more network performance info: ether_output()

Luigi Rizzo rizzo at
Tue May 1 15:20:48 UTC 2012

On Tue, May 01, 2012 at 10:27:42AM -0400, George Neville-Neil wrote:
> On Apr 20, 2012, at 15:03 , Luigi Rizzo wrote:
> > Continuing my profiling on network performance, another place
> > were we waste a lot of time is if_ethersubr.c::ether_output()
> > 
> > In particular, from the beginning of ether_output() to the
> > final call to ether_output_frame() the code takes slightly
> > more than 210ns on my i7-870 CPU running at 2.93 GHz + TurboBoost.
> > In particular:
> > 
> > - the route does not have a MAC address (lle) attached, which causes
> >  arpresolve() to be called all the times. This consumes about 100ns.
> >  It happens also with locally sourced TCP.
> >  Using the flowtable cuts this time down to about 30-40ns
> > 
> > - another 100ns is spend to copy the MAC header into the mbuf,
> >  and then check whether a local copy should be looped back.
> >  Unfortunately the code here is a bit convoluted so the
> >  header fields are copied twice, and using memcpy on the
> >  individual pieces.
> > 
> > Note that all the above happens not just with my udp flooding
> > tests, but also with regular TCP traffic.
> Hi Luigi,
> I'm really glad you're working on this.  I may have missed this in a thread
> but are you tracking these somewhere so we can pick them up and fix them?
> Also, how are you doing the measurements.

The measurements are done with tools/tools/netrate/netsend and
kernel patches to return from sendto() at various places in the
stack (from the syscall entry point down to the device driver).
A patch is attached. You don't really need netmap to run it,
it was just a convenient place to put the variables.

I am not sure how much we can "fix", there are multiple expensive
functions on the tx path, and probably also on the rx path.

My hope at least for the tx path is that we can find out a way to install a
"fastpath" handler in the socket.
When there is no handler installed (e.g. on the first packet or
unsupported protocols/interfaces) everything works as usual. Then
when the packet reaches the bottom of the stack, we try to update
the socket with a copy of the headers generated in the process, and
the name of the fastpath function to be called.  Next transmissions
will then be able to shortcut the stack and go straight to the
device output routine.

I don't have data on the receive path or good ideas on how to proceed -- the
advantage of the tx path is that traffic is implicitly classified,
whereas it might not be the case for incoming traffic, and classification
might be the expensive step.

Hopefully we'll have time to discuss this next week in ottawa.


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