linux-kernel - Re: [v2 RFC PATCH 0/9] Another Approach to Use PMEM as NUMA Node

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Date:   Wed, 1 May 2019 13:20:31 +0800
From:   Fengguang Wu <fengguang.wu@...el.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Yang Shi <yang.shi@...ux.alibaba.com>, mgorman@...hsingularity.net,
        riel@...riel.com, hannes@...xchg.org, akpm@...ux-foundation.org,
        dave.hansen@...el.com, keith.busch@...el.com,
        dan.j.williams@...el.com, fan.du@...el.com, ying.huang@...el.com,
        ziy@...dia.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [v2 RFC PATCH 0/9] Another Approach to Use PMEM as NUMA Node

On Thu, Apr 18, 2019 at 11:02:27AM +0200, Michal Hocko wrote:
>On Wed 17-04-19 13:43:44, Yang Shi wrote:
>[...]
>> And, I'm wondering whether this optimization is also suitable to general
>> NUMA balancing or not.
>
>If there are convincing numbers then this should be a preferable way to
>deal with it. Please note that the number of promotions is not the only
>metric to watch. The overal performance/access latency would be another one.

Good question. Shi and me aligned today. Also talked with Mel (but
sorry I must missed some points due to poor English listening). It
becomes clear that

1) PMEM/DRAM page promotion/demotion is a hard problem to attack.
There will and should be multiple approaches for open discussion
before settling down. The criteria might be balanced complexity,
overheads, performance, etc.

2) We need a lot more data to lay solid foundation for effective
discussions. Testing will be a rather time consuming part for
contributor. We'll need to work together to create a number of
benchmarks that can well exercise the kernel promotion/demotion paths
and gather the necessary numbers. By collaborating on a common set of
tests, we can not only amortize efforts, but also compare different
approaches or compare v1/v2/... of the same approach conveniently.

Ying has already created several LKP test cases for that purpose.
Shi and me plan to join the efforts, too.

Thanks,
Fengguang