linux-kernel - Re: [patch] mm: speedup in __early_pfn_to

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Date:	Mon, 18 Mar 2013 20:56:13 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Russ Anderson <rja@....com>
cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	tglx@...utronix.de, mingo@...hat.com, hpa@...or.com
Subject: Re: [patch] mm: speedup in __early_pfn_to_nid

On Mon, 18 Mar 2013, Russ Anderson wrote:

> When booting on a large memory system, the kernel spends
> considerable time in memmap_init_zone() setting up memory zones.
> Analysis shows significant time spent in __early_pfn_to_nid().
> 
> The routine memmap_init_zone() checks each PFN to verify the
> nid is valid.  __early_pfn_to_nid() sequentially scans the list of
> pfn ranges to find the right range and returns the nid.  This does
> not scale well.  On a 4 TB (single rack) system there are 308
> memory ranges to scan.  The higher the PFN the more time spent
> sequentially spinning through memory ranges.
> 
> Since memmap_init_zone() increments pfn, it will almost always be
> looking for the same range as the previous pfn, so check that
> range first.  If it is in the same range, return that nid.
> If not, scan the list as before.
> 
> A 4 TB (single rack) UV1 system takes 512 seconds to get through
> the zone code.  This performance optimization reduces the time
> by 189 seconds, a 36% improvement.
> 
> A 2 TB (single rack) UV2 system goes from 212.7 seconds to 99.8 seconds,
> a 112.9 second (53%) reduction.
> 
> Signed-off-by: Russ Anderson <rja@....com>

Acked-by: David Rientjes <rientjes@...gle.com>

Very nice improvement!
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