Re: Single core gets pegged on multi-core PPTP server

On Fri, Jan 20, 2012 at 12:16 AM, Eric Dumazet <eric.dumazet <at> gmail.com> wrote:
> Le jeudi 19 janvier 2012 à 16:35 -0600, Bradley Peterson a écrit :
>> Hello,
>>
>> I am trying to test the capacity of a linux PPTP server, both in
>> number of connections, and in packets per second.  I am using kernel
>> 2.6.38.8, with the ppp, pptp, and gre modules, and accel-pptp 0.8.3.
>> I have RPS, RFS, and XPS enabled on the network devices for SMP
>> support.
>>
>> But I'm seeing one CPU get pegged out with soft interrupt, while the
>> others are almost completely idle.
>>
>> In my current test, I'm starting 250 pptp connections from another
>> server, then running iperf across each connection.  The client machine
>> pegs out, sure, but I'm surprised the server pegs out a single CPU.
>> With RPS, I would expect softirq's to be more balanced.
>>
>> Where could the bottleneck be?  Do all ppp packets need to be
>> processed serially?
>>
>
> Hmmm, you need a more recent kernel or backport commit
> c6865cb3cc6f3c2857fa4c6f5fda2945d70b1e84
>    rps: Inspect GRE encapsulated packets to get flow hash
>
>    Crack open GRE packets in __skb_get_rxhash to compute 4-tuple hash on
>    in encapsulated packet.  Note that this is used only when the
>    __skb_get_rxhash is taken, in particular only when the device does
>    not compute provide the rxhash (ie. feature is disabled).