Subject: Re: performance: memcpy vs. __copy_tofrom_user

From: Benjamin Herrenschmidt <[email-masked]>
Date: Sun, 12 Oct 2008 15:05:37 +1100

> Would the examples (page copy, page clear) be an okay place to do it?
> These sections can't be preempted anyway (right?), and it's noted that
> doing it with AltiVec is a tad faster than using MMU tricks or standard
> copies?

I think typically page copying and clearing -are- preemptible. I'm not
sure what you mean by MMU tricks, but it's not clear whether using
altivec will result in any significant performance gain here,
considering the cost of enabling/disabling altivec (added to handling
the preemption issue).

However, nothing prevents you from trying to do it and we'll see what
the results are with hard numbers.

> In Scott's case, while "optimizing memcpy for 48byte blocks" was a joke,
> this is 3 load/stores in AltiVec, which as long as every SKB is 16
> byte aligned (is there any reason why it would not be? :)

In this case, the cost of enabling/saving/restoring altivec will far
outweight any benefit. In addition, skb's are often not well aligned due
to the alignment tricks done with packet headers.

> skb_clone might not be something you want to dump AltiVec into and would
> make a mess if an skb got extended somehow, but the principle is outlined
> in a very good document from a very long time ago;
> 
> http://www.motorola.com.cn/semiconductors/sndf/conference/PDF/AH1109.pdf
> 
> I think a lot of it still holds true as long as you really don't care
> about preemption under these circumstances (where network throughput
> is more important, and where AltiVec actually *reduces* CPU time, the
> overhead of disabling preemption is lower anyway). You could say the
> same about the RAID functions - I bet LatencyTOP has a field day when
> you're using RAID5 AltiVec.

RAID6 actually :-)

In any case, as I said, people are welcome to implement something that
can be put to the test and measured. If it proves beneficial enough, then
I see no reason not to merge it. Basically, enough talks, just do something
and we'll see whether it proves useful or not.

Cheers,
Ben.