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

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).
    
    This was tested by creating a single GRE tunnel between two 16 core
    AMD machines.  200 netperf TCP_RR streams were ran with 1 byte
    request and response size.
    
    Without patch: 157497 tps, 50/90/99% latencies 1250/1292/1364 usecs
    With patch: 325896 tps, 50/90/99% latencies 603/848/1169
    
    Signed-off-by: Tom Herbert [off-list ref]
    Signed-off-by: David S. Miller [off-list ref]



And make sure you disabled hardware rxhash (if your NIC provides it)