Messages in this thread |  | | Date | Fri, 8 Sep 2006 09:36:33 +0100 (IST) | From | Mel Gorman <> | Subject | Re: [PATCH 0/8] Avoiding fragmentation with subzone groupings v25 |
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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 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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