Re: [patch 0/4] ia64 SPARSEMEM

[David Mosberger <davidm@napali.hpl.hp.com>]

>>>>> On Fri, 27 May 2005 06:35:48 -0400, Bob Picco <bob.picco@hp.com> said:

  Bob> luck wrote:	[Thu May 26 2005, 06:03:29PM EDT]
  >> >Well worse case it would consume 2^(1(50-32)+3) (2 Mb).  I would hope that 
  >> >it's not configured for 28 SECTION_SIZE_BITS and 50 physical. This would
  >> >be excessive 2^((50-28)+3 = 32Mb and not advised.

  >> While you can tune a custom kernel for a particular system configuration,
  >> we might have to use the 50/28 configuration for the generic "defconfig"
  >> case ... that kernel should be bootable anywhere.   SGI needs (or will
  >> need) the "50" for total physical size, and other platforms may need the
  >> "28" (is that even small enough?  We currently have "granule" sizes of
  >> 16M and 64M to cope with odd holes in the physical address space ... so
  >> perhaps the section size might need to be even smaller: 24 or 26?  How did
  >> you come up with 28 as the low bound for SECTION_BITS?)

  Bob> It seems to be least costly in terms of consuming reserved
  Bob> pages. Of course reducing it decreases consuming reserved pages
  Bob> but at the cost of increasing the mem_section size.  Should 50
  Bob> have to be the defconfig value, then I'd recommend 30 for
  Bob> SECTION_SIZE_BITS.  So memory wise 8Mb -> 2^((50-30)+3) for
  Bob> mem_section.  Smaller than 30 SECTION_BITS for 50 bit physical
  Bob> consumes too much memory for mem_section.

This discussion just demonstrates what a pain such config parameters
are.  Such stuff need to be
self-adjusting/self-tuning/able-to-cover-any-configuration.  Virtual
mem-map seems to qualify much better here.

	--david