Re: 4g/4g for 2.6.6

[Ingo Molnar <mingo@elte.hu>]

* Linus Torvalds <torvalds@osdl.org> wrote:

> Quite frankly, a number of us are hoping that we can make them
> unnecessary. The cost of the 4g/4g split is absolutely _huge_ on some
> things, including basic stuff like kernel compiles.
>
> The only valid reason for the 4g split is that the VM doesn't always
> behave well with huge amounts of highmem. The anonvma stuff in
> 2.6.7-pre1 is hoped to make that much less of an issue.
>
> Personally, if we never need to merge 4g for real, I'll be really
> really happy. I see it as a huge ugly hack.

i agree with the hack part - but the performance aspect has been blown
out of proportion. 4:4 has the same cost on kernel compiles as highpte.
There are also real workloads where it actually helps performance.

also, the 4:4 overhead is really a hardware problem - and there are
x86-compatible CPUs (amd64) where the TLB flush problem has already been
solved: on amd64 the 4:4 feature has no noticeable overhead. So as long
as people opt for a 32-bit OS (even on a 64-bit CPU, for whatever weird
compatibility reason), 4:4 can be useful. For the other workloads i as
much hope as everyone else that people switch to a 64-bit OS on x86-64
ASAP!

also, while a quick transition to x86-64 will most likely happen, the
large installed base of big x86 boxes is a matter of fact too - and they
wont vanish into thin air. Also, there will always be specific user
workloads where lowmem grows to large values. Not to mention the fact
that 4:4 is a nice debugging/security tool as you cannot dereference
user pointers ;) - it has caught countless bugs already. Plus there are
specific workloads that want 4GB of userspace (and no, 3.5 GB wont do).

So 4:4 will have its niches where it will live on. We could argue on and
on how quickly 'x86 with more than 4GB of RAM' and 'x86 with 4GB of a
userspace' will become a niche, hopefully it happens fast but we've got
to keep our options open.

	Ingo