performance: memcpy vs. __copy_tofrom_user

[Benjamin Herrenschmidt]

> There should definitely be a nice API for an in-kernel AltiVec context
> save/restore. When preemption happens doesn't it do some equivalent of
> the userspace context switch? Why can't the preemption system take care
> of it?
> 
> At worst case you make the worst case latency bigger, but at best case
> you gain performance across the board.

Do you ? Can you prove this assertion with numbers ?

> One thing which is worrying me is that now that Ben has thrown down the
> gauntlet (note, I'm not going to be coding a line, but I know a man who
> can :) how on earth do we benchmark the differences here?

Precisely :-)

So again, let's start by having somebody pick up something that you
believe is worth altivec-ifying, eat the preempt_disable/enable for now,
and if we see that indeed, it's worth the pain, then we can look into
adding a way to context switch altivec in a kernel thread upon explicit
request or something like that.

As to how to benchmark the difference ? Well, I would suggest first a
couple of very simple things that give a good indication, and from
there, if it looks promising, we can torture more and see whether we can
find regressions etc..

For example, I personally use kernel compile times (with make -jN on
SMP), I find it a good overall exercise, but if you feel like a network
benchmark might be better at advertising your improvements, then go for
that too, though expect us to also do some other tests to verify they
didn't regress.

Cheers,
Ben.