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

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Date:	Thu, 21 Mar 2013 11:55:16 +0100
From:	Ingo Molnar <mingo@...nel.org>
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


* Russ Anderson <rja@....com> 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.

Nice speedup!

A minor nit, in addition to Andrew's suggestion about wrapping 
__early_pfn_to_nid():

> Index: linux/mm/page_alloc.c
> ===================================================================
> --- linux.orig/mm/page_alloc.c	2013-03-18 10:52:11.510988843 -0500
> +++ linux/mm/page_alloc.c	2013-03-18 10:52:14.214931348 -0500
> @@ -4161,10 +4161,19 @@ int __meminit __early_pfn_to_nid(unsigne
>  {
>  	unsigned long start_pfn, end_pfn;
>  	int i, nid;
> +	static unsigned long last_start_pfn, last_end_pfn;
> +	static int last_nid;

Please move these globals out of function local scope, to make it more 
apparent that they are not on-stack. I only noticed it in the second pass.

Thanks,

	Ingo
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