Andi Kleen wrote:
>
> On Sat, Sep 02, 2000 at 04:28:18PM -0600, Jeff V. Merkey wrote:
> >
> >
> > Alan Cox wrote:
> > >
> > > 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.
> >
> > You stated in an earlier message you copied the data when you caclulated
> > the TCPIP checksum? No you say you don't. Perhaps I misunderstood.
>
> Linux always does a single copy for TCP, and the checksum is folded into
> that. Doing just the checksum alone wouldn't be much less costly.
>
> [Note this is only true for 2.4 in the fast path, 2.2 RX usually does
> checksum and copy-to-user separated, unless you have hardware RX checksumming
>
> For TX we always do a single copy checksum out of user space or out of the
> page cache when you use sendfile or mmap]
This makes sense.
Jeff
>
> -Andi
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