svn commit: r280759 - head/sys/netinet

[Gleb Smirnoff]

On Sat, Mar 28, 2015 at 10:23:13AM -0700, John-Mark Gurney wrote:
J> > On Fri, Mar 27, 2015 at 01:26:59PM +0000, Fabien Thomas wrote:
J> > F> Author: fabient
J> > F> Date: Fri Mar 27 13:26:59 2015
J> > F> New Revision: 280759
J> > F> URL: https://svnweb.freebsd.org/changeset/base/280759
J> > F> 
J> > F> Log:
J> > F>   On multi CPU systems, we may emit successive packets with the same id.
J> > F>   Fix the race by using an atomic operation.
J> > F>   
J> > F>   Differential Revision:	https://reviews.freebsd.org/D2141
J> > F>   Obtained from:	emeric.poupon at stormshield.eu
J> > F>   MFC after:	1 week
J> > F>   Sponsored by:	Stormshield
J> > 
J> > The D2141 says that benchmarking were done in presence of IPSEC, which
J> > of course is the bottleneck and performance of this instruction can't
J> > be benchmarked in its presence. Anyway, I believe that results of
J> > right benchmark would still show little difference between atomic and
J> > non-atomic increment of a shared value.
J> > 
J> > I think we can use per-cpu ID counters, each CPU incrementing its
J> > own. If we start with random values, then probability of two packets with
J> > the same ID emitting at the allowed timeframe will be acceptably small.
J> 
J> Please do not use per-cpu id counters.. That will just push the
J> duplicate ids to being more rare, but just as much of a problem...
J> 
J> Please read:
J> https://tools.ietf.org/html/rfc6864
J> 
J> And then implement one hased upon source/dest/protocol...

I know about this RFC, but using per-cpu ID counter if a low hanging
fruit and is improvement. What is important not only improvement in
terms of lowering probability of collision, but also easy improvement in
performance, since now global ip_id increment is a cache line thrasher.

So, I don't see how existense of the RFC blocks introducing an easy
improvement. And if anyone works on implementing the RFC, he shouldn't
care what is in head before his work, either global or per-cpu counter.

-- 
Totus tuus, Glebius.