Re: -mmX 4G patches feedback [numbers: how much performance impact]

["Martin J. Bligh" <mbligh@aracnet.com>]

> I agree as well it solves a real problem (i.e. 4G userspace), though the
> userbase that needs it is extremely limited and they're sure ok to run
> slower than to change their application to use shmfs (a special 4:4
> kernel may be ok, just like a special 2.5:1.5 may be ok, just like
> 3.5:0.5 was ok for similar reasons too), but the mass market doesn't
> need 4:4 and it will never need it, so it's bad to have the masses pay
> for this relevant worthless runtime overhead in various common
> workloads.

Yeah, it needs to be a separate kernel for huge blobby machines. I think
that's exactly what RH does, IIRC (> 16GB ?)
 
> Of course above I'm talking about 2.6-aa or 2.6-mjb. Clearly with
> kernels including rmap like 2.6 mainline or 2.6-mm or 2.6-mc or the
> 2.4-rmap patches you need 4:4 everywhere, even on a 4/8G box to avoid
> running out of normal zone in some fairly common and important workload.

Speaking of which, pte_highmem is stinking expensive itself. There's
probably a large class of workloads that'd work with out pte_highmem
if we had 4/4 split (or shared pagetables. Grrr ;-))

M.