linux-kernel - Re: gettimeofday() jumping into the future

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Message-ID: <46CD85AC.7050805@argo.co.il>
Date:	Thu, 23 Aug 2007 16:03:40 +0300
From:	Avi Kivity <avi@...o.co.il>
To:	Gerald Britton <gbritton@...mcom.org>
CC:	Michael Smith <msmith@...h.org>, linux-kernel@...r.kernel.org,
	Andy Wingo <wingo@...endo.com>
Subject: Re: gettimeofday() jumping into the future

Gerald Britton wrote:
> On Thu, Aug 23, 2007 at 01:08:27PM +0200, Michael Smith wrote:
>   
>> Hi,
>>
>> We've been seeing some strange behaviour on some of our applications
>> recently. I've tracked this down to gettimeofday() returning spurious
>> values occasionally.
>>
>> Specifically, gettimeofday() will suddenly, for a single call, return
>> a value about 4398 seconds (~1 hour 13 minutes) in the future. The
>> following call goes back to a normal value.
>>     
>
> I have seen this as well (on a 2.6.20.4 kernel).  The value returned was
> always identical each time the glitch occured (just a little over 4398
> seconds).  I saw it watching packet receive timestamps and on the system in
> question, it would generally hit this problem around once a minute.  When
> moving forward to a 2.6.21 kernel, the problem seemed to go away (also back
> to 2.6.17, unfortunately I didn't have any sample points inbetween).
> I didn't have free time to spend bisecting attempting to find when the
> behavior started or stopped.
>
>   

That value, in nanoseconds, is 0x3fffd3a4c00.  The next second is 
0x40038d51600.  Is the wraparound at (0x400 << 32) significant?

-- 
error compiling committee.c: too many arguments to function

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