On Fri, 2009-12-18 at 23:07 +0100, Ingo Molnar wrote: > * Steve Rago <sar@xxxxxxxxxxxx> wrote: > > > > > On Fri, 2009-12-18 at 20:41 +0100, Ingo Molnar wrote: > > > * Steve Rago <sar@xxxxxxxxxxxx> 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 The examples I cited are not tunables. They are characteristics of the systems we use. I can't squeeze more than 1Gb/s out of my gigabit Ethernet connection; I can't make my 2GHz CPU compute any faster; I am limited by these components to the performance I can attain. Writing software that performs well in all combinations, especially to take advantage of the myriad of combinations, is difficult at best. The tunable introduced in the patch is a compromise to writing a much more complicated adaptive algorithm that most likely wouldn't have access to all of the information it needed anyway. Steve > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
