Re: Periodic rant about SCHED_ULE

On 3/31/23, Tomoaki AOKI <junchoon@dec.sakura.ne.jp> wrote:
> On Mon, 27 Mar 2023 16:47:04 +0200
> Mateusz Guzik <mjguzik@gmail.com> wrote:
>
>> On 3/27/23, Mateusz Guzik <mjguzik@gmail.com> wrote:
>> > On 3/25/23, Mateusz Guzik <mjguzik@gmail.com> wrote:
>> >> On 3/23/23, Mateusz Guzik <mjguzik@gmail.com> wrote:
>> >>> On 3/22/23, Mateusz Guzik <mjguzik@gmail.com> wrote:
>> >>>> On 3/22/23, Steve Kargl <sgk@troutmask.apl.washington.edu> wrote:
>> >>>>> On Wed, Mar 22, 2023 at 07:31:57PM +0100, Matthias Andree wrote:
>> >>>>>>
>
>    (snip)
>
>> >
>> > 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:
>> https://people.freebsd.org/~mjg/.junk/ule-poc-hacks-dont-use.diff
>>
>> sysctl kern.sched.slice_nice=0
>> sysctl kern.sched.preempt_thresh=400 # arbitrary number higher than any
>> prio
>>
>> --
>> Mateusz Guzik <mjguzik gmail.com>
>
> 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
> ends.)
>

thanks for testing

the patch is just the bare minimum to prove what the problem is. due
to that it runs into problems a real fix will not

however, even with the real fix i can't promise optimal behavior

> But unfortunately, the patch cannot be applied to stable/13, as some
> prerequisite commits are not MFC'ed.

there are no prerequisites for merging a proper fix

-- 
Mateusz Guzik <mjguzik gmail.com>