[RFC PATCH] ring: adding TPAUSE instruction to ring dequeue

[Morten Brørup]

> From: David Coyle [mailto:david.coyle at intel.com]
> Sent: Wednesday, 3 May 2023 13.39
> 
> This is NOT for upstreaming. This is being submitted to allow early
> comparison testing with the preferred solution, which will add TAPUSE
> power management support to the ring library through the addition of
> callbacks. Initial stages of the preferred solution are available at
> http://dpdk.org/patch/125454.
> 
> This patch adds functionality directly to rte_ring_dequeue functions to
> monitor the empty reads of the ring. When a configurable number of
> empty reads is reached, a TPAUSE instruction is triggered by using
> rte_power_pause() on supported architectures. rte_pause() is used on
> other architectures. The functionality can be included or excluded at
> compilation time using the RTE_RING_PMGMT flag. If included, the new
> API can be used to enable/disable the feature on a per-ring basis.
> Other related settings can also be configured using the API.

I don't understand why DPDK developers keep spending time on trying to invent methods to determine application busyness based on entry/exit points in a variety of libraries, when the application is in a much better position to determine busyness. All of these "busyness measuring" library extensions have their own specific assumptions and weird limitations.

I do understand that the goal is power saving, which certainly is relevant! I only criticize the measuring methods.

For reference, we implemented something very simple in our application framework:
1. When each pipeline stage has completed a burst, it reports if it was busy or not.
2. If the pipeline busyness is low, we take a nap to save some power.

And here is the magic twist to this simple algorithm:
3. A pipeline stage is not considered busy unless it processed a full burst, and is ready to process more packets immediately. This interpretation of busyness has a significant impact on the percentage of time spent napping during the low-traffic hours.

This algorithm was very quickly implemented. It might not be perfect, and we do intend to improve it (also to determine CPU Utilization on a scale that the end user can translate to a linear interpretation of how busy the system is). But I seriously doubt that any of the proposed "busyness measuring" library extensions are any better.

So: The application knows better, please spend your precious time on something useful instead.

@David, my outburst is not directed at you specifically. Generally, I do appreciate experimenting as a good way of obtaining knowledge. So thank you for sharing your experiments with this audience!

PS: If cruft can be disabled at build time, I generally don't oppose to it.

-Morten