On 26 Jan 2011, at 18:29, mdf@FreeBSD.org wrote:
>> I suppose an important question is now often we see this actually failing
>
> I don't believe we've ever seen a memory failure relating to sysctls
> at Isilon and we've been using the equivalent of this code for a few
> years. Our machines aren't low memory but they are under memory
> pressure sometimes.
The kinds of cases I worry about are things like the tcp connection monitoring sysctls. Most systems have a dozen, hundred, or a thousand connections. Some have half a million or a million. If we switched to requiring wiring every page needed to store that list, it would do terrible things to the system. So really what I have in mind is: either we handle cases like that well, or we put in a clear warning and have obvious failure modes to catch the cases where it didn't work out. In practice, I think we would not want to switch the tcpcb/inpcb sysctl for this reason, but as people say "ah, this is convenient" we need to make sure it's handled well, and easy to debug problems when they do arise.
Robert