Re: [PATCH] improve the performance of large sequential write NFS workloads

Ingo Molnar
2009-12-18T22:07:58Z

* Steve Rago <sar-a+KepyhlMvJWk0Htik3J/w@public.gmane.org> wrote:

>
> On Fri, 2009-12-18 at 20:41 +0100, Ingo Molnar wrote:
> > * Steve Rago <sar-a+KepyhlMvJWk0Htik3J/w@public.gmane.org> wrote:
> >
> > > > Also, I don't think this needs to have a sysctl, it should just work.
> > >
> > > The sysctl is a *good thing* in that it allows the eager writeback behavior
> > > to be tuned and shut off if need be. I can only test the changes on a
> > > finite set of systems, so better safe than sorry.
> >
> > This issue has been settled many years ago and that's not what we do in the
> > Linux kernel. We prefer patches to core code where we are reasonably sure they
> > result in good behavior - and then we fix bugs in the new behavior, if any.
> >
> > (Otherwise odd sysctls would mushroom quickly and the system would become
> > untestable in practice.)
> >
> > Ingo
>
> I don't disagree, but "that's not what we do" hardly provides insight into
> making the judgment call. [...]

I gave you an example of the problems that arise, see the last sentence above.

> [...] In this case, the variety of combinations of NFS server speed, NFS
> client speed, transmission link speed, client memory size, and server memory
> size argues for a tunable parameter, because one value probably won't work
> well in all combinations. Making it change dynamically based on these
> parameters is more complicated than these circumstances call for, IMHO.

So having crappy tunables is the reason to introduce even more tunables? I
think you just gave a good second example of why we dont want sysctls for
features like this.

Ingo