performance: memcpy vs. __copy_tofrom_user

Dominik Bozek domino at
Thu Oct 9 22:41:15 EST 2008

Paul Mackerras wrote:
> Dominik Bozek writes:
>> Actually I made couple of other tests on that mpc8313. Most of them are
>> to ugly to publish them, but... My problem is that I have to boost the
>> gigabit interface on the mpc8313. I made simple substitution and
>> __copy_tofrom_user was used instead of memcpy. I know, it's wrong, but I
>> speedup that way the network interface for about 10%.
> Very interesting.  Can you work out where memcpy is being called on
> the network data?  I wouldn't have expected that.

I'm not the fastest, but I will. Just need some time.

> There is actually no strong reason not to use __copy_tofrom_user as
> memcpy, in fact, as long as we are sure that source and destination
> are both cacheable.

My board doesn't have graphics, sound,...  so I don't know if and how it
touch that subsystems, but for sure ext2 fail. Interesting because it
was a ramdisk. Remember, that it was very tricky test, don't make any
wrong conclusion out of it.


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