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<== Date ==> | <== Thread ==> |
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Subject: | Latency between NI LabVIEW RT (cRIO) and EPICS on RT Linux |
From: | "Johannes Spinneken (EGI)" <[email protected]> |
To: | "[email protected]" <[email protected]> |
Date: | Tue, 6 Feb 2018 19:33:13 +0000 |
Hi I am fairly new to EPICS and have been testing an application with the following setup:
The setup works in principle, but I am seeing higher latencies than I was expecting. I was hoping to achieve latencies in the low ms order (say 1 – 2ms data round trip from cRIO to RT Linux back to cRIO).
In practice, I observe round-trip timings of order 10ms. Two questions:
(i)
What is the best timing (latency) that can generally be achieved using EPICS? Is there something fundamental in the protocol that prohibits data exchange in the low ms order? I understand that this
will of course depend on target hardware / software, but a ballpark figure would be good as “expectation management”. A typical ping between cRIO and Linux RT takes approx. 0.2ms, so the hardware seems clearly capable of <1ms timings.
(ii)
Does anyone have experience with running EPICS on LabVIEW RT / cRIO? I suspect some (most) of the latency is due to the way in which NI handles TCP traffic. I already discovered that data is normally
buffered for 10ms (or 8 kB). This can be avoided by using the Shared Variable Data Flush VI: http://zone.ni.com/reference/en-XX/help/371361H-01/lvcomm/flushsharedvar/ which indeed reduced round-trip timings by around 8-10ms. However, I seem to be unable to push timings down further below the 10ms round-trip mark. Any guidance would
be much appreciated. Many thanks! Best wishes Johannes Johannes Spinneken PhD FRSA Principal Systems Engineer Evergreen Innovations UK & USA +1 916 266 3709 (US cell) +44 7868 923583 (UK mobile) |