Date: Wed, 26 Jan 2011 13:14:15 -0800 From: mdf@FreeBSD.org To: "Robert N. M. Watson" <rwatson@freebsd.org> Cc: svn-src-head@freebsd.org, svn-src-all@freebsd.org, src-committers@freebsd.org Subject: Re: svn commit: r217830 - head/share/man/man9 Message-ID: <AANLkTim3M7GzNmV-M9%2BPhtVRv5EK3u9AsPz=YaESv7y5@mail.gmail.com> In-Reply-To: <161C86E9-A24C-4E71-90C6-26C3B47ACC1B@freebsd.org> References: <201101251739.p0PHdqKX044842@svn.freebsd.org> <alpine.BSF.2.00.1101260929430.44308@fledge.watson.org> <AANLkTimqyPYah5=yWHVxf3Us4=cBYKGkb0oyAE%2B7R-%2Bt@mail.gmail.com> <12EB1BEA-F0AF-4B2A-AFEB-9C38C7994FA8@freebsd.org> <AANLkTimn0te0NKR%2BusYC6CzxUVVaP%2BnpZKstsw1mWC7o@mail.gmail.com> <161C86E9-A24C-4E71-90C6-26C3B47ACC1B@freebsd.org>
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On Wed, Jan 26, 2011 at 1:10 PM, Robert N. M. Watson <rwatson@freebsd.org> wrote: > > On 26 Jan 2011, at 18:29, mdf@FreeBSD.org wrote: > >>> I suppose an important question is now often we see this actually faili= ng >> >> 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. =A0Our 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 monit= oring sysctls. Most systems have a dozen, hundred, or a thousand connection= s. Some have half a million or a million. If we switched to requiring wirin= g 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, th= is 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. For situations like this where there is a lot of output but it doesn't need to be serialized by a lock held across the whole data fetch, then yes, using sbuf_new_for_sysctl() would wire more memory. Thanks, matthew
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