[Cake] low bandwidth default params best effort vs voice latency.

Jonathan Morton chromatix99 at gmail.com
Sat Mar 4 15:27:57 EST 2017

Okay, I think I’ve worked out what is happening.

At 250KB/s, it takes 6ms to get one 1500-byte bulk packet down the pipe.  This is unavoidable, so having a bulk flow competing with your game traffic will always increase your peak latency by that much.

With three independent game streams in play, it’s possible for them all to transmit simultaneously *and* to coincide with a bulk packet having just been sent.  With overheads, it will take a total of 8.5ms to get all four packets through.  This corresponds nicely to your best-effort results; you’re getting very close to the theoretical best performance there.

So Diffserv marking actually can’t improve your performance in this particular case.  But it shouldn’t make it worse either.  You’re actually seeing a nearly 6ms increase in peak latency, which corresponds neatly to an additional bulk packet ending up ahead of the game traffic in the queue.

That’s not supposed to happen, but I think I can see how it *can* happen with the current Diffserv logic.  It’s a weighted DRR, much like what is used between flow queues a little further down - but it *doesn’t* have a special bonus for sparse tins.  That’s something I clearly need to fix.

To help remind me, please do open an issue on the Github project.

 - Jonathan Morton

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