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copied!<p>The prio facility will simply send the highest priority packet available at the time when it is sending packets (typically as soon as the previous packet has been sent unless there are no packets waiting to go out).</p> <p>Your test relies on the packets having been placed in the queue by the appropriate program's processes on each machine, and received packets having been retrieved from port on each machine. </p> <p>Any scheduling delays which affect the time that a process gets on either machine might affect the process's ability to place a message on the queue or to retrieve and process a message form the queue. It sounds like you have loaded at least one of the machines to test for this but my experience is that machine loading will definitely affect measured latency like this (in the order of milliseconds not microseconds) so it might be worth repeating this with both machines loaded with high priority tasks.</p> <p>The other thing to check is the timestamp you are using to measure the latency - is it the time the echoed message is actually received at the client machine or the time your program processes it. If the latter then you are measuring not just the network latency, but also the time between the message being received and your program getting a slice of the processor and getting to the point where you check the time - see <a href="http://wiki.wireshark.org/Timestamps" rel="nofollow">http://wiki.wireshark.org/Timestamps</a>.</p> <p>As an aside, I don't think you will be able to to get guaranteed micro second level responsiveness without a real time OS like mechanism. On the other hand if your application is VoIP like then you will usually be OK up to about 200 millisecond latency.</p>

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