Re: Periodic rant about SCHED_ULE

On Thu, Mar 30, 2023 at 8:37 AM Mateusz Guzik <mjguzik@gmail.com> 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.

An elided simple algorithm for big.LITTLE, from Larry McVoy.. if you
run for an entire quantum, flag preference for big core.  If you run
for less or get punted off, flag for little core preference.

> 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,
> 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 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.
>
> 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].
>
> 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
> 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,
> but would not cover other programs (say a pdf reader). it may be it is
> good enough though
>
> --
> Mateusz Guzik <mjguzik gmail.com>
>