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SubjectRe: i686 SMP systems with more then 12 GB ram with 2.4.x kernel ?
Date
On Saturday, 29. December 2001 18:45, Alan cox wrote:
> > Are there some i686 SMP systems with more then 12 GB ram out there ?
>
> Very very few.
>
> > Is there a known problem with 2.4.x kernel and such systems ?
>
> Several 8)
>
> Hardware limits:
> - 36bit addressing mode on x86 processors is slower
> - Many device drivers cant handle > 32bit DMA
> - The CPU can't efficiently map all that memory at once
>
> Software:
> - The block I/O layer doesn't cleanly handle large systems
> - The page struct is too big which puts undo loads on the
> memory that the CPU can map
> - We don't discard page tables when we can and should
> - We should probably switch to a larger virtual page size
> on big machines.
>
> The ones that actually bite hard are the block I/O layer and the page
> struct size. Making the block layer handle its part well is a 2.5 thing.
>
> > It looks like as the buffer_heads would fill the low memory up,
> > whether there is sufficient memory available or not, as long as
> > there is sufficient high memory for caching.
>
> That may well be happening. The Red Hat supplied 7.2 and 7.2 errata kernels
> were tested on 8Gb, I don't know what else larger.
>
> Because much of the memory cannot be used for kernel objects there is an
> imbalance in available resources and its very hard to balance them sanely.
> I'm not sure how many 8Gb+ machines Andrea has handy to tune the VM on
> either.

I think Andrea has access to some. Maybe SAP?

Have you tried with 2.4.17rc2aa2?
ftp://ftp.us.kernel.org/pub/linux/kernel/people/andrea/kernels/v2.4/2.4.17rc2aa2.bz2

The 10_vm-21 part applies clean to 2.4.17 (final), too.
I have it running without a glitch. But sadly a way smaller (much smaller)
system...;-)

Regards,
Dieter

--
Dieter Nützel
Graduate Student, Computer Science

University of Hamburg
Department of Computer Science
@home: Dieter.Nuetzel@hamburg.de
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