DP performance

[Danial Thom]

--- Steve Shorter <steve at xxxxxxxxxxxx> wrote:

> On Mon, Nov 28, 2005 at 10:15:55AM -0800,
> Matthew Dillon wrote:
> > :What kind of "benefits" would be realized
> for
> > :systems being used primary as a
> router/bridge,
> > :given that its almost 100% kernel usage?
> > :
> > :DT
> > 
> >     Routing packets doesn't take much cpu
> unless you are running a gigabit
> >     of actual bandwidth (or more).  If you
> aren't doing anything else with 
> >     the machine then the cheapest AMD XP will
> do the job.
> > 
> 
> 	We've found the bottle neck for routers is CPU
> cycles neccessary to
> process NIC hardware interrupts. At least for
> OBSD. Interupt mitigation, and
> I suppose POLLING on Dragonfly may help but it
> isn't supported on all 
> hardware AFAIK.

Polling is pretty dumb with modern NICs, as most
have built-in interrupt moderation that does the
work of polling without all of the overhead (by
generating interrupts with user-definable forced
separation). At least Intels do; if others don't
then thats enough reason not to use them. Doing
500K pps with a 10K interrupts/second setting is
better than you could ever do with polling, and
the results quite good.

Dealing with the NICs (processing packets, I/Os,
etc) is what uses the cycles, and the stack (a
bridge machine can do twice as many packets as a
"router" for example). For network processing
true separation of transmit and receive is
probably the only way to realize networking gains
for non TCP/UDP operations. Slicing up the stack
will only slow things down compared to UP (think
of a full-speed relay race against a guy that
doesn't get tired...the guy without the hand-offs
will always win.) The best  you can probably do
is match the UP performance; but idealy have a
bunch of cpu power left over. So maybe you'd have
a UP machine that can do 800K pps and be on the
edge of livelock, and a DP machine that can do
750K pps but is still usable at the user level.

Danial



	
		
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