Re: [RFC][PATCH 0/4] ZERO PAGE again v2

[Linus Torvalds <torvalds@linux-foundation.org>]

On Tue, 7 Jul 2009, Nick Piggin wrote:
> 
> I just wouldn't like to re-add significant complexity back to
> the vm without good and concrete examples. OK I agree that
> just saying "rewrite your code" is not so good, but are there
> real significant problems? Is it inside just a particuar linear
> algebra library or something  that might be able to be updated?

The thing is, ZERO_PAGE really used to work very well.

It was not only useful for simple "I want lots of memory, and I'm going to 
use it pretty sparsely" (which _is_ a very valid thing to do), but it was 
useful for TLB benchmarking, and for cache-efficient "I'm going to write 
lots of zeroes to files", and for a number of other uses.

You can talk about TLB pressure all you want, but the fact is, quite often 
normal cache effects dominate - and ZERO_PAGE is _wonderful_ for sharing 
cachelines (which is why it was so useful for TLB performance testing: map 
a huge area, and you know that there will be no cache effects, only TLB 
effects).

There are actually very few cases where TLB effects are the primary ones - 
they tend to happen when you have truly random accesses that have no 
locality even on a small case. That's pretty rare. Even things that depend 
on sparse arrays etc tend to mainly _access_ the parts it works on (ie you 
may have allocated hundreds of megs of memory to simplify your memory 
management, but you work on only a small part of it).

So it's not just "people actually use it". It really was a useful feature, 
with valid uses. We got rid of it, but if we can re-introduce it cleanly, 
we definitely should.

I don't understand why you fight it. If we can do it well (read: without 
having fork/exit cause endless amounts of cache ping-pongs due to touching 
'struct page *'), there are no downsides that I can see. It's not like 
it's a complicated feature.

		Linus

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