[Codel] FQ_Codel lwn draft article review

Rick Jones rick.jones2 at hp.com
Mon Nov 26 16:05:53 EST 2012

On 11/23/2012 04:07 PM, Toke Høiland-Jørgensen wrote:
> "Paul E. McKenney" <paulmck at linux.vnet.ibm.com> writes:

>> Also, I know what ICMP is, but the UDP variants are new to me.  Could
>> you please expand the "EF", "BK", "BE", and "CSS" acronyms?
> The UDP ping times are simply roundtrips/second (as measured by netperf)
> converted to ping times. The acronyms are diffserv markings, i.e.
> EF=expedited forwarding, BK=bulk (CS1 marking), BE=best effort (no
> marking). The UDP ping tests tend to not work so well on a loaded link,
> however, since netperf stops sending packets after detecting
> (excessive(?)) loss. Which is why you see only see the UDP ping times on
> the first part of the graph.

In a "classic" netperf UDP_RR test, where there is only one 
request/response (transaction) in flight at one time, the test will come 
to a halt on the first packet loss - of either a request or a response. 
  Netperf has no retransmission mechanism for UDP.

If one is using "burst mode" then the test will continue so long as 
there is at least one "transaction" outstanding.  However, one cannot 
then simply invert transactions per second to get seconds per transaction.

That is why I tend to use TCP_RR - the retransmission mechanism of TCP 
will kick-in to keep the test going.

In theory, netperf could be tweaked to set SO_RCVTIMEO at some 
high-but-not-too-high level (from the command line?).  It could then 
keep the test limping along I suppose (with gaps), but I don't want 
anything terribly complicated going-on in netperf - otherwise one might 
as well use TCP_RR anyway.

rick jones

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