Jes Sorensen wrote:
> You can't DMA directly from a file cache page unless you have a
> network card that does scatter/gather DMA and surprise surprise,
> 80-90% of the cards on the market don't support this. Besides that you
> need to do copy-on-write if you want to be able to do zero copy on
> write() from user space, marking data copy on write is *expensive* on
> x86 SMP boxes since you have to modify the tlb on all
> processors. On top of that you have to look at the packet size, for
> small packets a copy is often a lot cheaper than modifying the page
> tables, even on UP systems so you need a copy/break scheme here.
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
quite a lot of RAM and some intelligence. An Alteon could do it, with
extra RAM or an impressively underloaded network.
(for example) http://www.digital.com/info/DTJS05/
-- Jamie
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