[omniORB] Latencies - Oneway vs. Roundtrip

[Helge Penne]

Jim Redman wrote:

> I have a counter-intuitive latency problem and would like some input and
> validation of what I think I'm seeing.  This is strictly latency, not
> throughput.   Running between two NT 4.0 system with 2.7.0 I see latencies
> for oneway messages that appear to approach 100ms.  When I change the same
> calls to roundtrip the latencies drop below the NT measurable time (about
> 10ms?).
>
> Has anyone seen anything similar or different, that is, can anyone confirm
> or deny my numbers.

I'm doing some benchmarking at this very moment, and just discovered this
latency myself.  The test uses a "publish/subscribe" pattern to distribute a
packet of data from a server to a client application.  I ran both programs on
the same NT 4.0 computer with a packet rate of 100Hz, and saw latencies of up
to 200ms when using oneway calls.  With ordinary calls, the latency is hardly
ever measurable using GetTickCount.

What was really revealing was that the latency would start at approx. 40ms,
and then increase by 10ms for each call until it would reach 200 ms.  It would
then fall back to 40ms and start over.

My theory is that omniORB buffers my oneway calls and sends them in larger
packages every 200ms (5Hz).  This explains every symptom that I have seen, and
also fits in nicely with my previous experience with VisiBroker.  VisiBroker
also buffers such requests by default, but there is an option in BOA_init that
lets you turn off this buffering and force these requests to be sent
immediately.

This leads up to a very important question: How do we turn off this buffering
in omniORB?  This is quite an important question for my project.  Has anybody
done this before?

--
Helge Penne (helge.penne@datarespons.no)
Senior Software Development Engineer
Data Respons AS, Postboks 489, N-1323 Høvik, Norway
Phone: +47 67112081 / 22719957 (work/home)