[Beowulf] ...Re: Benchmark results

[Josip Loncaric]

Jim Lux wrote:
> 
> At 05:18 PM 1/7/2004 -0700, Josip Loncaric wrote:
> 
>>  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...
> 
> 
> Most of the more recent implementations of NTP clients can do this sort 
> of thing, sending messages both ways across the link to measure the 
> latency (and latency distribution) 

I love NTP, but in its current state it does not deliver microsecond 
level synchronization.

I just checked my old cluster (which runs NTP) and the compute nodes are 
off by 500-25000 microseconds.  Actually, most are within 1000 
microseconds, but about 20% are off by up to 25000 microseconds.  This 
happens over Fast Ethernet where ntpdate usually reports timing 
dispersion of 0-20 microseconds (based on 7 ping-pong measurements).  In 
other words, network timing is quite precise, but we'd need about 1000 
times better clock synchronization.

NTP may need to be streamlined and tuned for clusters...

Sincerely,
Josip

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