[Cerowrt-devel] [Bloat] Comcast upped service levels -> WNDR3800 can't cope...

Aaron Wood woody77 at gmail.com
Mon Sep 1 16:25:08 EDT 2014

> > But this doesn't really answer the question of why the WNDR has so much
> lower a ceiling with shaping than without.  The G4 is powerful enough that
> the overhead of shaping simply disappears next to the overhead of shoving
> data around.  Even when I turn up the shaping knob to a value quite close
> to the hardware's unshaped capabilities (eg. 400Mbps one-way), most of the
> shapers stick to the requested limit like glue, and even the worst offender
> is within 10%.  I estimate that it's using only about 500 clocks per packet
> *unless* it saturates the PCI bus.
> >
> > It's possible, however, that we're not really looking at a CPU
> limitation, but a timer problem.  The PowerBook is a "proper" desktop
> computer with hardware to match (modulo its age).  If all the shapers now
> depend on the high-resolution timer, how high-resolution is the WNDR's
> timer?
> Both good questions worth further exploration.

Doing some napkin math and some spec reading, I think that the memory bus
is a likely factory.  The G4 had a fairly impressive memory bus for the day
(64-bit?).  The WNDR3800 appears to be used in an x16 configuration (based
on the numbers on the memory parts).  It may have *just* enough bw to push
concurrent 3x3 802.11n through the software bridge interface, which
short-circuits a lot of processing (IIRC).

The typical way I've seen a home router being benchmarked for the
"marketing numbers" is to flow tcp data to/from a wifi client to a wired
client.  Single socket is used, for a uni-directional stream of data.  So
long as they can hit peak rates (peak MCS), it will get marked as good for
"up to 900Mbps!!" or whatever they want to say.

The small cache of the AR7161 vs. the G4 is another issue (32KB vs. 2MB)
the various buffers for fq_codel and htb may stay in L2 on the G4, but
there simply isn't room in the AR7161 for that, which puts further pressure
on the bus.

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