Date: Mon, 12 Feb 2018 17:29:50 -0700 From: Alan Somers <asomers@freebsd.org> Cc: Mike Pumford <michaelp@bsquare.com>, FreeBSD <freebsd-stable@freebsd.org> Subject: Re: Clock occasionally jumps backwards on 11.1-RELEASE Message-ID: <CAOtMX2jXk40O2jTSwmJQ-f=mrREzvc7jjZ4=mxBRDavwbxG5mg@mail.gmail.com> In-Reply-To: <CAOtMX2hNxRYR0Q3WxhM64wj5bNq2-wcOoSTxn_wixMiTQPseFA@mail.gmail.com> References: <CAOtMX2iCkurg8HXn7KD9AbrPcDVSRN-jK4MR%2BgFMAd%2BOFEdpow@mail.gmail.com> <0b170dae-b816-ea49-3516-40bfd1deaa2a@bsquare.com> <CAOtMX2hNxRYR0Q3WxhM64wj5bNq2-wcOoSTxn_wixMiTQPseFA@mail.gmail.com>
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On Tue, Jan 23, 2018 at 8:40 AM, Alan Somers <asomers@freebsd.org> wrote: > On Tue, Jan 23, 2018 at 3:48 AM, Mike Pumford <michaelp@bsquare.com> > wrote: > >> On 22/01/2018 17:07, Alan Somers wrote: >> >>> Since upgrading my jail server to 11.1-RELEASE, the clock occasionally >>> jumps backwards by 5-35 minutes for no apparent reason. Has anybody seen >>> something like this? >>> >>> Details >>> ===== >>> >>> * Happens about once a day on my jail server, and has happened at least >>> once on a separate bhyve server. >>> >>> * The jumps almost always happen between 1 and 3 AM, but I've also seen >>> them happen at 06:30 and 20:15. >>> >>> That's the window when the period scripts are run which if you have a >> default configuration and a lot of jails will put the system under a lot of >> stress. >> > > That did not fail to escape my notice. However, none of the jails' > periodic jobs involve the clock in any way. And I wouldn't think that a > high CPU load could cause clock drift, could it? This isn't Windows XP, > after all. > > >> * I'm using the default ntp.conf file. >>> >>> Are you running ntpd inside the jail or on the jail host? On my jail >> systems (which are 10.3 and 11.1) I run ntpd out the jail host (outside all >> jails) and not inside the jails and the jails then get the accurate time as >> the underlying host has accurate time. >> > > Only on the host. > > New info: there is a possibility that my NFS server is hanging for > awhile. That would explain my problem's timing. However, ntpd shouldn't > be accessing any NFS shares, and I wouldn't think that a hung NFS server > should be able to pause the clock. I'm doing a new experiment that should > be more informative. But I'll have to wait until the problem recurs to > learn anything. > I have a little more data now. The problem happens much more frequently than I originally realized, but usually for just a few seconds at a time. It looks like the system is hanging for awhile and then recovering. Or at least, the clocks are hanging. The only other possibility would be for both the realtime _and_ monotonic clocks to jump backwards. In any case, the problem is not ntpd's fault. I don't know what could cause a system to hang for up to 30 minutes without crashing, and I'm not sure how to tell unless it happens during working hours. I'll send another update if I learn more. -Alan
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