linux-kernel - Re: [PATCH] mm: memmap_init_zone() performance improvement

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Message-ID: <20121008151656.GM29125@suse.de>
Date:	Mon, 8 Oct 2012 16:16:56 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	Mike Yoknis <mike.yoknis@...com>
Cc:	mingo@...hat.com, akpm@...ux-foundation.org,
	linux-arch@...r.kernel.org, mmarek@...e.cz, tglx@...utronix.de,
	hpa@...or.com, arnd@...db.de, sam@...nborg.org, minchan@...nel.org,
	kamezawa.hiroyu@...fujitsu.com, mhocko@...e.cz,
	linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH] mm: memmap_init_zone() performance improvement

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@...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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