Date: Thu, 30 Mar 2023 20:50:31 +0200 From: Mateusz Guzik <mjguzik@gmail.com> To: Kevin Bowling <kevin.bowling@kev009.com> Cc: freebsd-hackers@freebsd.org Subject: Re: Periodic rant about SCHED_ULE Message-ID: <CAGudoHGNRQudBUj-XCNb=AirOgWn%2Bs0mi96v3axK4ZZSSdybZA@mail.gmail.com> In-Reply-To: <CAK7dMtAHOpq579Hb_Ar3e-VaYZGfawAxBMDoM0Zt0T_=UD5Jgw@mail.gmail.com> References: <a401e51a-250a-64a0-15cb-ff79bcefbf94@m5p.com> <8173cc7e-e934-dd5c-312a-1dfa886941aa@FreeBSD.org> <8cfdb951-9b1f-ecd3-2291-7a528e1b042c@m5p.com> <c3f5f667-ba0b-c40c-b8a6-19d1c9c63c5f@FreeBSD.org> <ZBtRJhNHluj5Nzyk@troutmask.apl.washington.edu> <CAGudoHEj%2BkoaYhkjzDE5KX9OsCno=X5M_E3z9uwg6Pg7dtqTsA@mail.gmail.com> <CAGudoHHxTT-Cn11zcFB3ZwF76UcRUv=QS28RLgzd=hVehTy0Kg@mail.gmail.com> <CAGudoHGoh30O-3O0jjwevDvP43-ykUt6JUDiwRNW918VZfybhA@mail.gmail.com> <CAGudoHEWfy61XSMhXdYOrKWVotuC0Kc6NSWiaaZCy6aQhbvXoQ@mail.gmail.com> <CAGudoHFPqz_LtsVNnz4P2gyKXz5Z8hU%2Bv6QYGizm4%2BDtZRn8Yg@mail.gmail.com> <CAGudoHGzBjXjXZFs%2BqZJUS-M6VeX5=LB2ifRLP7hFBZXPvqP7g@mail.gmail.com> <CAK7dMtAsBehP2cy6cn31Z%2BSo6T2Q_mpN5ibEmYMNPOkWQHk8FA@mail.gmail.com> <CAK7dMtAHOpq579Hb_Ar3e-VaYZGfawAxBMDoM0Zt0T_=UD5Jgw@mail.gmail.com>
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On 3/30/23, Kevin Bowling <kevin.bowling@kev009.com> wrote:
> On Thu, Mar 30, 2023 at 11:29=E2=80=AFAM Kevin Bowling <kevin.bowling@kev=
009.com>
> wrote:
>>
>> On Thu, Mar 30, 2023 at 8:37=E2=80=AFAM 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] =3D 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.
>
> One other random anecdote. Windows 11 uses window focus to highly
> boost scheduling priority in an obviously effective way. I have no
> idea how difficult something like that would be to fit into the unix
> world.
>
I thought about doing something like that, but I consider it dodgy.
Imagine you play some crap from youtube while messing around in a text
editor -- I'm pretty sure the former is more prone to disturbance from
scheduling changes.
Anyhow after sending the above e-mail an actual solution hit me: the X
server can tell the kernel what processes connect to it over the unix
socket, which again very well may be good enough.
In the reports I got I found pulseaudio, this one may need to be
patched in a similar manner.
--=20
Mateusz Guzik <mjguzik gmail.com>
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