preemtible Kernel

Hello,

From the introduction of kernel 2.6 on, the kernel can be configured to 
be preemtible. This promises better latency.

Many did not use this feature in the past: Why?

When I look at Fedora's 7 kernel source configuration (with 'make 
menuconfig'), the kernel is also not preemtible, but "voluntarily 
preemtible": What does that mean?

thanks and greets
Boris

> Many did not use this feature in the past: Why?

I mean "Many distributors did not use...", of course!

Greets Boris

[Fernando Apesteguía]

On 9/26/07, Boris Glawe  wrote:
> > Many did not use this feature in the past: Why?

Non preemptible kernels are suited for servers
Voluntary preempt. kernels for desktops
Full preempt kernels for low latency desktops.

It's just a matter of how many people are going to use the
distribution and with what purpose . I guess that a Red Hat Advanced
Server delivers a non preempt. kernel.

Cheers
> I mean "Many distributors did not use...", of course!
>
> Greets Boris
>
> --
> fedora-list mailing list
> [e-mail]
> To unsubscribe: https://www.redhat.com/mailman/listinfo/fedora-list

Fernando Apesteguía schrieb:
> On 9/26/07, Boris Glawe  wrote:
> > > Many did not use this feature in the past: Why?
>
> Non preemptible kernels are suited for servers
> Voluntary preempt. kernels for desktops
> Full preempt kernels for low latency desktops.
>
> It's just a matter of how many people are going to use the
> distribution and with what purpose . I guess that a Red Hat Advanced
> Server delivers a non preempt. kernel.
>
> Cheers

Thanks for the reply!

So, what is the difference between "voluntary preemtible" kernels und 
"preemtible" kernels (what is a low latency desktop?). Wwho is doing 
what voluntarily here?

Would it be worth to recompile the kernel with "preemptible kernel" 
enabled, as I'm using my linux installation on  a desktop machine? Or is 
the gain only measurable but not noticeable?

Thanks and greets

Boris
> > I mean "Many distributors did not use...", of course!
> >
> > Greets Boris
> >
> > --
> > fedora-list mailing list
> > [e-mail]
> > To unsubscribe: https://www.redhat.com/mailman/listinfo/fedora-list

[Fernando Apesteguía]

On 9/26/07, Boris Glawe  wrote:
> Fernando Apesteguía schrieb:
> > On 9/26/07, Boris Glawe  wrote:
> > > > Many did not use this feature in the past: Why?
> >
> > Non preemptible kernels are suited for servers
> > Voluntary preempt. kernels for desktops
> > Full preempt kernels for low latency desktops.
> >
> > It's just a matter of how many people are going to use the
> > distribution and with what purpose . I guess that a Red Hat Advanced
> > Server delivers a non preempt. kernel.
> >
> > Cheers
>
> Thanks for the reply!
>
> So, what is the difference between "voluntary preemtible" kernels und
> "preemtible" kernels (what is a low latency desktop?). Wwho is doing
> what voluntarily here?

I can't say it exactly, but as far as I know "preemptible" is a more
fine grained preemption. Low latency basically produces (without
running into details) that your systems behave smooth and all the
applications and windows seem to be very responsive. This is intended
for games, music, movies and more. For example when you don't want
your movie to be stopped at some points because you run ls -R in a
terminal.

A server, on the other hand, has other requirements: speed doing calcs
for example or serving connections, but the server is not intended to
be responsive _in terms of user experience_ (click here, there open a
new window, open a music player, flash movies...)
> Would it be worth to recompile the kernel with "preemptible kernel"
> enabled, as I'm using my linux installation on  a desktop machine? Or is
> the gain only measurable but not noticeable?

It would be worth or not depending of the kind of use you give to your
sytem. If you can see movies, listen music, you have a good web
experince and more, maybe it doesn't worth.
But if you have some problems like the ones above, you should give it a try.

If your systems acts like a server and you want to get all the
performance you can, then you should try to disable preemption.

Cheers

Cheers
> Thanks and greets
>
> Boris
> > > I mean "Many distributors did not use...", of course!
> > >
> > > Greets Boris
> > >
> > > --
> > > fedora-list mailing list
> > > [e-mail]
> > > To unsubscribe: https://www.redhat.com/mailman/listinfo/fedora-list
>
> --
> fedora-list mailing list
> [e-mail]
> To unsubscribe: https://www.redhat.com/mailman/listinfo/fedora-list

Ok, thanks for all the replys. I think I'll stick with the current kernel!

greets boris

[Frode Petersen]

Boris Glawe wrote:
> Fernando Apesteguía schrieb:
> > Non preemptible kernels are suited for servers
> > Voluntary preempt. kernels for desktops
> > Full preempt kernels for low latency desktops.
>
> Would it be worth to recompile the kernel with "preemptible kernel" 
> enabled, as I'm using my linux installation on  a desktop machine? Or is 
> the gain only measurable but not noticeable?

If you use your computer for music production, you might run into 
problems with audible clicks in the sound if you don't use a real time 
kernel, limiting i.e. the number of tracks you can record or play back 
at the same time. There are other factors as well, so depending on HOW 
you make music you might not need full preemtion. Other people know more 
about the practical ramifications than I do, though.

If you'd like to make music on your Fedora system, check out the 
PlanetCCRMA website, which also have customized kernels and so on.

http://ccrma.stanford.edu/planetccrma/software/

There might be other areas with similar needs, but most of them are 
probably not of very mainstream use(?).

Frode

[Gilboa Davara]

On Wed, 2007-09-26 at 14:22 +0200, Boris Glawe wrote:
> Hello,
>
> From the introduction of kernel 2.6 on, the kernel can be configured to 
> be preemtible. This promises better latency.
> Many did not use this feature in the past: Why?
>
> When I look at Fedora's 7 kernel source configuration (with 'make 
> menuconfig'), the kernel is also not preemtible, but "voluntarily 
> preemtible": What does that mean?
>
> thanks and greets
> Boris

As the name suggests (and I'm doing a massive simplification... so don't
shoot me) involuntary preemption means: "Unless strictly stated
other-wise, your code can be preempted any time, any place" while
voluntary preemption (roughly) means: "When you call certain kernel
functions the CPU might get rescheduled from under your feet".

Needless to say, involuntary preemption can wreck havoc on code that was
never designed with preemption in mind. (Mostly
resource-sharing-dead-locks and code re-entries)
As such, making the kernel preempt-able ready takes a lot of time (to
change the code behavior)  and a lot of testing. (To fix all the bugs)

- Gilboa