Re: [RFC] mm: support multi_freearea to the reduction of external fragmentation

Vlastimil Babka
2021-04-16T11:06:52Z

On 4/14/21 4:38 AM, lipeifeng@oppo.com wrote:
> From: lipeifeng <lipeifeng@oppo.com>
>
> This patch would "sort" the free-pages in buddy by pages-PFN to concentrate
> low-order-pages allocation in the front area of memory and high-order-pages
> allcation on the contrary so that few memory-pollution in the back area of
> memory and the probablity of high-order-pages allocation would be increased
> significantly.
> -----------------------------------------------------------------------
>
> 1) Divide memory into several segments by pages-PFN
> "Multi_freearea" would divide memory into FREE_AREA_COUNTS segments
> by pages-PFN,each memory-segment corresponds to a free_area.
>
> Example: machine(4G of physical memery) and FREE_AREA_COUNTS(4):
> page-PFN:0x0 0x40000(1G) 0x80000(2G) 0xc0000(3G) 0xFFFFF(4G)
> |------------|--------------|--------------|-------------|
> free_area: [0][] [1][] [2][] [3][]
>
> NOTE: Selecting the corresponding freearea when pages are freed back
> to buddy:
> - pages-PFN[0, free_area_segment[0].max_pfn] -> free_area[0][]
> - pages-PFN[free_area_segment[flc - 1].max_pfn,
> free_area_segment[flc].max_pfn] -> free_area[flc][]
> (flc > 0)
>
> By this way, all pages in the same segment/free_area is within a
> certain range of pages-PFN.
>
> 2) Select the corresponding freearea to alloc-pages
> "Multi_freearea" would select the corresponding free_area by the
> allocation-order when alloc-pages.
> - order < HIGH_ORDER_TO_FLC:
> free_area[0] -> ... -> free_area[FREE_AREA_COUNTS - 1]
> - order >= HIGH_ORDER_TO_FLC:
> free_area[FREE_AREA_COUNTS - 1] -> ... -> free_area[0]
>
> Example:
> The machine(4G of physical memery) and FREE_AREA_COUNTS(4)
> and HIGH_ORDER_TO_FLC(3).
> If user allocs page(order = 0),it would take page from
> free_area[0][] first, if that fails,try free_area[1][] and so on.
> If user allocs page(order = 4),it would take page from
> free_area[3][] first, if that fails,try free_area[2][] and so on.
>
> By this way,low-order pages will be concentrated in the front area
> of memory.Because of few memory-pollution in the back area of memory,
> the sussessful probablity of high-order allocation would be improved.
>
> 3) Adjust the location of free-pages in the free_list
> "Multi_freearea" would place free-pages in the head of free_list if
> pages-PFN is smaller than free_area_segment[flc]->median_pfn and in
> the tail of free_list on the contrary.
>
> Example:
> page-PFN: free_area_segment[flc]->median_pfn
> |
> free_list: page->page->page->...|...page->page->page
> pages-PFN:| < median_pfn | >= median_pfn |
>
> Because it would take pages from the head of the freelist first in
> buddy system,the free-pages in the tail are more likely to keep in the
> buddy system.The closer the PFN of pages kept in buddy system, the
> greater the probablity of merging that into high-order pages.

I think this part 3) would be worth to be tried separately first, as it's not a
big change compared to the other ones.