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

[Mel Gorman <mgorman@suse.de>]

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@hp.com>

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;
}

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.

-- 
Mel Gorman
SUSE Labs

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