BQL and HTB are not really comparible things.
all the BQL does is to change the definition of the length of a buffer from X
packets to X bytes.
using your example, 1000 packets of 1500 bytes is 1.5MB or 120ms at 100Mb. But
if you aren't transmitting 1500 byte packets, and are transmitting 75 byte
packets instead, it's only 6ms worth of buffering.
The bottom line is that sizing buffers by packets doesn't work.
HTB creates virtual network interfaces that chop up the available bandwidth of
the underlying device. I believe that if the underlying device supports BQL, HTB
is working on byte length allocations, not packet counts.
fq_codel doesn't have fixed buffer sizes, it takes a completely different
approach that works much better in practice.
The document that you found is actually out of date. Rather than trying to tune
each thing for optimum performance and then measureing things, just benchmark
the stock, untuned setup that you have and the simple fq_codel version without
any tweaks and see if that does what you want. You can then work on tweaking
things from there, but the improvements will be minor compared to doing the
switch in the first place.
A good tool for seeing the performance (throughput and latency) is
netperf-wrapper. Set it up and just test the two configs. The RRUL test is
especially good at showing the effects of the switch.
David Lang
On Fri, 27 Mar 2015, Bill Ver Steeg (versteb) wrote:
Date: Fri, 27 Mar 2015 21:45:11 +0000
From: "Bill Ver Steeg (versteb)" <[email protected]>
To: "[email protected]" <[email protected]>
Subject: [Bloat] setting queue depth on tail drop configurations of
pfifo_fast
Bloaters-
I am looking into how Adaptive Bitrate video algorithms interact with the
various queue management schemes. I have been using the netperf and netperf
wrapper tools, along with the macros to set the links states (thanks Toke and
Dave T). I am using HTB rather than BQL, which may have something to do with
the issues below. I am getting some interesting ABR results, which I will
share in detail with the group once I write them up.
I need to set the transmit queue length of my Ubuntu ethernet path while
running tests against the legacy pfifo_fast (tail drop) algorithm. The
default value is 1000 packets, which boils down to 1.5 MBytes. At 100 Mbps,
this gives me a 120ms tail drop buffer, which is big, but somewhat reasonable.
When I then run tests at 10 Mbps, the buffer becomes a 1.2 second bloaty
buffer. When I run tests at 4 Mbps, the buffer becomes a 3 second extra-bloaty
buffer. This gives me some very distinct ABR results, which I am looking into
in some detail. I do want to try a few more delay values for tail drop at 4
Mbps.
https://www.bufferbloat.net/projects/codel/wiki/Best_practices_for_benchmarking_Codel_and_FQ_Codel
says to set txqueuelen to the desired size, which makes sense. I have tried
several ways to do this on Ubuntu, with no glory. The way that seems it should
have worked was "ifconfig eth8 txqueuelen 100". When I then check the
txqueuelen using ifconfig, it looks correct. However, the delay measurements
still stay up near 3 seconds under load. When I check the queue depth using
"tc -s -d qdisc ls dev ifb_eth8", it shows the very large backlog in
pfifo_fast under load.
So, has anybody recently changed the ethernet/HTB transmit packet queue size
for pfifo_fast in Ubuntu? If so, any pointers? I will also try to move over to
BQL and see if that works better than HTB...... I am not sure that my ethernet
drivers have BQL support though, as they complain when I try to load it as the
queue discipline.
Thanks in advance
Bill VerSteeg
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