Re: large page patch (fwd) (fwd)

[Linus Torvalds]

On Sat, 3 Aug 2002, David Mosberger wrote:
>
> Your point about wanting databases have access to giant pages even
> under memory pressure is a good one.  I had not considered that
> before.  However, what we really are talking about then is a security
> or resource policy as to who gets to allocate from a reserved and
> pinned pool of giant physical pages.

Absolutely. We can't allow just anybody to allocate giant pages, since
they are a scarce resource (set up at boot time in both Ingo's and Intels
patches - with the potential to move things around later with additional
interfaces).

>			  You don't need separate system
> calls for that: with a transparent superpage framework and a
> privileged & reserved giant-page pool, it's trivial to set up things
> such that your favorite data base will always be able to get the giant
> pages (and hence the giant TLB mappings) it wants.  The only thing you
> lose in the transparent case is control over _which_ pages need to use
> the pinned giant pages.  I can certainly imagine cases where this
> would be an issue, but I kind of doubt it would be an issue for
> databases.

That's _probably_ true. There aren't that many allocations that ask for
megabytes of consecutive memory that wouldn't want to do it. However,
there might certainly be non-critical maintenance programs (with the same
privileges as the database program proper) that _do_ do large allocations,
and that we don't want to give large pages to.

Guessing is always bad, especially since the application certainly does
know what it wants.

		Linus