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SubjectRe: zero-copy TCP


Alan Cox wrote:
>
> > > Sounds like Linux - one DMA and one copy to user space.
> >
> > Alan, Please. I'm in your code and there are copies all over the
> > place. I agree you have a "fast path" for most stuff, but there's all
>
> There arent copies all over the case for the paths that occur. Like 99.999%
> of the time. Fragmented packets dont happen except for NFS (which is a rather
> broken protocol anyway).

There are.

>
> One DMA, one copy to user space
>
> > kinds of handles lookups, linear list searching like
> >
> > while (x)
> > {
> > x = x->next
> > }
>
> timers are constructed to be close to O(1), the tcp hash isnt a linear lookup,
> the socket operations from user space use file-> dereferences not a lookup

It is is there's a hash collision.

>
> > nothing in TCPIP except at the stream head. Why do you need to copy
> > data anyway to checksum an IP packet anyway? I noticed you do the right
> > thing and keep the headers and data as separate fragments during header
> > construction, so why do you need to copy data for checksumming?
>
> We dont copy for checksumming. We fold the single user space copy and the
> checksum operation into one path, because on any modern CPU it costs precisely
> the same to copy as to copy/checksum.
>
> I don't think you've actually sat and instrumented the TCP code

In Linux, no, in Netware, yes. I'm in your TCP code now and it's
fairly large.

Jeff



>
> Alan
>
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