Re: [RFC] Remove unswappable anonymous pages off the LRU

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 15 Feb 2007 19:50:45 -0800 (PST) Christoph Lameter <clameter@sgi.com> wrote:

> On Thu, 15 Feb 2007, Andrew Morton wrote:
> 
> > > Maybe we could somehow splite up page->flags into 4 separate bytes?
> > > Updating one byte would not endanger the other bytes in the other 
> > > sets?
> > 
> > yipes.  I'm not sure that'd work?
> 
> Are all arches able to do atomic ops on bytes?

I think they are, but you only wanted three bits.  I don't think we'll be
able to convert eight bits into a 256-value scalar efficiently.

> > compare-and-swap-in-a-loop could be used, I guess.  With the obvious problem..
> 
> Yucks. There seems to be no easy solution.
>  
> > I do think that those two swsusp flags are low-hanging-fruit.  It'd be
> > trivial to vmalloc a bitmap or use a radix-tree-holding-longs, but I have a
> > vague feeling that there were subtle issues with that.  Still, Something
> > Needs To Be Done.
> 
> I tinkered with some similar radical ideas lately. Maybe a bit vector
> could be used instead? For 1G of memory we would need 
> 
> 2^(30 - PAGE_SHIFT / 8 = 2^(30-12-3) = 2^15 = 32k bytes of a bitmap.
> 
> Seems to be reasonable?
> 

32k per bit per gig, yes.  Better for large PAGE_SIZE.  More cachemisses.

But will it come unstuck for machines which have a super-sparse pfn space?

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