[systemd-devel] Designing a scheduler interface

[Colin Walters]

I'm going to set a high bar for you here:

On Thu, Jan 13, 2011 at 12:29 PM, Daniel Poelzleithner
<poelzi at poelzi.org> wrote:
>
> I'm currently implementing a dynamic linux kernel optimizer called
> ulatencyd [1]. In my opinion the desktop experience (which applies to
> servers as well) can be much improved by dynamically adjusting the the
> kernel. Having a very fair scheduler is a very good thing, but this is
> not the best experience for a user. The user for example expects the
> current used program to be as fast as possible, not some random
> background task getting the same cpu usage.

So it's fine to experiment with a project like this, but I think
ultimately it's not a good idea unless you have a reliable benchmark
and numbers for it.  Otherwise, how do you know you're actually making
things better in general?  This is a very complicated domain.  Have
you (or anyone) for example looked at what happens if we were to move
tasks between cgroups frequently?  What kind of kernel locks does that
take?  Etc.  If you're not careful, I could easily imagine making
things *worse*.

Basically, don't optimize without performance numbers to back it up.
Anecdotes about "updatedb" running in the background are OK as a basis
for experimentation, but maybe the right fix is just a strategic
"ionice/nice" in a few places in the OS for those things, rather than
a daemon.