Freebsd IP Forwarding performance (question, and some info) [7-stable, current, em, smp]

Bruce Evans brde at
Mon Jul 7 19:15:46 UTC 2008

On Tue, 8 Jul 2008, Bruce Evans wrote:

> On Mon, 7 Jul 2008, Andre Oppermann wrote:
>> Bruce Evans wrote:
>>> So it seems that the major overheads are not near the driver (as I already
>>> knew), and upper layers are responsible for most of the cache misses.
>>> The packet header is accessed even in monitor mode, so I think most of
>>> the cache misses in upper layers are not related to the packet header.
>>> Maybe they are due mainly to perfect non-locality for mbufs.
>> Monitor mode doesn't access the payload packet header.  It only looks
>> at the mbuf (which has a structure called mbuf packet header).  The mbuf
>> header it hot in the cache because the driver just touched it and filled
>> in the information.  The packet content (the payload) is cold and just
>> arrived via DMA in DRAM.
> Why does it use ntohs() then? :-).  From if_ethersubr.c:

> ...
> % 	eh = mtod(m, struct ether_header *);
> Point outside of mbuf header.
> % 	etype = ntohs(eh->ether_type);
> First access outside of mbuf header.
> ...
> % % 	/* Allow monitor mode to claim this frame, after stats are updated. 
> */
> % 	if (ifp->if_flags & IFF_MONITOR) {
> % 		m_freem(m);
> % 		return;
> % 	}
> Finally return in monitor mode.
> I don't see any stats update before here except for the stray if_imcasts
> one.

There are some error stats with printfs, but I've never seen these do
anything except with a buggy sk driver.

Testing verifies that accessing eh above gives a cache miss.  Under
~5.2 receiving on bge0 at 397 kpps:

-monitor: 17% idle 19 cm/p  (18% less idle than under -current)
  monitor: 66% idle  8 cm/p  (17% less idle than under -current)
+monitor: 71% idle  7 cm/p  (idle time under -current not measured)

+monitor is monitor mode with the exit moved to the top of ether_input().

If the cache miss takes the time measured by lmbench2 (42 ns), then
397 k of these per second gives 17 ms or 1.7% CPU, which is vaguely
consistent with the improvement of 5% by not taking this cache miss.
Avoiding most of the 19 cache misses should give much more than a
5% improvement.  Maybe -current gets its 17% improvement by avoiding

More userland stats weirdness in userland:
- in monitor mode, em0 gives byte counts delayed while bge0 gives byte
   counts always 0.
- netstat -I <interface> 1 seems to be broken in ~5.2 in all modes -- it
   gives output for interfaces with drivers but no hardware.

All this is for UP.  An SMP kernel on the same UP system loses < 5% for at
least tx.


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