john stultz wrote:
On Wed, 2005-08-24 at 16:46 -0700, George Anzinger wrote:
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.
Yes, clearly a forward knowledge of the NTP adjustment is necessary for
gettimeofday(), because after the NTP adjustment has been accumulated
into xtime, there's nothing left for gettimeofday to adjust (its already
been applied). :)
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)
Well, in my example, the ntp_adjustment is a fixed nanosecond offset, so
it would be added to the nanosecond offset from the last tick (which is
how the current code works). If you are doing scaling (as you have in
the equation above), then the problem goes away, since you can apply the
adjustment consistently through any interval.
Until the end of the correction time...
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.
Ok, so your forcing gettimeofday to be interval aware, so its applying
different fixed NTP adjustments to different chunks of the current
interval. The issue of course is if you're using fixed adjustments, is
that you have to have n ntp adjustments for n intervals, or you have to
apply the same ntp adjustment to multiple intervals.
Uh, are you saying that one ntpd call can set up several different
adjustments? I was assuming that any given call would set up either a
fixed adjustment for ever or a fixed adjustment to be applied for a
fixed number of ticks (or until so much correcting was done, which, in
the end is the same thing at this point in the code).
If ntpd has to come back to change the adjustment, I am assuming that
some kernel action can be taken at that time to sync the xtime clock and
the gettimeofday reading of it. I.e. we would only have to keep track
of one adjustment with a possible pre specified end time.
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.
If you argue that we only need two adjustments, why not argue for only
one? You're saying have one adjustment that you apply for the first
tick's worth of time, and a second adjustment that you apply for the
following N ticks' worth of time in the interval. Why the odd base
case?
Correct me if I am wrong here, but I am assuming that ntpd can ask for
an adjustment of X amount which the kernel changes into N adjustments of
X/N amount spread over the next N ticks. This means that, once we have
done N ticks, we MUST stop doing the correction. Life would be easier
if ntpd only asked for rate changes which are to be applied until the
next ntpd call (which, as I understand it, it can ALSO do). Of course,
the issue of missed ticks kicks in here and is the ONLY reason we need
to do two corrections, i.e. we need only one ntp correction if we have
not missed a tick.
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.
Indeed.
thanks
-john
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