Re: 2.4.23aa2 (bugfixes and important VM improvements for the high end)

[Ingo Molnar <mingo@elte.hu>]

* Andrea Arcangeli <andrea@suse.de> wrote:

> [...] but I'm quite confortable to say that up to 16G (included) 4:4
> is worthless unless you've to deal with the rmap waste IMHO. [...]

i've seen workloads on 8G RAM systems that easily filled up the ~800 MB
lowmem zone. (it had to do with many files and having them as a big
dentry cache, so yes, it's unfixable unless you start putting inodes
into highmem which is crazy. And yes, performance broke down unless most
of the dentries/inodes were cached in lowmem.)

as i said - it all depends on the workload, and users are amazingly
creative at finding all sorts of workloads. Whether 4:4 or 3:1 is thus
workload dependent.

should lowmem footprint be reduced? By all means yes, but only as long
as it doesnt jeopardize the real 64-bit platforms. Is 3:1 adequate as a
generic x86 kernel for absolutely everything up to and including 16 GB? 
Strong no. [not to mention that 'up to 16 GB' is an artificial thing
created by us which wont satisfy an IHV that has a hw line with RAM up
to 32 or 64 GB. It doesnt matter that 90% of the customers wont have
that much RAM, it's a basic "can it scale to that much RAM" question.]

so i think the right answer is to have 4:4 around to cover the bases -
and those users who have workloads that will run fine on 3:1 should run
3:1.

(not to mention the range of users who need 4GB _userspace_.)

but i'm quite strongly convinced that 'getting rid' of the 'pte chain
overhead' in favor of questionable lowmem space gains for a dying
(high-end server) platform is very shortsighted. [getting rid of them
for purposes of the 64-bit platforms could be OK, but the argumentation
isnt that strong there i think.]

	Ingo