model"
(kernel/power/energy_model.c) today is.

On Mon, Aug 10, 2020 at 09:32:18AM +0200, David Hildenbrand wrote:
> I guess one universal approach is by measuring access times ... not what
> we might be looking for :)

I don't think it's an unreasonable approach, but measurement accuracy
and speed will determine if it's actually practical. There are
examples of this kind of approach elsewhere too. For example, MCTOP
[1] which aims to provide topology-aware OS abstractions, uses cache
latency measurements to infer the topology.

1. https://timharris.uk/papers/2017-eurosys-mctop.pdf

Regards,

- Pekka