Re: Periodic rant about SCHED_ULE

From: Peter <>
Date: Thu, 23 Mar 2023 20:50:35 UTC
> Yes, you've all heard it before, but I've just reverified on FreeBSD

Yes, this is part of our culture: we need this periodic rant every
once and again.

> 13.1-RELEASE-p7 that SCHED_ULE gives terrible performance for "make
> buildworld" in the presence of a totally compute-bound job (misc/dnetc

Yes, this is well known, and works-as-designed. The reason is that a
totally compute-bound job can only be preempted, it cannot be
scheduled. A /mostly/ compute-bound job however does occasional system
calls, and at that point it will be scheduled.

Then, the design specifics of sched-ULE is that a preempted job will
be re-positioned by the scheduler to it's former place, i.e. as said
above, it will NOT be scheduled. In other words: it just continues to
run until the quantum is empty.
And that is the reason why your "totally compute-bound" job gets more
compute than the competing one with i/o.

You can actually ignore the responses with the naive questions here,
because what you observe is what the code says that it has to happen.
So, when I ran into this issue many years ago, I just fixed it, and my
machines run fine since then.

The only problem is: nobody wants the patch.

(And no, I don't say that patch would solve all problems, but it did
solve mine, and one could start from there.)