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SubjectRe: i686 SMP systems with more then 12 GB ram with 2.4.x kernel ?
On Wed, Jan 02, 2002 at 01:17:59PM -0800, Gerrit Huizenga wrote:
> I don't know if there are real examples of large memory systems
> exhausting the ~1 GB of kernel virtual address space on machines
> with > 12-32 GB of physical memory (we had this problem in PTX which
> created the need for a larger kernel virtual address space in some
> contexts).

The ~800MB or so of kernel address space is exhausted with struct page
entries around 48GB of physical memory or so

SGI's original highmem patch switched page tables on entry to kernel
space, so there is code already tested that we can borrow. But I'm
not sure if it's worth it as the overhead it adds makes life really
suck: we would lose the ability to use global pages, as well as always
encounter tlb misses on the kernel<->userspace transition. PAE shows
up as a 5% performance loss on normal loads, and this would make it
worse. We're probably better off implementing PSE. Of course, making
these kinds of choices is hard without actual statistics of the
usage patterns we're targetting.

> Would be nice to have a config option like "CONFIG_PCI_36" to imply
> that all devices on a PAE system were able to access all of memory,
> globally removing the need for bounce buffering and allowing a native
> PCI setup for mapping memory addresses...

That would be neat.

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