Re: [Lhms-devel] [PATCH 0/7] Fragmentation Avoidance V19

[Linus Torvalds <torvalds@osdl.org>]

On Thu, 3 Nov 2005, Arjan van de Ven wrote:

> On Thu, 2005-11-03 at 07:36 -0800, Martin J. Bligh wrote:
> > >> Can we quit coming up with specialist hacks for hotplug, and try to solve
> > >> the generic problem please? hotplug is NOT the only issue here. Fragmentation
> > >> in general is.
> > >> 
> > > 
> > > Not really it isn't. There have been a few cases (e1000 being the main
> > > one, and is fixed upstream) where fragmentation in general is a problem.
> > > But mostly it is not.
> > 
> > Sigh. OK, tell me how you're going to fix kernel stacks > 4K please. 
> 
> with CONFIG_4KSTACKS :)

2-page allocations are _not_ a problem.

Especially not for fork()/clone(). If you don't even have 2-page 
contiguous areas, you are doing something _wrong_, or you're so low on 
memory that there's no point in forking any more. 

Don't confuse "fragmentation" with "perfectly spread out page 
allocations". 

Fragmentation means that it gets _exponentially_ more unlikely that you 
can allocate big contiguous areas. But contiguous areas of order 1 are 
very very likely indeed. It's only the _big_ areas that aren't going to 
happen.

This is why fragmentation avoidance has always been totally useless. It is
 - only useful for big areas
 - very hard for big areas

(Corollary: when it's easy and possible, it's not useful).

Don't do it. We've never done it, and we've been fine. Claiming that 
fork() is a reason to do fragmentation avoidance is invalid.

		Linus
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