Date: Tue, 13 Aug 2002 09:30:04 -0700 (PDT) From: John Polstra <jdp@polstra.com> To: freebsd-bugs@FreeBSD.org Subject: Re: kern/41552: TCP timers' sysctl's overflow Message-ID: <200208131630.g7DGU494028149@freefall.freebsd.org>
next in thread | raw e-mail | index | archive | help
The following reply was made to PR kern/41552; it has been noted by GNATS. From: John Polstra <jdp@polstra.com> To: bug-followup@freebsd.org Cc: bde@zeta.org.au Subject: Re: kern/41552: TCP timers' sysctl's overflow Date: Tue, 13 Aug 2002 09:24:19 -0700 (PDT) In article <200208131220.g7DCK5eQ076224@freefall.freebsd.org>, Bruce Evans <bde@zeta.org.au> wrote: > On Tue, 13 Aug 2002, G.P. de Boer wrote: > > Now the question pops up if setting HZ -is- unusual. I can imagine that > > there are many admins around who turned on polling for extra > > performance/robustness and tuned option HZ because LINT says so. > > Garrett clarified that setting hz to more than 1000 breaks more than the > TCP timer sysctls. It violates an RFC. I don't think that's a valid argument. Our hz value is an implementation detail which shouldn't have anything to do with RFCs. The nugget of truth buried in what Garrett said is that increasing hz tickles a _bug_ in our TCP implementation, which in turn causes the RFC to be violated. (The bug is the direct use of hz rather than the use of a scaled version of it.) The proper solution is to fix that bug, not to restrict the value of hz artificially. In my opinion, 100 ticks per second is a ridiculously low value for hz on modern systems. Even a PII/400 can run at hz=10000 without significant overhead. (I.e., the overhead can hardly be measured.) There are plenty of reasonable applications that utterly rely on elevated hz values. Dummynet is just one example. Note, I don't think the fix referenced in this PR should be merged into the security branches anyway, since it is not security related. John To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-bugs" in the body of the message
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?200208131630.g7DGU494028149>