From: Jonathan Morton <chromatix99@gmail.com>
To: cake@lists.bufferbloat.net
Subject: [Cake] Cake fix: diffserv4 priority quanta
Date: Mon, 4 May 2015 06:58:08 +0300 [thread overview]
Message-ID: <7D56E9A0-E611-47D6-996D-798700F7D050@gmail.com> (raw)
My preferred cake fix is Death By Chocolate - and it’s been quite a while since I’ve had any. But this will have to do for now.
While drafting a paper to go with the code, I had to go back and think about the rationale for quite a few things, and this revealed that the quanta used for the priority layer in diffserv4 mode were incorrect. There are two sets of quanta here:
- The one used when the allocated bandwidth for the class has not been exceeded, which sets a priority balance. The ratios here need to be at least sufficient to allow any higher-priority class to force any lower-priority class out of the higher class's allocation; in practice it’s only necessary to consider adjacent pairs for this purpose.
- The one used when the allocated bandwidth for the class *has* been exceeded, which sets a bandwidth balance. The ratios here should correspond to the effective allocations when *all* classes are fully saturated; transitions between the bandwidth and priority balance states take care of any other case. The effective saturated allocation is the class’s own allocation *minus* the allocation of the class above it.
The basic allocations of the four classes in diffserv4 are:
Background: 100%
Best Effort: 15/16
Video: 3/4
Voice: 1/4
The effective saturated allocations are therefore:
Background: 1/16
Best Effort: 3/16
Video: 1/2
Voice: 1/4
It is now obvious that the quantum ratio between Best Effort and Background needs to be at least 16:1 in the priority balance, and should be 3:1 in the bandwidth balance. Ratios of 4:1 for the elevated-priority classes in priority balance are sufficient.
The quanta for the diffserv8 and precedence modes are already sane (although the bandwidth-balanced quantum for the highest class turns out low, but this doesn't matter in practice; exercise for the reader is to work out why). It helped, there, that the bandwidth allocations proceed geometrically (7:8 ratio in each step up) and are thus easier to calculate intuitively.
I’ve just pushed a corresponding patch to the net-next derived repo. I don’t have push access to the out-of-tree version.
- Jonathan Morton
reply other threads:[~2015-05-04 3:58 UTC|newest]
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