March/April issue of acmqueue


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Distributed Computing

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Originally published in Queue vol. 13, no. 4
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Comments

(newest first)

Elios | Sat, 07 Nov 2015 09:29:52 UTC

Hi Andrew,

Thanks for the nice post. That's a great sum-up of problems in the design and implementation of distributed low latency systems.

I'm working on a distributed low-latency market data distribution system. In this system, one of the biggest challenge is how to measure its latency which is supposed to be several micro seconds.

In our previous system, the latency is measured in an end-to-end manner. We take timestamp in milli seconds on both publisher and subscriber side and record the difference between them. This works but we are aware that the result is not accurate because even with servers having clock synchronized with NTP, users complain sometimes that negative latency is observed.

Given we are reducing the latency to micro seconds, the end-to-end measurement seems to be too limited (it should be better with PTP but we can't force our users to support PTP in their infrastructure) and thus we are trying to get a round-trip latency. However, I can see immediately several cons with this method :

- extra complexity to configure and implement the system because we need to ensure two-way communication. - we can't deduce the end-to-end latency from the round trip one because the loads on both direction are not the same. (we want to send only some probes and get them back)

Do you have some experiences on the round-trip latency measurement and if so could you please share some best practices ?

Thanks a lot, Helios


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