[c-nsp] Increasing hold-queue to alleviate microbursts with small hardware queues

Phil Mayers p.mayers at imperial.ac.uk
Fri Aug 17 04:37:45 EDT 2012

On 08/17/2012 07:02 AM, John Neiberger wrote:
> This has come up a few times recently. We continue to run into new
> situations where we see lots of output queue drops on 6748 blades,
> especially in cases where a 10g link is feeding a 1g link. We see OQDs
> long before the interface approaches anything close to line rate on
> average.

As I'm sure you're aware, but just for the archives, average rate is 
pretty irrelevant in this scenario. All that matters is the burst rate 
at the ingress (10 gig) port.

6748 cards have ~1.3Mb of per-port output buffer, which at 9Gbit/sec (10 
in minus 1 out) will fill in ~1.1 milliseconds. At some fraction of that 
load (e.g. 5 in, 1 out) it'll scale appropriately.

If you are doing QoS with the default queue parameters, you'll get 
(possibly much) less than that.

> bursty traffic. Is it possible that the solution to this particular
> problem is this simple?

As others have said, no. It won't do anything.

You're sure you've either disabled QoS or reconfigured the queues to 
allocate space appropriately i.e. in proportion to the offered load?

TBH microburst problems on this platform don't come up on the list very 
often, as far as I can see. It's usually the lower-end catalysts.

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