svn commit: r273112 - head/sys/dev/ixgbe

[Luigi Rizzo]

On Thu, Nov 06, 2014 at 03:05:30PM +0400, Alexander V. Chernikov wrote:
> On 15.10.2014 05:22, Adrian Chadd wrote:
> > Author: adrian
> > Date: Wed Oct 15 01:22:56 2014
> > New Revision: 273112
> > URL: https://svnweb.freebsd.org/changeset/base/273112
> >
> > Log:
> >    Set the DROP_EN bit before the RX queue is brought up and active.
> >    
> >    He noticed issues setting this bit in SRRCTL after the queue was up,
> >    so doing it from the sysctl handler isn't enough and may not actually
> >    work correctly.
> While investigating/measuring ixgbe performance w/ and without flow control
> I noticed that disabling fc entirely (e.g. disabling 512K skid buffer 
> and turning drop_en on)
> not only makes things a bit worse, but also makes NIC stop accounting 
> tail-drops as errors
> at all.
> It is a bit unhandy with HW counters, since you, for example, see 2M 
> packets being received
> in netstat, but in reality system receives only 1M (And there is no easy 
> way to determine exact
> count).

i'd like to point out that the default does not change (flow control
is still enabled by default) and the previous handling of DROP_EN
was ineffective.

One may argue that FC and DROP_EN should be set independently,
and i am all for it. In fact that would be the right thing to do
in terms of POLA.

This said, i do not understand what kind of performance degradation
you experienced, can you be more specific ?

cheers
luigi