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

From: Fengguang Wu
Date: Wed May 01 2019 - 01:21:19 EST

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.