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

Linus Torvalds
2005-11-03T16:52:38Z

On Thu, 3 Nov 2005, Mel Gorman wrote:

> On Thu, 3 Nov 2005, Linus Torvalds wrote:
>
> > 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).
> >
>
> Unless you are a user that wants a large area when it suddenly is useful.

No. It's _not_ suddenly useful.

It might be something you _want_, but that's a totally different issue.

My point is that regardless of what you _want_, defragmentation is
_useless_. It's useless simply because for big areas it is so expensive as
to be impractical.

Put another way: you may _want_ the moon to be made of cheese, but a moon
made out of cheese is _useless_ because it is impractical.

The only way to support big areas is to have special zones for them.

(Then, we may be able to use the special zones for small things too, but
under special rules, like "only used for anonymous mappings" where we
can just always remove them by paging them out. But it would still be a
special area meant for big pages, just temporarily "on loan").

Linus