Re: Periodic rant about SCHED_ULE

From: Mark Johnston <>
Date: Thu, 30 Mar 2023 20:08:29 UTC
On Thu, Mar 30, 2023 at 05:36:54PM +0200, Mateusz Guzik wrote:
> I looked into it a little more, below you can find summary and steps forward.
> First a general statement: while ULE does have performance bugs, it
> has better basis than 4BSD to make scheduling decisions. Most notably
> it understands CPU topology, at least for cases which don't involve
> big.LITTLE. For any non-freak case where 4BSD performs better, it is a
> bug in ULE if this is for any reason other than a tradeoff which can
> be tweaked to line them up. Or more to the point, there should not be
> any legitimate reason to use 4BSD these days and modulo the bugs
> below, you are probably losing on performance for doing so.
> Bugs reported in this thread by others and confirmed by me:
> 1. failure to load-balance when having n CPUs and n + 1 workers -- the
> excess one stays on one the same CPU thread continuously penalizing
> the same victim. as a result total real time to execute a finite
> computation is longer than in the case of 4BSD
> 2. unfairness of nice -n 20 threads vs threads going frequently off
> CPU (e.g., due to I/O) -- after using only a fraction of the slice the
> victim has to wait for the cpu hog to use up its entire slice, rinse
> and repeat. This extends a 7+ minute buildkernel to over 67 minutes,
> not an issue on 4BSD
> I did not put almost any effort into investigating no 1. There is code
> which is supposed to rebalance load across CPUs, someone(tm) will have
> to sit through it -- for all I know the fix is trivial.
> Fixing number 2 makes *another* bug more acute and it complicates the
> whole ordeal.
> Thus, bug reported by me:
> 3. interactivity scoring is bogus -- originally introduced to detect
> "interactive" behavior by equating being off CPU with waiting for user
> input. One part of the problem is that it puts *all* non-preempted off
> CPU time into one bag: a voluntary sleep. This includes suffering from
> lock contention in the kernel, lock contention in the program itself,

Note that time spent off-CPU on a turnstile is not counted as sleeping
for the purpose of interactivity scoring, so this observation applies
only to sx, lockmgr and sleepable rm locks.  That's not to say that this
behaviour is correct, but it doesn't apply to some of the most contended
locks unless I'm missing something.

> file I/O and so on, none of which has bearing on how interactive or
> not the program might happen to be. A bigger part of the problem is
> that at least today, the graphical programs don't even act this way to
> begin with -- they stay on CPU *a lot*.

I think this statement deserves more nuance.  I was on a video call just
now and firefox was consuming about the equivalent of 20-30% of a CPU
across all threads.  What kind of graphical programs are you talking
about specifically?

> I asked people to provide me with the output of: dtrace -n
> 'sched:::on-cpu { @[execname] = lquantize(curthread->td_priority, 0,
> 224, 1); }' from their laptops/desktops.
> One finding is that most people (at least those who reported) use firefox.
> Another finding is that the browser is above the threshold which would
> be considered "interactive" for vast majority of the time in all
> reported cases.

That is not true of the output that I sent.  There, most of the firefox
thread samples are in the interactive range [88-135].  Some show an even
higher priority, maybe due to priority propagation.

> I booted a 2 thread vm with xfce and decided to click around. Spawned
> firefox, opened a file manager (Thunar) and from there I opened a
> movie to play with mpv. As root I spawned make -j 2 buildkernel. it
> was not particularly good :)
> I found that mpv spawns a bunch of threads, most notably 2 distinct
> threads for audio and video output. The one for video got a priority
> of 175, while the rest had either 88 or 89 -- the lowest for
> timesharing not considered interactive [note lower is considered
> better].

Presumably all of the video decoding was done in software, since you're
running in a VM?  On my desktop, mpv does not consume much CPU and is
entirely interactive.  Your test suggests that you expect ULE to
prioritize a CPU hog, which doesn't seem realistic absent some
scheduling hints from the user or the program itself.  Problem 2 is the
opposite problem: timesharing CPU hogs are allowed to starve other
timesharing threads.

> At the same time the file manager who was left in the background kept
> doing evil syscall usage, which as a result bouncing between a regular
> timesharing priority and one which made it "interactive", even though
> the program was not touched for minutes.
> Or to put it differently, the scheduler failed to recognize that mpv
> is the program to prioritize, all while thinking the background time
> waster is the thing to look after (so to speak).
> This brings us to fixing problem 2: currently, due to the existence of
> said problem, the interactivity scoring woes are less acute -- the
> venerable make -j example is struggling to get CPU time, as a result
> messing with real interactive programs to a lesser extent. If that
> gets fixed, we are in a different boat altogether.
> I don't see a clean solution.
> Right now I'm toying with the idea of either:
> 1. having programs explicitly tell the kernel they are interactive

I don't see how this can work.  It's not just traditional "interactive"
programs that benefit from this scoring, it applies also to network
servers and other programs which spend most of their time sleeping but
want to handle requests with low latency.

Such an interface would also let any program request soft realtime
scheduling without giving up the ability to monopolize CPU time, which
goes against ULE's fairness goals.

> 2. adding a scheduler hook to /dev/dsp -- the observation is that if a
> program is producing sound it probably should get some cpu time in a
> timely manner. this would cover audio/video players and web browsers,

On my system at least firefox doesn't open /dev/dsp, it sends audio
streams to pulseaudio.

> but would not cover other programs (say a pdf reader). it may be it is
> good enough though

I think some more thorough analysis, using tools like schedgraph or
KUtrace[1], is needed to characterize the problems you are reporting
with interactivity scoring.  It's also not clear how any of this would
address the problem that started this thread, wherein two competing
timesharing (i.e., non-interactive) workloads get uneven amounts of CPU

There is absolutely room for improvement in ULE's scheduling decisions.
It seems to be common practice to tune various ULE parameters to get
better interactive performance, but in general I see no analysis
explaining /why/ exactly they help and what goes wrong with the default
parameter values in specific workloads.  schedgraph is a very useful
tool for this sort of thing.

Such tools also required to rule out bugs in ULE itself, when looking at
abnormal scheduling behaviour.  Last year some scheduler races[2] were
fixed that apparently hurt system performance on EPYC quite a bit.  I
was told privately that applying those patches to 13.1 improved IPSec
throughput by ~25% on EPYC, and I wouldn't be surprised if there are
more improvements to be had which don't involve modifying core
heuristics of the scheduler.  Either way this requires deeper analysis
of ULE's micro-level behaviour; I don't think "interactivity scoring is
bogus" is a useful starting point.

    Dick Sites' tracing framework, ported to FreeBSD.  I have some WIP
    to integrate parts of it into the base system and ports tree but
    haven't yet had a good chunk of time to devote to it.  I hope to get
    it done soon.  Please let me know if you'd like to try it.

[2] There were several more, but:
    are the ones that affected ULE specifically.