[ntp:questions] Network latency questions

Kasper Pedersen spam at kasperkp.dk
Mon May 31 16:59:51 UTC 2010

On 05/31/2010 11:16 AM, Frans Grotepass wrote:

> These delays (even with local network) will make the solution impossible.
> Sorry again for posing the questions here. I know this is a blatant off topic
> post, but getting the details from the internet is a little more difficult and
> there are so many people here with the knowledge at hand. Thanks for the help.

And it will likely be impossible even with priority tagging.
I did an experiment using cheap switches and a frame generator, having
the frame generator alternate between priority tagged and untagged packets.

Measurement results (skip down to Result #2):


Using tagging I can keep the one way delay below 400us, but that leaves
only 200us, and if you budget 100us tx latency in each nic (strangely tx
latency seems to be much more variable than the much shorter un-coalesced
rx latency), then there is no time left for actually doing work.

/Kasper Pedersen

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