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From: Vlastimil Babka <>
To: Mike Kravetz <>,,,
Cc: Reinette Chatre <>,
	Michal Hocko <>,
	Christopher Lameter <>,
	Guy Shattah <>,
	Anshuman Khandual <>,
	Michal Nazarewicz <>,
	David Nellans <>,
	Laura Abbott <>, Pavel Machek <>,
	Dave Hansen <>,
	Andrew Morton <>
Subject: Re: [PATCH v2 0/4] Interface for higher order contiguous allocations
Date: Mon, 21 May 2018 14:00:04 +0200	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On 05/04/2018 01:29 AM, Mike Kravetz wrote:
> Vlastimil and Michal brought up the issue of allocation alignment.  The
> routine will currently align to 'nr_pages' (which is the requested size
> argument).  It does this by examining and trying to allocate the first
> nr_pages aligned/nr_pages sized range.  If this fails, it moves on to the
> next nr_pages aligned/nr_pages sized range until success or all potential
> ranges are exhausted.

As I've noted in my patch 3/4 review, in fact nr_pages is first rounded
up to an order, which makes this simpler, but suboptimal. I think we
could perhaps assume that nr_pages that's a power of two should be
aligned as such, and other values of nr_pages need no alignment? This
should fit existing users, and can be extended to explicit alignment
when such user appears?

> If we allow an alignment to be specified, we will
> need to potentially check all alignment aligned/nr_pages sized ranges.
> In the worst case where alignment = PAGE_SIZE, this could result in huge
> increase in the number of ranges to check.
> To help cut down on the number of ranges to check, we could identify the
> first page that causes a range allocation failure and start the next
> range at the next aligned boundary.  I tried this, and we still end up
> with a huge number of ranges and wasted CPU cycles.

I think the wasted cycle issues is due to the current code structure,
which is based on the CMA use-case, which assumes that the allocations
will succeed, because the areas are reserved and may contain only
movable allocations

      - performs only very basic pfn validity and belongs-to-zone checks
       for (pfn per pageblock) - the main cycle
           has_unmovable_pages() - cancel if yes
           move_freepages_block() - expensive!
etc (not important)

So I think the problem is that in the main cycle we might do a number of
expensive move_freepages_block() operations, then hit a block where
has_unmovable_pages() is true, cancel and do more expensive
undo_isolate_page_range() operations.

If we instead first scanned the range with has_unmovable_pages() and
only start doing the expensive work when we find a large enough (aligned
or not depending on caller) range, it should be much faster and there
should be no algorithmic difference between aligned and non-aligned case.


  parent reply	other threads:[~2018-05-21 12:00 UTC|newest]

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-05-03 23:29 Mike Kravetz
2018-05-03 23:29 ` [PATCH v2 1/4] mm: change type of free_contig_range(nr_pages) to unsigned long Mike Kravetz
2018-05-18  9:12   ` Vlastimil Babka
2018-05-18 22:01     ` Mike Kravetz
2018-05-03 23:29 ` [PATCH v2 2/4] mm: check for proper migrate type during isolation Mike Kravetz
2018-05-18 10:32   ` Vlastimil Babka
2018-05-21 23:10     ` Mike Kravetz
2018-05-22  7:07       ` Vlastimil Babka
2018-05-03 23:29 ` [PATCH v2 3/4] mm: add find_alloc_contig_pages() interface Mike Kravetz
2018-05-21  8:54   ` Vlastimil Babka
2018-05-21 23:48     ` Mike Kravetz
2018-05-22 16:41       ` Reinette Chatre
2018-05-22 20:35         ` Mike Kravetz
2018-05-23 11:18         ` Vlastimil Babka
2018-05-23 18:07           ` Reinette Chatre
2018-05-28 13:12             ` Vlastimil Babka
2018-05-03 23:29 ` [PATCH v2 4/4] mm/hugetlb: use find_alloc_contig_pages() to allocate gigantic pages Mike Kravetz
2018-05-21 12:00 ` Vlastimil Babka [this message]
2018-05-22  0:15   ` [PATCH v2 0/4] Interface for higher order contiguous allocations Mike Kravetz

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