Re: [RFC - 0/9] Generic timekeeping subsystem (v. B5)

[George Anzinger]

john stultz wrote:
> On Tue, 2005-08-23 at 17:29 -0700, George Anzinger wrote:
>
> > Roman Zippel wrote:
> >
> > > Hi,
> > >
> > > On Tue, 23 Aug 2005, john stultz wrote:
> > >
> > >
> > >
> > > > I'm assuming gettimeofday()/clock_gettime() looks something like:
> > > >  xtime + (get_cycles()-last_update)*(mult+ntp_adj)>>shift
> > >
> > >
> > > Where did you get the ntp_adj from? It's not in my example.
> > > gettimeofday() was in the previous mail: "xtime + (cycle_offset * mult +
> > > error) >> shift". The difference between system time and reference 
> > > time is really important. gettimeofday() returns the system time, NTP 
> > > controls the reference time and these two are synchronized regularly.
> > > I didn't see that anywhere in your example.
>
>
> > If I read your example right, the problem is when the NTP adjustment 
> > changes while the two clocks are out of sync (because of a late tick). 
>
>
> Not quite. The issue that I'm trying to describe is that if, we
> inconsistently calculate time intervals in gettimeofday and the timer
> interrupt, we have the possibility for time inconsistencies.
>
> The trivial example using the current code would be something like:
>
> Again with my 2 cyc per tick clock, HZ=1000.
>
> gettimeofday():
> 	xtime + offset_ns
>
> timer_interrupt:
> 	xtime += tick_length + ntp_adj
> 	offset_ns = 0
>
> 0:  gettimeofday:  0 + 0 = 0 ns
> 1:  gettimeofday:  0 + 500k ns = 500k ns
> 2:  gettimeofday:  0 + 1M ns = 1M ns
> 2:  timer_interrupt:  
> 2:  gettimeofday:  1M ns + 0 ns = 1M ns
> 3:  gettimeofday:  1M ns + 500k ns = 1.5M ns
> 4:  gettimeofday:  1M ns + 1M ns = 2 ns
> 4:  timer_interrupt (using -500ppm adjustment)
> 4:  gettimeofday:  1,999,500 ns + 0 ns = 1,999,500 ns
At point 4 you are introducing a NEW ntp adjustment.  This, I submit, 
needs to actually have been introduced to the system prior to the 
interrupt at point 2 with the first xtime change at point 4.  However, 
gettimeofday() should be aware of it from the interrupt at point 2 and 
be doing corrections from that time forward.  Thus when the point 4 
interrutp happens xtime will be the same at the gettimeofday a ns earlier.

Likewise, gettimeofday() needs to know when to stop apply the correction 
so that if a tick is late, it will apply the correction only for those 
times that it was needed.  This, could be done by figuring the offset 
thusly:

offset = (offset from last tick to end of ntp period * ntp_adj1) + 
(offset from end of ntp period to now)

I suppose it is possible that the latter part of the offset is also 
under a different ntp correction which would mean a "* ntp_adj2" is 
needed.  I would argue that only two terms are needed here regardless of 
how late a tick is.  This is because, I would expect the ntp system call 
to sync the two clocks.  This means in your example, the ntp call would 
have been made at, or prior to the timer interrupt at 2 and this is the 
same edge that gettimeofday is to used to start applying the correction.


> > It would appear that gettimeofday would need to know that the NTP 
> > adjustment is changing  (and to what).  It would also appear that this 
> > is known by the ntp code and could be made available to gettimeofday. 
> > If it is changing due to an NTP call, that system call, itself, 
> > should/must force synchronization.  So the only case gettimeofday needs 
> > to worry/know about is that an adjustment is to change at time X to 
> > value Y.  Also, me thinks there is only one such change that can be 
> > present at any given time.
>
>
> Well, in many arches gettimeofday() works around the above issue by
> capping the offset_ns value as such:

I think this may have been done with only usec gettimeofday.  Now that 
we have clock_gettime() returning nsec, we need to be a bit more careful.
> gettimeofday:
> 	xtime + min(offset_ns, tick_len + ntp_adj)
>
> The problem with this is that when we have lost or late ticks, or if we
> are using dynamic ticks you have granularity problems.
--
George Anzinger   george@xxxxxxxxxx
HRT (High-res-timers):  http://sourceforge.net/projects/high-res-timers/
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