[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

[LDM #XXW-990118]: LDM - Not your average latency problem


> >I infer the following from the MRTG and the rtstats(1) plots for
> >SonomaTech: 1) the SonomaTech LDM restarted around noon WDT on the 18th;
> >2) it saturated the pipe; 3) it didn't catch up until just before 1700
> >WDT; 4) throughout the subsequent night, the pipe was occationally
> >saturated; and 5) the pipe became saturated again around the start of the
> >workday at SonomaTech.
> The pipe becomes saturated during daylight hours because the NIMAGE feed
> volume has a diurnal cycle....... it's based largely on visible satellite
> imagery and so it increases during the day.
> >Judging from this and the good performance of the LDM in sending the same
> >data-products from Rossby to here, I think it's safe to say that
> >SonomaTech's pipe just isn't big enough to receive the data-products that
> >it's requesting.
> Yes, this was never in doubt. But they should be able to get half their
> pipe in volume without much latency, and that is the main thing I've been
> trying to figure.  I added back in the NIMAGE feed yesterday to prove a
> point. Without the NIMAGE feed, we were having large latencies ( for
> unknown reasons, I would argue). After the NIMAGE feed was added, and the
> feeds were combined, we got much better performance.
> >Merging all the requests into one was done to validate the hypothesis that
> >a SonomaTech router was favoring higher-volume LDM connections at the
> >expense of lower-volume ones.  That appears to be the case.
> Hmm...could it be a router CPU problem? But if we lower volume a bunch
> (without NIMAGE), you would think it could handle it, even if the feeds
> were split. OK, I'll give you a chance to make a prediction...maybe you can
> get Tom in on it and we can have an office pool.
> GIVEN: the current good performance of the LDM at STI (aside from the
> expected latencies due to their small pipe), and considering their current
> feed volume, and the fact that their feed types are combined in one entry...
> PREDICT: what will happen if we request the feeds separately, AND we do not
> request the NIMAGE feed (a reduction in volume by more than half). Now
> myself, and I'm guessing most others that have played around with the LDM,
> would never predict that large latencies would develop and certain feeds
> might just stop coming in. Any takers? This is the fundamental thing that I
> have been unable to understand.

Unfortunately, you're proposing to change two parameters simultaneously (volume 
and number of connections).  I think I can predict the outcome when only one is 
changed.  If only the volume is decreased, then I expect the average latency to 
decrease.  If only the number of connections is increased, then I expect the 
average latency to *increase* due to a hypothesized problem with Sonomatech's 
IP-mapping router managing its connection tables.

Feel free to test this out -- but I'd only change one parameter at a time.

> Again, I appreciate your indulgence, you can ignore me anytime your
> patience has been exhausted. :-)

Compared to my impatience with myself, you're a piece of cake.  :-)

Steve Emmerson

Ticket Details
Ticket ID: XXW-990118
Department: Support LDM
Priority: Normal
Status: Closed

NOTE: All email exchanges with Unidata User Support are recorded in the Unidata inquiry tracking system and then made publicly available through the web. If you do not want to have your interactions made available in this way, you must let us know in each email you send to us.