Date: Thu, 21 Mar 2013 11:55:16 +0100
From: Ingo Molnar <mingo@kernel.org>
To: Russ Anderson <rja@sgi.com>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, tglx@linutronix.de,
        mingo@redhat.com, hpa@zytor.com
Subject: Re: [patch] mm: speedup in __early_pfn_to_nid
Message-ID: <20130321105516.GC18484@gmail.com>
References: <20130318155619.GA18828@sgi.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20130318155619.GA18828@sgi.com>
User-Agent: Mutt/1.5.21 (2010-09-15)
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 2077
Lines: 53


* Russ Anderson <rja@sgi.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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/