Re: [PATCH v8] mm/mempolicy: Weighted Interleave Auto-tuning

[Joshua Hahn]

Hello Andrew,

Perhaps at this point it is best to just send a v9? : -)
The v9 will include: 2 code fixlets, as suggested by Ying Huang
And adding tags from both Honggyu and Ying.

I just thought it would be difficult to track 4 fixlets, but please let me
know whatever works best for you! Thank you always for your help. I hope you
have a great day!
Joshua

On Mon,  5 May 2025 11:23:28 -0700 Joshua Hahn <joshua.hahnjy@xxxxxxxxx> wrote:

> On machines with multiple memory nodes, interleaving page allocations
> across nodes allows for better utilization of each node's bandwidth.
> Previous work by Gregory Price [1] introduced weighted interleave, which
> allowed for pages to be allocated across nodes according to user-set ratios.
> 
> Ideally, these weights should be proportional to their bandwidth, so
> that under bandwidth pressure, each node uses its maximal efficient
> bandwidth and prevents latency from increasing exponentially.
> 
> Previously, weighted interleave's default weights were just 1s -- which
> would be equivalent to the (unweighted) interleave mempolicy, which goes
> through the nodes in a round-robin fashion, ignoring bandwidth information.
> 
> This patch has two main goals:
> First, it makes weighted interleave easier to use for users who wish to
> relieve bandwidth pressure when using nodes with varying bandwidth (CXL).
> By providing a set of "real" default weights that just work out of the
> box, users who might not have the capability (or wish to) perform
> experimentation to find the most optimal weights for their system can
> still take advantage of bandwidth-informed weighted interleave.
> 
> Second, it allows for weighted interleave to dynamically adjust to
> hotplugged memory with new bandwidth information. Instead of manually
> updating node weights every time new bandwidth information is reported
> or taken off, weighted interleave adjusts and provides a new set of
> default weights for weighted interleave to use when there is a change
> in bandwidth information.
> 
> To meet these goals, this patch introduces an auto-configuration mode
> for the interleave weights that provides a reasonable set of default
> weights, calculated using bandwidth data reported by the system. In auto
> mode, weights are dynamically adjusted based on whatever the current
> bandwidth information reports (and responds to hotplug events).
> 
> This patch still supports users manually writing weights into the nodeN
> sysfs interface by entering into manual mode. When a user enters manual
> mode, the system stops dynamically updating any of the node weights,
> even during hotplug events that shift the optimal weight distribution.
> 
> A new sysfs interface "auto" is introduced, which allows users to switch
> between the auto (writing 1 or Y) and manual (writing 0 or N) modes. The
> system also automatically enters manual mode when a nodeN interface is
> manually written to.
> 
> There is one functional change that this patch makes to the existing
> weighted_interleave ABI: previously, writing 0 directly to a nodeN
> interface was said to reset the weight to the system default. Before
> this patch, the default for all weights were 1, which meant that writing
> 0 and 1 were functionally equivalent. With this patch, writing 0 is invalid.
> 
> [1] https://lore.kernel.org/linux-mm/20240202170238.90004-1-gregory.price@xxxxxxxxxxxx/
> 
> Suggested-by: Yunjeong Mun <yunjeong.mun@xxxxxx>
> Suggested-by: Oscar Salvador <osalvador@xxxxxxx>
> Suggested-by: Ying Huang <ying.huang@xxxxxxxxxxxxxxxxx>
> Suggested-by: Harry Yoo <harry.yoo@xxxxxxxxxx>
> Reviewed-by: Harry Yoo <harry.yoo@xxxxxxxxxx>
> Co-developed-by: Gregory Price <gourry@xxxxxxxxxx>
> Signed-off-by: Gregory Price <gourry@xxxxxxxxxx>
> Signed-off-by: Joshua Hahn <joshua.hahnjy@xxxxxxxxx>
> ---