linux-kernel - Re: [PATCH 5/5] hugetlb: add support for gigantic page allocation at runtime

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Date:	Thu, 17 Apr 2014 16:00:39 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Luiz Capitulino <lcapitulino@...hat.com>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	mtosatti@...hat.com, aarcange@...hat.com, mgorman@...e.de,
	andi@...stfloor.org, davidlohr@...com, rientjes@...gle.com,
	isimatu.yasuaki@...fujitsu.com, yinghai@...nel.org,
	riel@...hat.com, n-horiguchi@...jp.nec.com, kirill@...temov.name
Subject: Re: [PATCH 5/5] hugetlb: add support for gigantic page allocation
 at runtime

On Thu, 10 Apr 2014 13:58:45 -0400 Luiz Capitulino <lcapitulino@...hat.com> wrote:

> HugeTLB is limited to allocating hugepages whose size are less than
> MAX_ORDER order. This is so because HugeTLB allocates hugepages via
> the buddy allocator. Gigantic pages (that is, pages whose size is
> greater than MAX_ORDER order) have to be allocated at boottime.
> 
> However, boottime allocation has at least two serious problems. First,
> it doesn't support NUMA and second, gigantic pages allocated at
> boottime can't be freed.
> 
> This commit solves both issues by adding support for allocating gigantic
> pages during runtime. It works just like regular sized hugepages,
> meaning that the interface in sysfs is the same, it supports NUMA,
> and gigantic pages can be freed.
> 
> For example, on x86_64 gigantic pages are 1GB big. To allocate two 1G
> gigantic pages on node 1, one can do:
> 
>  # echo 2 > \
>    /sys/devices/system/node/node1/hugepages/hugepages-1048576kB/nr_hugepages
> 
> And to free them all:
> 
>  # echo 0 > \
>    /sys/devices/system/node/node1/hugepages/hugepages-1048576kB/nr_hugepages
> 
> The one problem with gigantic page allocation at runtime is that it
> can't be serviced by the buddy allocator. To overcome that problem, this
> commit scans all zones from a node looking for a large enough contiguous
> region. When one is found, it's allocated by using CMA, that is, we call
> alloc_contig_range() to do the actual allocation. For example, on x86_64
> we scan all zones looking for a 1GB contiguous region. When one is found,
> it's allocated by alloc_contig_range().
> 
> One expected issue with that approach is that such gigantic contiguous
> regions tend to vanish as runtime goes by. The best way to avoid this for
> now is to make gigantic page allocations very early during system boot, say
> from a init script. Other possible optimization include using compaction,
> which is supported by CMA but is not explicitly used by this commit.

Why aren't we using compaction?

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