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

Ingo Molnar
2005-11-02T12:00:48Z

* Gerrit Huizenga <gh@us.ibm.com> wrote:

>
> On Wed, 02 Nov 2005 11:41:31 +0100, Ingo Molnar wrote:
> >
> > * Gerrit Huizenga <gh@us.ibm.com> wrote:
> >
> > > > generic unpluggable kernel RAM _will not work_.
> > >
> > > Actually, it will. Well, depending on terminology.
> >
> > 'generic unpluggable kernel RAM' means what it says: any RAM seen by the
> > kernel can be unplugged, always. (as long as the unplug request is
> > reasonable and there is enough free space to migrate in-use pages to).
>
> Okay, I understand your terminology. Yes, I can not point to any
> particular piece of memory and say "I want *that* one" and have that
> request succeed. However, I can say "find me 50 chunks of memory
> of your choosing" and have a very good chance of finding enough
> memory to satisfy my request.

but that's obviously not 'generic unpluggable kernel RAM'. It's very
special RAM: RAM that is free or easily freeable. I never argued that
such RAM is not returnable to the hypervisor.

> > reliable unmapping of "generic kernel RAM" is not possible even in a
> > virtualized environment. Think of the 'live pointers' problem i outlined
> > in an earlier mail in this thread today.
>
> Yeah - and that isn't what is being proposed here. The goal is to
> ask the kernel to identify some memory which can be legitimately
> freed and hasten the freeing of that memory.

but that's very easy to identify: check the free list or the clean
list(s). No defragmentation necessary. [unless the unit of RAM mapping
between hypervisor and guest is too coarse (i.e. not 4K pages).]

Ingo