Re: [PATCH] mm: memmap_init_zone() performance improvement

[Ni zhan Chen]

On 10/08/2012 11:16 PM, Mel Gorman wrote:
> On Wed, Oct 03, 2012 at 08:56:14AM -0600, Mike Yoknis wrote:
> > memmap_init_zone() loops through every Page Frame Number (pfn),
> > including pfn values that are within the gaps between existing
> > memory sections.  The unneeded looping will become a boot
> > performance issue when machines configure larger memory ranges
> > that will contain larger and more numerous gaps.
> >
> > The code will skip across invalid sections to reduce the
> > number of loops executed.
> >
> > Signed-off-by: Mike Yoknis <mike.yoknis@xxxxxx>
> This only helps SPARSEMEM and changes more headers than should be
> necessary. It would have been easier to do something simple like
>
> if (!early_pfn_valid(pfn)) {
> 	pfn = ALIGN(pfn + MAX_ORDER_NR_PAGES, MAX_ORDER_NR_PAGES) - 1;
> 	continue;
> }

So if present memoy section in sparsemem can have 
MAX_ORDER_NR_PAGES-aligned range are all invalid?
If the answer is yes, when this will happen?

> because that would obey the expectation that pages within a
> MAX_ORDER_NR_PAGES-aligned range are all valid or all invalid (ARM is the
> exception that breaks this rule). It would be less efficient on
> SPARSEMEM than what you're trying to merge but I do not see the need for
> the additional complexity unless you can show it makes a big difference
> to boot times.

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