[Beowulf] precise synchronization of system clocks

Lux, James P james.p.lux at jpl.nasa.gov
Thu Oct 2 09:37:58 EDT 2008

Rgb wrote:
> I understand inductive surge when powering up, I understand in detail
> browning out a primary power transformer, but I think those are
> different issues and irrelevant here.
Inductive surge -> magnetizing current in large iron core inductors (depends
on where you are in line frequency cycle at "switch on")
Sag from overload -> impedance (both R and L) in transformer and wires from
transformer to load. 2% voltage drop is the NEC guideline for "in premises
wires".. From panel to load. The voltage at the utility entrance could
probably be +/- 5% at any given time.

> So far, using my trusty Kill-a-Watt on real world nodes, I haven't seen
> more than a 30% differential draw loaded to unloaded.  Large parts of
> the CPU require power at all times to function.  Memory, for example,
> both on and offboard.  Nearly everything inside a computer has a
> nontrivial idle draw, plus (sure) peak draw when it or one of its
> subsystems are in use.

Very much true.  DRAM needs refresh, for instance.

> Exceptions are modern laptops -- with variable speed clocks, they draw
> much less idling than they do at speed, in part because power (idle or
> otherwise) IS very nearly proportional to CPU clock in at least parts of
> the system.  And I don't really know how the latest designs do in this
> Multicores, of course, may function like hybrid cars, and somehow run
> more nearly idle when they are idle.  But I'd have to hear from someone
> who slapped a KaW on an actual system and clocked it from idle (solidly
> post-boot, running the OS, at idle "equilibrium") to loaded (running
> flat out on e.g. a benchmark suite that loads all cores and/or the
> memory etc.).  Has anyone actually done this and observed (say) a 2 or 3
> to 1 increase in power draw loaded to idle?  50W idle to 200W loaded in
> 1 second?  150W idle to 200W loaded is more like what I've seen...

Don't forget that the power supply efficiency drops dramatically when DC
load drops, too. They don't spend a penny more on sophisticated design than
required to get that "energy star" rating, and that has more to do with
having a good "low power hibernate" mode than good efficiency at 25% load.

>> In normal  practice I doubt that this is an issue but synchronization in the
>> extreme is interesting in its details and side effects.
> I completely agree with this, both parts.  Although if one IS bumping
> from 50->200W "instantly" on not even an entire cluster but just all the
> nodes on a single circuit, that's popping over a KW on a 20A line --
> ballpark where one MIGHT see something inductive (although as I said,
> probably nothing that the power supply capacitor(s) cannot buffer,
> although I'm too tired to compute the number of joules (watt-seconds)
> one can probably deliver and what RC probably is, etc).  Popping
> multiple (as in 10+) KW in less than a 60 Hz cycle would very likely be
> hard on the primary, no doubt about it.

If one considers that a single wire is about 1 uH/meter (typical electrical
wiring will be much less, because it's a pair, with currents flowing
opposite directions), the series L might be a few tens of uH.  At, say, 20
A, there's just not much energy stored there.

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