linux-kernel - Re: [PATCH v5 01/11] mm/mempolicy: implement the sysfs-based weighted

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Date: Tue, 2 Jan 2024 21:46:38 -0500
From: Gregory Price <gregory.price@...verge.com>
To: "Huang, Ying" <ying.huang@...el.com>
Cc: Gregory Price <gourry.memverge@...il.com>, linux-mm@...ck.org,
	linux-doc@...r.kernel.org, linux-fsdevel@...r.kernel.org,
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Subject: Re: [PATCH v5 01/11] mm/mempolicy: implement the sysfs-based
 weighted_interleave interface

On Tue, Jan 02, 2024 at 03:41:08PM +0800, Huang, Ying wrote:
> Think about the default weight value via HMAT/CDAT again.  It may be not
> a good idea to use "1" as default even for now.
> 
> For example,
> 
> - The memory bandwidth of DRAM is 100GB, whose default weight is "1".
> 
> - We hot-plug CXL.mem A with memory bandwidth 20GB.  So, we change the
>   weight of DRAM to 5, and use "1" as the weight of CXL.mem A.
> 
> - We hot-plug CXL.mem B with memory bandwidth 10GB.  So, we change the
>   weight of DRAM to 10, the weight of CXL.mem A to 2, and use "1" as the
>   weight of CXL.mem B.
> 
> That is, if we use "1" as default weight, we need to change weights of
> nodes frequently because we haven't a "base" weight.  The best candidate
> base weight is the weight of DRAM node.  For example, if we set the
> default weight of DRAM node to be "16" and use that as the base weight,
> we don't need to change it in most cases.  The weight of other nodes can
> be set according to the ratio of its memory bandwidth to that of DRAM.
> 
> This makes it easy to set the default weight via HMAT/CDAT too.
> 
> What do you think about that?
> 

Giving this more thought.

Hotplug should be an incredibly rare event. I don't think swapping defaults
"frequently" is a real problem we should handle.

It's expected that dynamic capacity devices will not cause a node to
hotplug, but instead cause a node to grow/shrink.

Seems perfectly fine to rebalance weights in response to rare events.

~Gregory