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


On Fri, 4 Nov 2005, Ingo Molnar wrote:
>
> just to make sure i didnt get it wrong, wouldnt we get most of the
> benefits Andy is seeking by having a: boot-time option which sets aside
> a "hugetlb zone", with an additional sysctl to grow (or shrink) the pool
> - with the growing happening on a best-effort basis, without guarantees?

Boot-time option to set the hugetlb zone, yes.

Grow-or-shrink, probably not. Not in practice after bootup on any machine
that is less than idle.

The zones have to be pretty big to make any sense. You don't just grow
them or shrink them - they'd be on the order of tens of megabytes to
gigabytes. In other words, sized big enough that you will _not_ be able to
create them on demand, except perhaps right after boot.

Growing these things later simply isn't reasonable. I can pretty much
guarantee that any kernel I maintain will never have dynamic kernel
pointers: when some memory has been allocated with kmalloc() (or
equivalent routines - pretty much _any_ kernel allocation), it stays put.
Which means that if there is a _single_ kernel alloc in such a zone, it
won't ever be then usable for hugetlb stuff.

And I don't want excessive complexity. We can have things like "turn off
kernel allocations from this zone", and then wait a day or two, and hope
that there aren't long-term allocs. It might even work occasionally. But
the fact is, a number of kernel allocations _are_ long-term (superblocks,
root dentries, "struct thread_struct" for long-running user daemons), and
it's simply not going to work well in practice unless you have set aside
the "no kernel alloc" zone pretty early on.

Linus
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