Re: Periodic rant about SCHED_ULE

From: Tomoaki AOKI <>
Date: Fri, 31 Mar 2023 12:57:51 UTC
On Mon, 27 Mar 2023 16:47:04 +0200
Mateusz Guzik <> wrote:

> On 3/27/23, Mateusz Guzik <> wrote:
> > On 3/25/23, Mateusz Guzik <> wrote:
> >> On 3/23/23, Mateusz Guzik <> wrote:
> >>> On 3/22/23, Mateusz Guzik <> wrote:
> >>>> On 3/22/23, Steve Kargl <> wrote:
> >>>>> On Wed, Mar 22, 2023 at 07:31:57PM +0100, Matthias Andree wrote:
> >>>>>>


> >
> > I repeat the setup: 8 cores, 8 processes doing cpu-bound stuff while
> > niced to 20 vs make -j buildkernel
> >
> > I had a little more look here, slapped in some hacks as a POC and got
> > an improvement from 67 minutes above to 21.
> >
> > Hacks are:
> > 1. limit hog timeslice to 1 tick so that is more eager to bail
> > 2. always preempt if pri < cpri
> >
> > So far I can confidently state the general problem: ULE penalizes
> > non-cpu hogs for blocking (even if it is not their fault, so to speak)
> > and that bumps their prio past preemption threshold, at which point
> > they can't preempt said hogs (despite hogs having a higher priority).
> > At the same time hogs use their full time slices, while non-hogs get
> > off cpu very early and have to wait a long time to get back on, at
> > least in part due to inability to preempt said hogs.
> >
> > As I mentioned elsewhere in the thread, interactivity scoring takes
> > "voluntary off cpu time" into account. As literally anything but
> > getting preempted counts as "voluntary sleep", workers get shafted for
> > going off cpu while waiting on locks in the kernel.
> >
> > If I/O needs to happen and the thread waits for the result, it most
> > likely does it early in its timeslice and once it's all ready it waits
> > for background hogs to get off cpu -- it can't preempt them.
> >
> > All that said:
> > 1. "interactivity scoring" (see sched_interact_score)
> >
> > I don't know if it makes any sense to begin with. Even if it does, it
> > counts stuff it should not by not differentiating between deliberately
> > going off cpu (e.g., actual sleep) vs just waiting for a file being
> > read. Imagine firefox reading a file from disk and being considered
> > less interactive for it.
> >
> > I don't have a solution for this problem. I *suspect* the way to go
> > would be to explicitly mark xorg/wayland/whatever as "interactive" and
> > have it inherited by its offspring. At the same time it should not
> > follow to stuff spawned in terminals. Not claiming this is perfect,
> > but it does eliminate the guessing game.
> >
> > Even so, 4BSD does not have any mechanism of the sort and reportedly
> > remains usable on a desktop just by providing some degree of fairness.
> >
> > Given that, I suspect the short term solution would whack said scoring
> > altogether and focus on fairness (see below).
> >
> > 2. fairness
> >
> > As explained above doing any offcpu-time inducing work instantly
> > shafts threads versus cpu hogs, even if said hogs are niced way above
> > them.
> >
> > Here I *suspect* position to add in the runqueue should be related to
> > how much slice was left when the thread went off cpu, while making
> > sure that hogs get to run eventually. Not that I have a nice way of
> > implementing this -- maybe a separate queue for known hogs and picking
> > them every n turns or similar.
> >
> Aight, now that I had a sober look at the code I think I cracked the case.
> The runq mechanism used by both 4BSD and ULE provides 64(!) queues,
> where the priority is divided by said number and that's how you know
> in which queue to land the thread.
> When deciding what to run, 4BSD uses runq_choose which iterates all
> queues from the beginning. This means threads of lower priority keep
> executing before the rest. In particular cpu hog lands with a high
> priority, looking worse than make -j 8 buildkernel and only running
> when there is nothing else ready to get the cpu. While this may sound
> decent, it is bad -- in principle a steady stream of lower priority
> threads can starve the hogs indefinitely.
> The problem was recognized when writing ULE, but improperly fixed --
> it ends up distributing all threads within given priority range across
> the queues and then performing a lookup in a given queue. Here the
> problem is that while technically everyone does get a chance to run,
> the threads not using full slices are hosed for the time period as
> they wait for the hog *a lot*.
> A hack patch to induce the bogus-but-better 4BSD behavior of draining
> all runqs before running higher prio threads drops down build time to
> ~9 minutes, which is shorter than 4BSD.
> However, the right fix would achieve that *without* introducing
> starvation potential.
> I also note the runqs are a massive waste of memory and computing
> power. I'm going to have to sleep on what to do here.
> For interested here is the hackery:
> sysctl kern.sched.slice_nice=0
> sysctl kern.sched.preempt_thresh=400 # arbitrary number higher than any prio
> -- 
> Mateusz Guzik <mjguzik>

Thanks for the patch.
Applied on top of main, amd64 at commit
9d33a9d96f5a2cd88d0955b5b56ef5058b1706c1, setup 2 sysctls as you
mentioned and tested as below

  *Play flac files by multimedia/audacious via audio/virtual_oss
  *Running www/firefox (not touched while testing)
  *Forcibly build lang/rust
  *Play games/aisleriot

at the same time.
games/aisleriot runs slower than the situation lang/rust is not in
build, but didn't "freeze" and audacious normally played next music on
playlist, even on lang/rust is building codes written in rust.

This is GREAT advance!
Without the patch, compiling rust codes eats up almost 100% of ALL
cores, and games/aisleriot often FREEZES SEVERAL MINUTES, and
multimedia/audacious needs to wait for, at worst, next music for FEW
MINUTES. (Once playback starts, the music is played normally until it

But unfortunately, the patch cannot be applied to stable/13, as some
prerequisite commits are not MFC'ed.
Missing commits are at least as below. There should be more, as I
gave up further tracking and haven't actually merged them to test.

 commit	954cffe95de1b9d70ed804daa45b7921f0f5c9da [1]
   ule: Simplistic time-sharing for interrupt threads.

 commit	fea89a2804ad89f5342268a8546a3f9b515b5e6c [2]
   Add sched_ithread_prio to set the base priority of an interrupt

 commit	85b46073242d4666e1c9037d52220422449f9584 [3]
   Deduplicate bus_dma bounce code.




Tomoaki AOKI    <>