[Beowulf] ...Re: Benchmark results

Josip Loncaric josip at lanl.gov
Thu Jan 8 10:44:42 EST 2004

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...


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