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SubjectRe: -mmX 4G patches feedback [numbers: how much performance impact]
--On Thursday, April 08, 2004 23:59:46 +0200 Andrea Arcangeli <andrea@suse.de> wrote:

> On Wed, Apr 07, 2004 at 11:24:16PM -0700, Martin J. Bligh wrote:
>> Instead of fiddling with tuning knobs, I'd prefer to just do the UKVA
>> idea I've proposed before, and let each process have their own pagetables
>> mapped permanently ;-)
>
> that will have you pay for pte-highmem even in non-highmem machines.
> I'm always been against your above idea ;) It can speedup mmap a bit for
> some uncommon case but I believe your slowdown comes from the page faults after
> exeve and startup not from mmap with the kernel compile, and worst of
> all for non-highmem too (no sysctl or tuning knob can save you then).
> Amittedly some mmap intensive workload can get a slight speedup compared
> to pte-highmem but I don't think it's common and it has the potential of
> slowing down the page faults especially in short lived tasks even w/o
> highmem.

You mean the page-faults for the pagetable mappings themselves? I wouldn't
have thought that'd make an impact - at least I don't see how it could be
worse than pte_highmem. And as we could make it conditional on highmem
anyway (or even CONFIG_64GB, I'm pretty sure 4GB machines don't need it),
I don't think it matters (ie you'd just turn it on instead of pte_highmem).

But you're right, we do need to take that into consideration.

> What I found attractive was the persistent kmap in userspace, but that
> idea breaks with threading, and Andrew found another way that is to make
> the page fault interruptible so it doesn't seem very worthwhile anymore
> even w/o threading.

Yeah, I've given up on that one ;-) The main use for it was pagetables
anyway, and we can do that without the threading problems.

M.

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