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

Rob Landley
2005-11-04T15:50:58Z

On Thursday 03 November 2005 21:26, Blaisorblade wrote:
> > I was hoping that since the file was deleted from disk and is already
> > getting _some_ special treatment (since it's a longstanding "poor man's
> > shared memory" hack), that madvise wouldn't flush the data to disk, but
> > would just zero it out. A bit optimistic on my part, I know. :)
>
> I read at some time that this optimization existed but was deemed obsolete
> and removed.
>
> Why obsolete? Because... we have tmpfs! And that's the point. With
> DONTNEED, we detach references from page tables, but the content is still
> pinned: it _is_ the "disk"! (And you have TMPDIR on tmpfs, right?)

If I had that kind of control over environment my build would always be
deployed in (including root access), I wouldn't need UML. :)

(P.S. The default for Ubuntu "Horny Hedgehog" is no. The only tmpfs mount
is /dev/shm, and /tmp is on / which is ext3. Yeah, I need to upgrade my
laptop...)

> I guess you refer to using frag. avoidance on the guest

Yes. Moot point since Linus doesn't want it.

> (if it matters for
> the host, let me know). When it will be present using it will be nice, but
> currently we'd do madvise() on a page-per-page basis, and we'd do it on
> non-consecutive pages (basically, free pages we either find or free or
> purpose).

Might be a performance issue if that gets introduced with per-page
granularity, and how do you avoid giving back pages we're about to re-use?
Oh well, bench it when it happens. (And in any case, it needs a tunable to
beat the page cache into submission or there's no free memory to give back.
If there's already such a tuneable, I haven't found it yet.)

Rob