Re: [RFC] [PATCH] cache pollution aware __copy_from_user_ll()

Arjan van de Ven
2005-08-15T07:21:57Z

On Sun, 2005-08-14 at 23:24 +0200, Ian Kumlien wrote:
> Hi, all
>
> I might be missunderstanding things but...
>
> First of all, machines with long pipelines will suffer from cache misses
> (p4 in this case).
>
> Depending on the size copied, (i don't know how large they are so..)
> can't one run out of cachelines and/or evict more useful cache data?

CPU caches are really big nowadays

>
> Ie, if it's cached from begining to end, we generally only need 'some
> of' the begining, the cpu's prefetch should manage the rest.

cpu prefetch isn't going to be fast enough. It helps some, but in the
end the cpu prefetch also has to wait for the ram, it doesn't make the
ram faster or free, it just takes a jumpstart on getting to it.

> I might, as i said, not know all about things like this and i also
> suffer from a fever but i still find Hiro's data interesting.

It is. It's good proof that you can make a big gain already by
converting a few key places to his excellent code. And neither me nor
Christoph are suggesting to ditch his effort! Instead we suggest that
what he is doing is useful for some cases and harmful for others, and
that it is quite easy to identify those cases and separate them from
eachother, and that thus as a result it is more optimal to have 2 apis,
one for each of the cases.