[Beowulf] precise synchronization of system clocks

NiftyOMPI Mitch niftyompi at niftyegg.com
Wed Oct 1 17:33:42 EDT 2008

On Wed, Oct 1, 2008 at 7:18 AM, Robert G. Brown <rgb at phy.duke.edu> wrote:

> On Tue, 30 Sep 2008, Nifty niftyompi Mitch wrote:

> One site service of interest is AC power.   A modern processor sitting
> in an idle state that then starts a well optimized loop will jump from
> a couple of watts to 100 watts in as many clocks as the set of pipelines
> is deep behind the instruction decode and instruction cache fill.  A 1000
> processor (4000 cores) might jump from 4000 watts to 100000 watts in the
> blink of an eye (err did the lights blink).   Buffer that dI/dT through
> the PS and it is less but still interesting on the mains which are
> synchronized.

Interesting.  I never have seen the lights blink although I don't run
> synchronous computations.  One wonders if the power supply capacitors
> (which should be quite large, I would think) don't soak up the
> transient, though, even on very large clusters.  Also, I think that the
> power differential is smaller than you are allowing for -- I don't think
> most idle processors draw "no" power...

The dI/dT  for processors can be quite high.
AMD Phenom™ X4 Quad-Core is listed as a 140 watt part (thermal)
it is unlikely that all 450 million transistors are active in an idle
loop.  Tom's Hardware list the idle power at 21 watts.  The speed at
which a modern processor can go from idle to full power is astonishing.
The local on board power supply regulation must respond very quickly. The
from 21 to 140 fit inside one half cycle of a 50/60 Hz AC mains service.  So
of one AC cycle the part can move from 21 to 140.... which is large when
multiplied by a 1000 node dual socket cluster.  I do know of clusters and
labs of workstations that power on hosts and disks in sequence to limit the
startup power surge.    Lots of us have been at it long enough to know  that

induction motors like elevators, refrigeration compressors and even vacuums
can hit the mains
hard enough to trigger errors.  My home vacuum does dim the lights a little

In normal  practice I doubt that this is an issue but synchronization in the
is interesting in its details and side effects.

       T o m   M i t c h e l l
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