> From: alan@lxorguk.ukuu.org.uk (Alan Cox)
> Date: Tue, 4 May 1999 19:01:21 +0100 (BST)
>
> > BTW is it really true, that NT transmitfile() does zero copy? I
> > strongly suspect, it does not.
>
> NT5 beta claims to
>
> They can avoid the extraneous copy, but what they cannot do with most
> PC networking cards is avoid touching the data since most cards do not
> provide a hardware checksumming facility.
>
> Most of this would suggest that their existing architecture passes
> mbuf-chain-like buffers to the networking drivers in NT, or some other
> kind of scatter-gather list like scheme. This is the only way they
> could do zero-copy without driver updates from all the networking card
> vendors.
>
> Later,
> David S. Miller
> davem@redhat.com
>
They can't. Many of the driver cards that NT uses use PIO. This, in
anybody's book, is a copy operation. There is one Intel board,
it has an i960 on it (don't remember the name), plus the regular
Intel chipset, that was designed for 'zero-copy' operation. There
is a NT driver for it. Even this is not 'zero-copy' although the
i960 does the copy instead of the ix86.
In the 'real world' about the only zero-copy that could work is
for datagrams, i.e., connectionless. If you are going to assemble
out-of-sequence and missing packets in 'user-land' I don't see
the advantage of zero-copy at all.
Cheers,
Dick Johnson
***** FILE SYSTEM WAS MODIFIED *****
Penguin : Linux version 2.2.6 on an i686 machine (400.59 BogoMips).
Warning : It's hard to remain at the trailing edge of technology.
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