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Date: Tue, 11 Mar 2008 09:14:10 +0100
From: Ingo Molnar <mingo@elte.hu>
To: Yinghai Lu <yhlu.kernel@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
       Christoph Lameter <clameter@sgi.com>,
       kernel list <linux-kernel@vger.kernel.org>,
       Andy Whitcroft <apw@shadowen.org>, Mel Gorman <mel@csn.ul.ie>
Subject: Re: [PATCH] mm: make mem_map allocation continuous.
Message-ID: <20080311081410.GA24240@elte.hu>
References: <86802c440803102322i74333718h60de0da329b139c8@mail.gmail.com>
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* Yinghai Lu <yhlu.kernel@gmail.com> wrote:

> [PATCH] mm: make mem_map allocation continuous.
> 
> vmemmap allocation current got
>  [ffffe20000000000-ffffe200001fffff] PMD ->ffff810001400000 on node 0
>  [ffffe20000200000-ffffe200003fffff] PMD ->ffff810001800000 on node 0
>  [ffffe20000400000-ffffe200005fffff] PMD ->ffff810001c00000 on node 0
>  [ffffe20000600000-ffffe200007fffff] PMD ->ffff810002000000 on node 0
>  [ffffe20000800000-ffffe200009fffff] PMD ->ffff810002400000 on node 0
> ...
> 
> there is 2M hole between them.
> 
> the rootcause is that usemap (24 bytes) will be allocated after every 2M
> mem_map. and it will push next vmemmap (2M) to next align (2M).
> 
> solution:
> try to allocate mem_map continously.
> 
> after patch, will get
>  [ffffe20000000000-ffffe200001fffff] PMD ->ffff810001400000 on node 0
>  [ffffe20000200000-ffffe200003fffff] PMD ->ffff810001600000 on node 0
>  [ffffe20000400000-ffffe200005fffff] PMD ->ffff810001800000 on node 0
>  [ffffe20000600000-ffffe200007fffff] PMD ->ffff810001a00000 on node 0
>  [ffffe20000800000-ffffe200009fffff] PMD ->ffff810001c00000 on node 0
> ...
> and usemap will share in page because of they are allocated continuously too.
> sparse_early_usemap_alloc: usemap = ffff810024e00000 size = 24
> sparse_early_usemap_alloc: usemap = ffff810024e00080 size = 24
> sparse_early_usemap_alloc: usemap = ffff810024e00100 size = 24
> sparse_early_usemap_alloc: usemap = ffff810024e00180 size = 24
> ...
> 
> so we make the bootmem allocation more compact and use less memory for usemap.
> 
> Signed-off-by: Yinghai Lu <yhlu.kernel@gmail.com>

very nice fix!

i suspect this patch should go via -mm.

>  	usemap = alloc_bootmem_node(NODE_DATA(nid), usemap_size());
> +	printk(KERN_INFO "sparse_early_usemap_alloc: usemap = %p size = %ld\n", usemap, usemap_size());

this should be in a separate patch.

	Ingo