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SubjectRe: [PATCH 0/8] Avoiding fragmentation with subzone groupings v25
On Thu, 7 Sep 2006, Andrew Morton wrote:

> On Thu, 7 Sep 2006 20:03:42 +0100 (IST)
> Mel Gorman <mel@csn.ul.ie> wrote:
>
>> When a page is allocated, the page-flags
>> are updated with a value indicating it's type of reclaimability so that it
>> is placed on the correct list on free.
>
> We're getting awful tight on page-flags.
>

Yeah, I know :(

> Would it be possible to avoid adding the flag? Say, have a per-zone bitmap
> of size (zone->present_pages/(1<<MAX_ORDER)) bits, then do a lookup in
> there to work out whether a particular page is within a MAX_ORDER clump of
> easy-reclaimable pages?
>

An early version of the patches created such a bitmap and it was heavily
resisted for two reasons. It put more pressure on the cache and it needed
to be resized during hot-add and hot-remove. It was the latter issue
people had more problems with. However, I can reimplement it if people
want to take a look. As I see it currently, there are five choices that
could be taken to avoid using an additional pageflag

1. Re-use existing page flags. This is what I currently do in a later
patch for the software suspend flags
pros: Straight-forward implementation, appears to use no additional flags
cons: When swsusp stops using the flags, anti-frag takes them right back
Makes anti-frag mutually exclusive with swsusp

2. Create a per-zone bitmap for every MAX_ORDER block
pros: Straight-forward implementation initially
cons: Needs resizing during hotadd which could get complicated
Bit more cache pressure

3. Use the low two bits of page->lru
pros: Uses existing struct page field
cons: It's a bit funky looking

4. Use the page->flags of the struct page backing the pages used
for the memmap.
pros: Similar to the bitmap idea except with less hotadd problems
cons: Bit more cache pressure

5. Add an additional field page->hintsflags used for non-critical flags.
There are patches out there like guest page hinting that want to
consume flags but not for any vital purpose and usually for machines
that have ample amounts of memory. For these features, add an
additional page->hintsflags
pros: Straight-forward to implement
cons: Increses struct page size for some kernel features.

I am leaning towards option 3 because it uses no additional memory but I'm
not sure how people feel about using pointer magic like this.

Any opinions?

--
Mel Gorman
Part-time Phd Student Linux Technology Center
University of Limerick IBM Dublin Software Lab
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