Re: zero-copy TCP

From: Jamie Lokier (
Date: Sun Sep 03 2000 - 15:21:14 EST

Alan Cox wrote:
> > I just thought I'd mention that you can do zero copy TCP in and out
> > *without* any page marking schemes. All you need is a network card with
> No
> > quite a lot of RAM and some intelligence. An Alteon could do it, with
> > extra RAM or an impressively underloaded network.
> PCI bus latency on reads kills you. Nice idea, but you need busses far better
> than those hanging off conventional computers

No, there are no PCI bus reads.

read/recv block while the NIC DMAs into user space main memory.

(Can't DMA earlier because we don't know the buffer address in advance
when using the standard socket API).

The NIC memory is never accessed directly. It's a cache of skbuff data
payloads, and the only access is by DMA, in those places where the
kernel stack normally does copy_to_user/copy_from_user.

-- Jamie
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