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Index: website/content/en/status/report-2024-07-2024-09/sched_prio-256_queue_runqueue.adoc
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+=== Scheduling Priorities: 256-queue Runqueues Sub-Project
+
+Contact: Olivier Certner <olce.freebsd.statusreports@certner.fr>
+
+The goal of the 256-queue runqueues sub-project is to fix FreeBSD's POSIX
+compliance in that different priority levels in the `SCHED_FIFO` or `SCHED_RR`
+scheduling classes must lead to immediate preemption by threads having higher
+priority.
+It is part of the bigger scheduling priorities revamp project aiming at
+rationalizing FreeBSD scheduling interfaces, including having consistent
+man:rtprio[2] and POSIX interface behaviors (where it makes sense), enhancing
+POSIX compliance, removing duplicate code and fixing existing bugs, and
+enhancing the non-standard parts both for better control and security.
+Expected benefits are increased usage of FreeBSD as a soft real-time platform,
+e.g., for video and audio processing in casual desktop uses to professional
+settings.
+Readers interested in this topic are invited to consult
+link:https://papers.freebsd.org/2024/asiabsdcon/certner-Scheduling_priorities_in_FreeBSD.files/AsiaBSDCon2024-Certner-Scheduling_priorities_in_FreeBSD-Article.pdf[AsiaBSDCon 2024's paper]
+and link:https://papers.freebsd.org/2024/eurobsdcon/certner-Scheduling_Priorities_and_FreeBSD.files/EuroBSDCon2024-Certner-Scheduling_Priorities_and_FreeBSD.pdf[EuroBSDCon 2024's slides]
+for a wider view, and to contact me for questions, feedbacks or discussions.
+
+Currently, priority levels specified either through the `prio` field of `struct
+rtprio` (man:rtprio[2] interface) or the `sched_priority` one of `struct
+sched_param`, for real-time scheduling classes (`RTP_PRIO_FIFO` and
+`RTP_PRIO_REALTIME` for the former, `SCHED_FIFO` and `SCHED_RR` for the latter)
+as well as idle-time ones (`RTP_PRIO_IDLE`), are conflated as follows: Each
+priority level that has the same quotient when divided by 4 is internally
+treated the same.
+In particular, threads being in the same such equivalence class but having
+higher priority will not preempt other threads in the same class, violating
+POSIX expectations for `SCHED_FIFO` and `SCHED_RR`.
+
+To remedy this situation, we have chosen an impacting internal change on the
+number of queues per runqueue, and defer to the above-mentioned EuroBSDCon
+2024's slides for more details.
+
+The switch to 256-queue runqueues having non-trivial impacts on the ULE
+scheduler, we have been analyzing it and tuning the scheduler to preserve its
+previous behavior with respect to anti-starvation and the effect of nice values.
+With the goal to perform further testing, we have revived Jeff Roberson's
+initial ULE's test tool, called `late` (currently available on
+link:https://github.com/OlCe2/late[GitHub]).
+
+All the modifications made as part of this sub-project are currently under
+review, starting with Phabricator's
+link:https://reviews.freebsd.org/D45387[review D45387] (click on the "Stack" tab
+to see the full series of reviews).
+
+In the course of this project, we have noticed that the effect of nice values is
+especially weak, and consequently have produced experimental patches to make
+their effect stronger.
+However, it is not clear at this point whether we can increase their effect
+satisfactorily enough in the current ULE setting.
+
+We have also started another scheduler project about adapting ULE to hybrid
+architectures, which might also trigger more drastic scheduler changes.
+
+Sponsor: The FreeBSD Foundation

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