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SubjectRe: [PATCH 0/8] Avoiding fragmentation with subzone groupings v25
On Fri, 8 Sep 2006, Peter Zijlstra wrote:

> On Fri, 2006-09-08 at 09:36 +0100, Mel Gorman wrote:
>> 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?
>
> If, as you stated in a previous mail, you'd like to have flags per
> MAX_ORDER block, you'd already have to suffer the extra cache pressure.
> In that case I vote for 4.
>

Originally, I wanted flags per MAX_ORDER block but I no longer have data
on whether this is a good idea or not. It could turn out that we steal
back and forth a lot when pageblock flags are used.

> Otherwise 3 sounds doable, we already hide PAGE_MAPPING_ANON in a
> pointer, so hiding flags is not new to struct page. It's just a question
> of how good the implementation will look, I hope you'll not have to
> visit all the list ops.
>

One way to find out for sure! I reckon I'll go off and implement options 3
and 4 as add-on patches that avoid the use of page->flags and see what
they look like. As you said, pointer magic in struct page is not new.

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