Re: clear_user_highpage()

["David S. Miller" <davem@redhat.com>]

On Wed, 11 Aug 2004 16:46:10 -0700 (PDT)
Linus Torvalds <torvalds@osdl.org> wrote:

> It sucks because it does bad things to cache behaviour. Sure, you'll move 
> the cost away from "clear_user_highpage", but the thing is, you will _not_ 
> move it into the idle time. What you will do is to move it into some 
> random time _after_ the idle time, when the idle thing has crapped all 
> over your caches.

It won't crap out the caches on sparc64 and any platform with cache-bypass-
on-miss stores.  I believe ia64 and opteron have similar mechanisms.

If the store misses the L2 cache it goes straight out to main memory,
it doesn't allocate cache lines anywhere in such cases.

I think ppc/ppc64 has this too... no, sorry, it has the data-cache allocate
line and zero instruction, which isn't what you want here.

> The thing is, you make your cache footprint per CPU _much_ bigger, and you 
> spread it out a lot over time too, so you make it even worse.
> 
> The clearing will then be totally hidden in the profiles, because you will
> have turned a nice and well-behaved "this is there the time goes" profile
> into a mush of "we're taking cache misses at random times, and we don't
> know why".

Therefore, I do not believe any of this is applicable.