Re: Linux 2.6.29

Matthew Garrett
2009-03-27T03:01:42Z

On Thu, Mar 26, 2009 at 06:25:19PM -0700, Andrew Morton wrote:
> On Thu, 26 Mar 2009 18:03:15 -0700 (PDT) Linus Torvalds <torvalds@linux-foundation.org> wrote:
> > You state that without any amount of data to back it up, as if it was some
> > kind of truism. It's not.
>
> I've seen you repeatedly fiddle the in-kernel defaults based on
> in-field experience. That could just as easily have been done in
> initscripts by distros, and much more effectively because it doesn't
> need a new kernel. That's data.

If there's a sensible default then it belongs in the kernel. Forcing
these decisions out to userspace just means that every distribution
needs to work out what these settings are, and the evidence we've seen
when they attempt to do this is that we end up with things like broken
cpufreq parameters because these are difficult problems. The simple
reality is that almost every single distribution lacks developers with
sufficient understanding of the problem to make the correct choice.

The typical distribution lifecycle is significantly longer than a kernel
release cycle. It's massively easier for people to pull updated kernels.

> Why does everyone just sit around waiting for the kernel to put a new
> value into two magic numbers which userspace scripts could have set?

If the distribution can set a globally correct value then that globally
correct value should be there in the first place!

> My /etc/rc.local has been tweaking dirty_ratio, dirty_background_ratio
> and swappiness for many years. I guess I'm just incredibly advanced.

And how have you got these values pushed into other distributions? Is
your rc.local available anywhere?

Linus is absolutely right here. Pushing these decisions out to userspace
means duplicated work in the best case - in the worst case it means most
users end up with the wrong value.

--
Matthew Garrett | mjg59@srcf.ucam.org