[ntp:questions] Network latency questions

David Mills mills at udel.edu
Fri May 28 03:31:38 UTC 2010


It's not the network latency that dominates the delay, but the FIFOs in 
the gigabit Ethernet chip (PCNET, etc.), which can hold several packets. 
This distorts the timestamps in both the transmit and receive sides. 
This is one of the things the interleaved modes were designed to cope 
with, but the fact not every packet generates an interrupt distorts even 
more. In some machines, specially Sunses, there can be an additional 
input delay of up to 2 microseconds.


Danny Mayer wrote:

>Frans Grotepass wrote:
>>Hi all,
>>Sorry for abusing my membership to this forum for this question.
>>We are busy with building an embedded application that must retrieve data very 
>>fast. The choice is to either have the data locally or go to a central 
>>server(pool) that contains the data. 
>>In evaluating the network option, I thought that the people here could 
>>possibly help me with the expected network latency for a Gb network via a 
>>switch. My gut feeling says that with increased load, the switch will bundle 
>>the traffic to the different nodes more and this will result in higher latency. 
>If you are asking a non-NTP question then this really is the wrong
>place. Our discussions of network latency are around the effects that it
>has on NTP calculations and the error budget.
>My only other comment is that local data is always faster (unless you
>have really slow disk drives) so why are you asking in the first place?
>It can't be just about the network latency.
>questions mailing list
>questions at lists.ntp.org

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