kern.sched.quantum: Creepy, sadistic scheduler

[Alban Hertroys]

> On 4 Apr 2018, at 2:52, Peter <pmc at citylink.dinoex.sub.org> wrote:
> 
> Occasionally I noticed that the system would not quickly process the
> tasks i need done, but instead prefer other, longrunning tasks. I
> figured it must be related to the scheduler, and decided it hates me.

If it hated you, it would behave much worse.

> A closer look shows the behaviour as follows (single CPU):

A single CPU? That's becoming rare! Is that a VM? Old hardware? Something really specific?

> Lets run an I/O-active task, e.g, postgres VACUUM that would

And you're running a multi-process database server on it no less. That is going to hurt, no matter how well the scheduler works.

> continuousely read from big files (while doing compute as well [1]):
> >pool        alloc   free   read  write   read  write
> >cache           -      -      -      -      -      -
> >  ada1s4    7.08G  10.9G  1.58K      0  12.9M      0
> 
> Now start an endless loop:
> # while true; do :; done
> 
> And the effect is:
> >pool        alloc   free   read  write   read  write
> >cache           -      -      -      -      -      -
> >  ada1s4    7.08G  10.9G      9      0  76.8K      0
> 
> The VACUUM gets almost stuck! This figures with WCPU in "top":
> 
> >  PID USERNAME   PRI NICE   SIZE    RES STATE    TIME    WCPU COMMAND
> >85583 root        99    0  7044K  1944K RUN      1:06  92.21% bash
> >53005 pgsql       52    0   620M 91856K RUN      5:47   0.50% postgres
> 
> Hacking on kern.sched.quantum makes it quite a bit better:
> # sysctl kern.sched.quantum=1
> kern.sched.quantum: 94488 -> 7874
> 
> >pool        alloc   free   read  write   read  write
> >cache           -      -      -      -      -      -
> >  ada1s4    7.08G  10.9G    395      0  3.12M      0
> 
> >  PID USERNAME   PRI NICE   SIZE    RES STATE    TIME    WCPU COMMAND
> >85583 root        94    0  7044K  1944K RUN      4:13  70.80% bash
> >53005 pgsql       52    0   276M 91856K RUN      5:52  11.83% postgres
> 
> 
> Now, as usual, the "root-cause" questions arise: What exactly does
> this "quantum"? Is this solution a workaround, i.e. actually something
> else is wrong, and has it tradeoff in other situations? Or otherwise,
> why is such a default value chosen, which appears to be ill-deceived?
> 
> The docs for the quantum parameter are a bit unsatisfying - they say
> its the max num of ticks a process gets - and what happens when
> they're exhausted? If by default the endless loop is actually allowed
> to continue running for 94k ticks (or 94ms, more likely) uninterrupted,
> then that explains the perceived behaviour - buts thats certainly not
> what a scheduler should do when other procs are ready to run.

I can answer this from the operating systems course I followed recently. This does not apply to FreeBSD specifically, it is general job scheduling theory. I still need to read up on SCHED_ULE to see how the details were implemented there. Or are you using the older SCHED_4BSD?
Anyway...

Jobs that are ready to run are collected on a ready queue. Since you have a single CPU, there can only be a single job active on the CPU. When that job is finished, the scheduler takes the next job in the ready queue and assigns it to the CPU, etc.

Now, that would cause a much worse situation in your example case. The endless loop would keep running once it gets the CPU and would never release it. No other process would ever get a turn again. You wouldn't even be able to get into such a system in that state using remote ssh.

That is why the scheduler has this "quantum", which limits the maximum time the CPU will be assigned to a specific job. Once the quantum has expired (with the job unfinished), the scheduler removes the job from the CPU, puts it back on the ready queue and assigns the next job from that queue to the CPU.
That's why you seem to get better performance with a smaller value for the quantum; the endless loop gets forcibly interrupted more often.

This changing of the active job however, involves a context switch for the CPU. Memory, registers, file handles, etc. that were required by the previous job needs to be put aside and replaced by any such resources related to the new job to be run. That uses up time and does nothing to progress the jobs that are waiting for the CPU. Hence, you don't want the quantum to be too small either, or you'll end up spending significant time switching contexts. That gets worse when the job involves system calls, which are handled by the kernel, which is also a process that needs to be switched (and Meltdown made that worse, because more rigorous clean-up is necessary to prevent peeks into sections of memory that were owned by the kernel process previously).

The "correct" value for the quantum depends on your type of workload. PostgreSQL's auto-vacuum is a typical background process that will probably (I didn't verify) request to be run at a lower priority, giving other, more important, jobs more chance to get picked from the ready queue (provided that the OS implements priority for the ready queue).
That is probably why your endless loop gets much more CPU time than the VACUUM process. It may be that FreeBSD's default value for the quantum is not suitable for your workload. Finding the one best suited to you is not particularly easy though - perhaps FreeBSD allows access to average job times (below quantum) that can be used to calculate a reasonable average from.

That said, SCHED_ULE (the default scheduler for quite a while now) was designed with multi-CPU configurations in mind and there are claims that SCHED_4BSD works better for single-CPU configurations. You may give that a try, if you're not already on SCHED_4BSD.

A much better option in your case would be to put the database on a multi-core machine.

> [1]
> A pure-I/O job without compute load, like "dd", does not show
> this behaviour. Also, when other tasks are running, the unjust
> behaviour is not so stongly pronounced.

That is probably because dd has the decency to give the reins back to the scheduler at regular intervals.

Alban Hertroys
--
If you can't see the forest for the trees,
cut the trees and you'll find there is no forest.