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Date:	Fri, 06 Apr 2007 08:25:48 -0600
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Benjamin Thery <benjamin.thery@...l.net>
Cc:	Daniel Lezcano <dlezcano@...ibm.com>,
	Linux Containers <containers@...ts.osdl.org>,
	netdev@...r.kernel.org
Subject: Re: L2 network namespace benchmarking (resend with Service Demand)

Benjamin Thery <benjamin.thery@...l.net> writes:

> Eric W. Biederman wrote:

>> A couple of random thoughts in trying to understand the numbers you are
>> seeing.
>>
>> - Checksum offloading?
>>
>>   You have noted that with the bridge netfilter support disabled you
>>   are still seeing additional checksum overhead.  Just like you are
>>   seeing in the routing case.
>>
>>   Is it possible the problem is simply that etun doesn't support
>>   checksum offloading, while your normal test hardware does?
>
> Looks like you are 100% correct.
> I feel a bit stupid I didn't think about this "small" difference between real
> NIC and etun.
>
> If I turn off checksum offloading on my physical NIC, the checksum "overhead"
> (load) measured by oprofile is about the same in both case: when running netperf
> through a real NIC or through an etun tunnel first.

Interesting.  You can also 'enable' checksum offloading when using etun with
ethtool.  Which should just tell the kernel not to do checksumming.  A
bad idea in general but it might be useful in confirming where the
performance overhead is coming from, and when used with routing I
believe it is safe.  When used with bridging I don't know.

Thinking about it the ideal situation is to preserve skb->ip_summed it
if came from another device, instead of unconditionally setting it.
I need to take a good hard look at etun_xmit and make certain we
are dotting all of the i's and crossing all of the t's for best
performance and compatibility with the rest of the network stack.

Eric
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