Re: -mm merge plans -- anti-fragmentation

[Dave McCracken]

On Tuesday 10 July 2007, Mel Gorman wrote:
> >  Mel's page allocator work.  Might merge this, but I'm still not hearing
> >  sufficiently convincing noises from a sufficient number of people over
> > this.
>
> This is a long on-going story. It bounces between people who say it's not a
> complete solution and everything should have the 100% ability to defragment
> and the people on the other side that say it goes a long way to solving
> their problem. I've cc'd some of the parties that have expressed any
> interest in the last year.

I find myself wondering what "sufficiently convincing noises" are.  I think we 
can all agree that in the current kernel order>0 allocations are a disaster.  
They simply aren't useable once the system fragments.  I think we can also 
all agree that 100% defragmentation is impossible without rewriting the 
kernel to avoid the hard-coded virtual->physical relationship we have now.

With that said, the only remaining question I see is whether we need order>0 
allocations.  If we do, then Mel's patches are clearly the right thing to do.  
They have received a lot of testing (if just by virtue of being in -mm for so 
long), and have shown to greatly increase the availability of order>0 pages.

The sheer list of patches lined up behind this set is strong evidence that 
there are useful features which depend on a working order>0.  When you add in 
the existing code that has to struggle with allocation failures or resort to 
special pools (ie hugetlbfs), I see a clear vote for the need for this patch.

Some object because order>0 will still be able to fail.  I point out that 
order==0 can also fail, though we go to great lengths to prevent it.  Mel's 
patches raise the success rate of order>0 to within a few percent of 
order==0.  All this means is callers will need to decide how to handle the 
infrequent failure.  This should be true no matter what the order.

I strongly vote for merging these patches.  Let's get them in mainline where 
they can do some good.

Dave McCracken
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