[erlang-bugs] R15B01 erlang:now() jumping ~24 days into the future

Tue Mar 5 20:20:40 CET 2013

The gist https://gist.github.com/garret-smith/5087169 is updated with a
slightly better version.  I was able to reproduce the jump in less than an
hour.  I also did some more things to perturb the timing code while the
test program was running.

Here is the latest info, everything I can think of that may have the
slightest effect:
 * R15B01 64-bit build
 * Pacific time zone (GMT -8)
 * Xeon E5405 in an HP DL160
 * no arguments to erl.exe
 * bursty, high CPU load, >75% memory used by other software
 * running Observer on the test VM displaying the "Load Charts" tab
 * made some small adjustments (~ 60 seconds) to the system clock while
running the tests - now() and os:timestamp() behaved as expected, initially
showing a delta and slowly converging
 * w32tm /resync to fix the system clock some time after perturbing it

The time jump in now() occurred when now() was ~9 seconds behind
os:timestamp() as reported by the new test program.

I'm starting to look at R16B now.

-Garret Smith

On Tue, Mar 5, 2013 at 8:37 AM, Garret Smith <garret.smith@REDACTED> wrote:

> I haven't seen anything unexpected in os:timestamp().  No jumps at all.
>
> CPU is an Intel Xeon X3430.
>
> I have reproduced it in the LosAngeles/Pacific Time (GMT -8) and US East
> coast time zone (GMT -5).
>
> I have not yet tried R16B.  I'll be starting that today.  I'm also trying
> to improve the test program, since it's taking quite a long time between
> jumps for me as well.  I'll let you know as soon as I have a better one.
>
> You have no idea how relieved I am that you are looking into this!
>
> Thanks,
> Garret Smith
>
>
> On Tue, Mar 5, 2013 at 3:06 AM, Patrik Nyblom <pan@REDACTED> wrote:
>
>>  Hi again...
>>
>> I'm not sure about one thing. What happens to os:timestamp() during these
>> jumps? Does it stay on track or does it also jump around?
>>
>> I've tried to reproduce it with your program, but has not yet succeeded.
>> Have you seen this on the R16B release as well?
>>
>> Is the hardware in any way fancy (like a lot of cores, some new processor
>> I don't have or something else?) or is there anything else special about
>> the machine? Also the time zone you're running in would be interesting, as
>> there is some time zone specific code there...
>>
>> I would really like to be able to reproduce it so you don't have to do
>> all the tests at your site, it might end up being really time consuming for
>> you if I make to many mistakes :)
>>
>> Cheers,
>> /Patrik
>>
>>
>>
>> On 03/05/2013 08:50 AM, Patrik Nyblom wrote:
>>
>> Hi!
>>
>> On 03/05/2013 02:26 AM, Garret Smith wrote:
>>
>>     I have been beating my head against a wall for weeks tracking down
>> spooky behaviour[sic] in one of our production systems.  I finally tracked
>> it down to "jumps" in the times returned by erlang:now(), causing all
>> timers in the system to expire at once.  I have witnessed this bug on
>> R15B01, both 64 and 32-bit versions running on Windows Server 2008 R2, both
>> on bare metal and VirtualBox VM.
>>
>>  The time jump is always around 2126000 seconds, or a little over 24
>> days.  The now() time does not try to converge with os:timestamp() as the
>> documentation suggests, and as I confirmed it does if you just change the
>> system clock.
>>
>>  Another VM running concurrently on the same machine but with little load
>> (diagnostic node & production node) did not time jump.
>>
>>  Higher load seems to make the time jumps happen more often.
>>
>>  Frequency between time jumps varies between seconds and hours, but when
>> a jump occurs, it is always 2126000 + (9 to 26) seconds.
>>
>>  I never see the jump in logfile timestamps that use os:timestamp() for
>> tagging log messages.  I had to start tracing a production node before I
>> caught the jump.  Here are some lines from a trace, where the timestamp in
>> trace_ts is printed using calendar:now_to_local_time() and then in raw
>> tuple format:
>>
>> 2013-4-16 21:40:1.993399|{1366,173601,993399}
>> 2013-4-16 21:40:1.993400|{1366,173601,993400}
>> 2013-5-11 12:13:41.986961|{1368,299621,986961}
>> 2013-5-11 12:13:41.986962|{1368,299621,986962}
>>
>>  then a bit later...
>>
>> 2013-5-11 12:36:19.955129|{1368,300979,955129}
>> 2013-5-11 12:36:19.955130|{1368,300979,955130}
>> 2013-6-5 3:9:49.538830|{1370,426989,538830}
>> 2013-6-5 3:9:49.538833|{1370,426989,538833}
>>
>>   Gah! That's obviously not supposed to happen...
>>
>>  I captured many such jumps over the course of a day or so.  Obviously
>> from the dates, 2 jumps happened before I started tracing.
>>
>>  I was able to reproduce the bug, though not as efficiently as my
>> production system, with the following sample program:
>> https://gist.github.com/garret-smith/5087169
>>
>> It took over an hour of runtime before the first time jump.  I am working
>> on a better way to reproduce it at the moment, but it's hard to test the
>> test with a bug so intermittent.
>>
>>  I am also testing various other VM versions.  My first hope was that
>> this was limited to the 64-bit version where we first encountered the
>> problem, but a change to the 32-bit version has only made the problem
>> happen less often, not eliminated it.
>>
>>  We never saw this bug with R14B03 which we were running previously to
>> R15B01.  However, system load is different so I can't make a direct
>> comparison.  I did notice a few significant updates to the Windows time
>> related code between R14B03 and R15:
>>
>>  git log sys_time.c
>>
>>  commit 46eb4359b05b220861453a869dc734480ec045a6
>> Author: Patrik Nyblom <pan@REDACTED>
>> Date:   Tue Dec 6 19:07:16 2011 +0100
>>
>>     Emulate localtime, gmtime and mktime to enable negative time_t
>>
>> commit 913f05af100e98a8665bbb6168e89fbcfe4ece75
>> Author: Bj<C3><B6>rn-Egil Dahlberg <egil@REDACTED>
>> Date:   Fri Dec 2 15:25:06 2011 +0100
>>
>>     Teach windows sys_localtime_r
>>
>>
>>   Yep, that's me... But even if I gave a totally weird time back from
>> those, the erlang:now logic should have stopped this from happening. I'll
>> try to reproduce using your example program. If nothing else helps, I'll
>> instrument a VM that gives som traces in the time code...
>>
>>  I am completely stumped.  What can I do next to help track down the
>> source of the bug?
>>
>>   Unfortunately, so am I. Especially weird that it's load related...
>> Maybe something is not locked as it should be...
>>
>>  Thanks,
>>  Garret Smith
>>
>> Thanks for reporting, I'll get back to you!
>>
>> Cheers,
>> /Patrik
>>
>>
>>
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