netdev - Re: L2 network namespaces + macvlan performances

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Date:	Mon, 09 Jul 2007 09:59:06 -0700
From:	Rick Jones <rick.jones2@...com>
To:	Benjamin Thery <benjamin.thery@...l.net>
Cc:	Linux Containers <containers@...ts.osdl.org>,
	netdev@...r.kernel.org, ebiederm@...ssion.com,
	Daniel Lezcano <dlezcano@...ibm.com>,
	Patrick McHardy <kaber@...sh.net>
Subject: Re: L2 network namespaces + macvlan performances

> Between the "normal" case and the "net namespace + macvlan" case, 
> results are  about the same for both the throughput and the local CPU 
> load for the following test types: TCP_MAERTS, TCP_RR, UDP_STREAM, UDP_RR.
> 
> macvlan looks like a very good candidate for network namespace in these 
> cases.
> 
> But, with the TCP_STREAM test, I observed the CPU load is about the
> same (that's what we wanted) but the throughput decreases by about 5%:
> from 850MB/s down to 810MB/s.
> I haven't investigated yet why the throughput decrease in the case.
> Does it come from my setup, from macvlan additional treatments, other? I 
> don't know yet

Given that your "normal" case doesn't hit link-rate on the TCP_STREAM, 
but it does with UDP_STREAM, it could be that there isn't quite enough 
TCP window available, particularly given it seems the default settings 
for sockets/windows are in use.  You might try your normal case with the 
test-specific -S and -s options to increase the socket buffer size:

netperf -H 192.168.76.1 -i 30,3 -l 20 -t TCP_STREAM -- -m 1400 -S 128K 
-S 128K

and see if that gets you link-rate.  One other possibility there is the 
use of the 1400 byte send - that probably doesn't interact terribly well 
with TSO.  Also, it isn't (?) likely the MSS for the connection, which 
you can have reported by adding a "-v 2" to the global options.  You 
could/should then use the MSS in a subsequent test, or perhaps better 
still use a rather larger send size for TCP_STREAM|TCP_MAERTS - I myself 
for no particular reason tend to use either 32KB or 64KB as the send 
size in the netperf TCP_STREAM tests I run.

A final WAG - that the 1400 byte send size interacted poorly with the 
Nagle algorithm since it was a sub-MSS send.  When Nagle is involved, 
things can be very timing-sensitive, change the timing ever so slightly 
and you can have a rather larger change in throughput. That could be 
dealt-with either with the larger send sizes mentioned above, or by 
adding a test-specific -D option to set TCP_NODELAY.

happy benchmarking,

rick jones
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