Re: Re: [External Mail] [RFC PATCH] mm/mempolicy: Weighted interleave auto-tuning

[Hyeonggon Yoo]

On 2024-12-14 4:57 AM, Joshua Hahn wrote:
> On Fri, 13 Dec 2024 15:19:20 +0900 Hyeonggon Yoo <hyeonggon.yoo@xxxxxx> wrote:
>
> > On 2024-12-11 06:54 AM, Joshua Hahn wrote:
> > > This patch introduces an auto-configuration for the interleave weights
> > > that aims to balance the two goals of setting node weights to be
> > > proportional to their bandwidths and keeping the weight values low.
> > > This balance is controlled by a value max_node_weight, which defines the
> > > maximum weight a single node can take.
> >
> > Hi Joshua,
> >
> > I am wondering how this is going to work for host memory + CXL memory
> > interleaving. I guess by "the ACPI table" you mean the ACPI HMAT or CXL
> > CDAT, both of which does not provide the bandwidth of host memory.
> > If this feature does not consider the bandwidth of host memory, manual
> > configuration will be inevitable anyway.
>
> Hi Hyeonggon,
>
> Thank you for reviewing my patch!

No problem :)

> As Gregory showed in his reply, I think it would be possible to get host bandwidth information
> using the ACPI HMAT.

You're right. I was wrong. I checked on our server, and its bandwidth 
information was valid for both local memory and CXL memory. Sorry for
the noise.

> [-----8<-----]
>
> > > +What:		/sys/kernel/mm/mempolicy/weighted_interleave/max_node_weight
> > > +Date:		December 2024
> > > +Contact:	Linux memory management mailing list <linux-mm@xxxxxxxxx>
> > > +Description:	Weight limiting / scaling interface
> > > +
> > > +		The maximum interleave weight for a memory node. When it is
> > > +		updated, any previous changes to interleave weights (i.e. via
> > > +		the nodeN sysfs interfaces) are ignored, and new weights are
> > > +		calculated using ACPI-reported bandwidths and scaled.
> > > +
> >
> > At first this paragraph sounded like "previously stored weights are
> > discarded after setting max_node_weight", but I think you mean
> > "User can override the default values, but defaults values are
> > calculated regardless of the values set by the user". Right?
>
> In the implementation, the first way you interpreted is the correct
> description. That is, if a user manually changes a ndoe weight,
> then updates the max_node_weight, the previous manual change will
> be overwritten by the newly scaled values.
>
> Does this behavior make sense?

Ok. then current implementation overwrites the node weights
previously set by the user.

I think it makes sense to re-scale all nodes and overwrite manually set 
node weights, because it's what the knob is meant to do, and the user 
still can override the weight by setting node weight after updating
max_node_weight.

By the way, could you please explain which part of this patch implements
this rule? IIUC it does not invalidate iw_table after updating these
default values, which makes get_il_weight() to use manully set node 
weights even after updating max_node_weight. (Or probably I just
misunderstood the code?)

> Have a great day!

Have a great day too :)

--
Hyeonggon