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

[John Neiberger]

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. Cisco has never recommended this when I've talked to them
about it, but I have seen a recommendation elsewhere to use something
like "hold-queue 1024 out" on the 1g interfaces to smooth out those
microbursts. I think that will add a tiny amount of latency to the
path, but would there be any other significant performance impacts?
Would this even do what we want? I'm not sure how it would interact
with the hardware queues. It seems as if it creates software-based
buffer space in front of the hardware output queues to store any
bursty traffic. Is it possible that the solution to this particular
problem is this simple?

I can only assume that since Cisco has never recommended trying this,
there must be a reason for it. Either it won't work at all or it has
unfortunate side effects. But I'm considering giving it a try.

Any thoughts?

Many thanks, as usual!

John