linux-kernel - Re: [PATCH] mm: make mem_map allocation continuous.

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Date:	Wed, 12 Mar 2008 09:40:36 -0700
From:	"Yinghai Lu" <yhlu.kernel@...il.com>
To:	"Mel Gorman" <mel@....ul.ie>
Cc:	"Ingo Molnar" <mingo@...e.hu>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	"Christoph Lameter" <clameter@....com>,
	"kernel list" <linux-kernel@...r.kernel.org>,
	"Andy Whitcroft" <apw@...dowen.org>
Subject: Re: [PATCH] mm: make mem_map allocation continuous.

On Wed, Mar 12, 2008 at 4:39 AM, Mel Gorman <mel@....ul.ie> wrote:
> On (11/03/08 09:14), Ingo Molnar didst pronounce:
>
>
> >
>  > * Yinghai Lu <yhlu.kernel@...il.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@...il.com>
>  >
>  > very nice fix!
>  >
>
>  Agreed, good work.
>
>
>  > 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.
>  >
>
>  Should this be KERN_DEBUG instead of KERN_INFO?
yes. to KERN_DEBUG or removed.
>
>  I don't have the original mail because I got unsubscribed from the lists
>  a few days ago and didn't notice (have been having mail issues) so
>  pardon awkward cut & pastes
>
>
>  > +/* section_map pointer array is 64k */
>  > +static __initdata struct page *section_map[NR_MEM_SECTIONS];
>
>  The size of this varies depending on architecture so the comment may be
>  misleading.  Maybe a comment like the following would be better?
>
>  /*
>   * The portions of the mem_map used by SPARSEMEM are allocated in
>   * batch and temporarily stored in this array. When sparse_init()
>   * completes, the array is discarded
>   */
Yes. for x86_64 is (1<<13)*8 at most. others should much less.
>
>  I can see why you file-scoped it because its too large for the stack but
>  would it be better to allocate it from bootmem instead? It is
>  available by the time you need to use the array.
Yes.
need to after another patch I sent out yesterday to make
free_bootmem_core could handle out of range inputs.

then could use alloc_bootmem and
for_each_online_node(node)
             free_bootmem_node(node, section_map, size);

will send delta to Andrew.

YH

YH
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