[ECOS] TCP/IP checksum routine performance

Grant Edwards grante@visi.com
Fri Apr 28 09:11:00 GMT 2000

On Fri, Apr 28, 2000 at 05:55:20PM +0200, Andrew Lunn wrote:

> > I've made a possibly dangerous assumption that the only time an
> > mbuf will contain an odd number of bytes is at the end of a
> > chain (IOW, a 16-bit word is never split across mbuf
> > boundaries).  
> My guess is you are quite safe with this assumption. This code
> came from a UNIX system which is quite likly to have DMA
> engines. A lot of DMA engines don't like transfering less than
> words, especially when in scatter/gather mode. Just to be safe
> you could add an assertion, so if it does blow up we know why.

Yea, I've got a test in there now.  I'm pretty sure that none
of the standard protocol code is going to try to split a word.
IP and TCP headers are always in their own mbufs, and I don't
think that TCP data segments are going to get split at odd byte
boundaries.  I also can't see any way that it could be made to
happen by user application code.  But, somebody went to a fair
amount of trouble to make the current routine handle it, so I'm
a bit uneasy.

> Probably a good idea to cross post this to the gcc list. Those readers
> may have a better idea whats going on inside gcc.

I'm going to try to generate a simpler, stand-alone source file
that demonstrates the phenomenon.  Right now, in order to get
the mbuf struct definition you end up with the entire set of
network headers pulled in just to compiler the checksum routine
(which isn't too convenient for the compiler-hacker).

Grant Edwards

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