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SubjectRe: [PATCH v2 0/4] Interface for higher order contiguous allocations
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On 05/21/2018 05:00 AM, Vlastimil Babka wrote:
> 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?

I'm good with that. I do believe that minimum alignment will be
pageblock size alignment (for > MAX_ORDER allocations).

>> 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
>
> find_alloc_contig_pages()
> __alloc_contig_pages_nodemask()
> contig_pfn_range_valid()
> - performs only very basic pfn validity and belongs-to-zone checks
> alloc_contig_range()
> start_isolate_page_range()
> for (pfn per pageblock) - the main cycle
> set_migratetype_isolate()
> has_unmovable_pages() - cancel if yes
> move_freepages_block() - expensive!
> __alloc_contig_migrate_range()
> 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.

Ok, I will give that a try.

Thanks again for looking at these.
--
Mike Kravetz

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