[Beowulf] ...Re: Benchmark results

[Josip Loncaric]

Jim Lux wrote:
> Josip Loncaric wrote:
> 
>> Here is my challenge to the Beowulf and MPI communities: Achieve 
>> microsecond-level *global* synchronization and tracking of all system 
>> clocks within a cluster, for application use via portable calls 
>> gettimeofday() and/or MPI_Wtime().
> 
> 
> Is added hardware on each node legal along with distributing some 
> periodic time mark (like a 1 pulse per second from a GPS receiver)?

I do not think this would be required, especially on clusters which 
already have a high performance network.  These days, 1/2 round trip 
time can be as low as 2.4 microseconds (Quadrics).  Even Fast Ethernet 
may prove sufficient (over time) if the delays are not too noisy.  BTW, 
the objective here is to support parallel timings within a cluster, so 
"global time" is still "local to this cluster only", although one may 
want to run NTP on the head node to stay close to the official time.

Back when VXIbus was introduced, I really liked its extra trigger lines 
which could synchronize multiple instruments within a few nanoseconds, 
but it is unlikely that people would distribute clock that way in a 
cluster.  Despite various software/hardware network layers and longer 
distances involved, it should be possible to get microsecond 
synchronization through the network by gradually tweaking local clock 
offset and drift...

Sincerely,
Josip

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