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Is there an optimal value for the output queue length and how does it affect system response?

Retired expert - Loki Jorgenson, Ph. D. EXPERT RESPONSE FROM: Retired expert - Loki Jorgenson, Ph. D.

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QUESTION POSED ON: 02 January 2004
Is there an optimal value for the output queue length and how does it affect system response? We are receiving poor system response from a pair of clustered x440's, which everyone wants to blame on the network. There doesn't appear to be significant traffic across the network, but the OQL is very high (>100K) on those boxes all the time. Is this significant? And if it is, what can I do?

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EXPERT RESPONSE
It isn't clear from your description what sort of network device we are talking about here. I will assume that the device is a router and an "X440" is a reference to the IBM server.

That said, the unequivocal answer is "yes" – router queue length can seriously affect performance in unexpected ways. The most typical is when queues are too large and the result can be dramatic increases in RTT/jitter, and sometimes destabilized TCP flows.

However, you seem to be referring to output queue lengths on the server interfaces, as opposed to input queues on a router. This I am less sure of since it sounds like something above Layer 3 and therefore part of the OS. And I can easily imagine that it is a symptom of something in the network.

I would need more details though. If you care to write back, I'll try to help further.


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