About SECTION_SIZE_BITS for Sparsemem

[mel@csn.ul.ie (Mel Gorman)]

On Tue, Jul 13, 2010 at 10:38:15AM +0100, Russell King - ARM Linux wrote:
> On Tue, Jul 13, 2010 at 10:26:58AM +0100, Mel Gorman wrote:
> > There is also an assumption that a section is fully populated or empty.
> 
> That is absolutely absurd. 

Arguably, violating the memory model by punching unexpected holes in it
is a also absurd.

> So, I have a platform which has 256MB at
> 64MB intervals in 4 chunks. I can fit 512kB to any slot. 

I'm afraid I'm not quite getting your example.

If the granularity of the banks is 64MB and the alignment is 256MB, I
don't see what hole you'd be punching anyway.

> It starts
> at 0x10000000.  Do I really need 1024 sparsemem sections to cater for
> this?
> 

Not necessarily, just don't punch holes within section boundaries when using
sparsemem. If mapping in a PageReserved page is not an option due to complexity
(it's possible by remapping pages in kernel space which x86 used to do for
discontig) then the size of mem_section as suggested by Minchan Kim would be
another option. This would increase the overhead of sparsemem in terms of space
and performance. - at worst by an amount matching the memory freed by memmap.

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab