Re: zero-copy TCP

From: Andi Kleen (
Date: Sat Sep 02 2000 - 17:39:38 EST

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]

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