Re: Half Duplex and Zero Copy IP

[Subba Rao]

On  0, Andrew Morton <[e-mail]> wrote:
> 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.

So, is the card set in half-duplex mode by default and full-duplex is a forced
option?

> 
> > 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?
> 
	http://www.fefe.de/linuxeth/

"To achieve gigabit throughput, it is important that the operating system does 
not copy the data in the packets before sending them (this is called zero-copy 
IP). Unfortunately, the kernel needs to put a header before the data in the
packet, so not copying the data to a buffer in kernel space means that the NIC
needs to be able to fetch the header from a different place in memory than the
user data in the packet. This is called scatter/gather and is necessary for
zero-copy IP." 

-- 

Subba Rao
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http://members.home.net/subba9/

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