Re: efficient copy_to_user and copy_from_user routines in Linux Kernel

From: Andrew Morton (
Date: Tue Jun 25 2002 - 12:03:38 EST

Mala Anand wrote:
> Here is a 2.5.19 patch that improves the performance of IA32 copy_to_user
> and copy_from_user routines used by :
> (1) tcpip protocol stack
> (2) file systems

This came up about a year back when zerocopy networking was merged.
Intel boxes started running more slowly purely because of the 8+8
alignment thing.

I changed tcp to use a different copy if either source or dest were
not eight-byte aligned, and found that the resulting improvement
across a mixed networking load was only 1%. Your numbers are higher,
so perhaps there are different alignments in the mix...

One question: have you tested on other CPU types? This problem is
very specific to Intel hardware. On AMD, the eight-byte alignement
artifact does not exist at all. It could be that your patch is not
desirable on such CPUs?

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