On Wednesday, January 26, 2011 4:14:15 pm m...@freebsd.org wrote: > On Wed, Jan 26, 2011 at 1:10 PM, Robert N. M. Watson > <rwat...@freebsd.org> wrote: > > > > On 26 Jan 2011, at 18:29, m...@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. > > > > But I think that problem exists today using sysctl for output, since > it's non-iterative. In fact, it's often worse today, because in > addition to the user-space buffer that needs to be large enough to > hold the output, the kernel needs to malloc(9) a buffer to hold it > before doing the one SYSCTL_OUT at the end that most routines I've > seen use.
Not always. I think in the case of the inpcb's what happens is that we hold enough references on objects that we can drop the locks while calling SYSCTL_OUT() without requiring us to 1) allocate a full duplicate of the buffer in KVM, or 2) wire the entire userland buffer. -- John Baldwin _______________________________________________ svn-src-head@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/svn-src-head To unsubscribe, send any mail to "svn-src-head-unsubscr...@freebsd.org"
