Re: [RFC PATCH 0/2] avoid external fragmentation related to migration fallback

[Xishi Qiu]

On 2016/2/1 21:53, Mel Gorman wrote:

> On Fri, Jan 29, 2016 at 10:03:52PM +0100, Vlastimil Babka wrote:
>>> Since the root cause is that fallbacks might frequently split order-2
>>> and order-3 pages of the other migration types. This patch tweaks
>>> fallback mechanism to avoid splitting order-2 and order-3 pages. while
>>> fallbacks happen, if the largest feasible pages are less than or queal to
>>> COSTLY_ORDER, i.e. 3, then try to select the smallest feasible pages. The
>>> reason why fallbacks prefer the largest feasiable pages is to increase
>>> fallback efficiency since fallbacks are likely to happen again. By
>>> stealing the largest feasible pages, it could reduce the oppourtunities
>>> of antoher fallback. Besides, it could make consecutive allocations more
>>> approximate to each other and make system less fragment. However, if the
>>> largest feasible pages are less than or equal to order-3, fallbacks might
>>> split it and make the upcoming order-3 page allocations fail.
>>
>> In theory I don't see immediately why preferring smaller pages for
>> fallback should be a clear win. If it's Unmovable allocations stealing
>> from Movable pageblocks, the allocations will spread over larger areas
>> instead of being grouped together. Maybe, for Movable allocations
>> stealing from Unmovable allocations, preferring smallest might make
>> sense and be safe, as any extra fragmentation is fixable bycompaction.
> 
> I strongly agree that spreading the fallback allocations over a larger
> area is likely to have a negative impact. Given the age of the kernel
> being tested, it would make sense to either rebase or at the very last
> backport the patches that affect watermark calculations and the
> treatment of high-order pages.
> 

Is it the feature of MIGRATE_HIGHATOMIC?

Thanks,
Xishi Qiu