Re: Half Duplex and Zero Copy IP

[Andrew Morton <akpm@zip.com.au>]

Subba Rao wrote:
> 
> Hello,
> 
> I have 2 3Com NICs on my system. They are 3c905C Tornado PCI cards.
> The drivers are compiled into the kernel (Slackware 8.0 with kernel 2.4.7).
> 
> One of the interfaces will be used as a sniffer interface (without IP address)
> and a very high traffic pipes. I do not wish to loose any packets coming to this
> interface. Is it better if I initialize the interface in HALF DUPLEX mode? If yes,
> how do I set the card to HALF DUPLEX mode? How can I find out the HW (NIC) settings
> on the system?

No, this will provide no benefit.

> Another question about 3Com NICs, do they perform zero-copy IP?

Linux's zerocopy infrastructure allows the sendfile() system call
to save a copy with NICs which have hardware checksumming and
scatter/gather.  3c905C is one such NIC.  Kernel is not generally
"zero copy", but large savings are available in certain situations.
NFS packet reassembly benefits from 905C's as well.

> I read that the performance improves a lot WITHOUT zero-copy IP.

Not right.  Where did you read that?

-