Re: [PATCH 4/5] Add a freeing threshold for the garbage collector.

[Andi Kleen <andi at firstfloor dot org>]

On Mon, Oct 10, 2011 at 12:20:53PM +0200, Richard Guenther wrote:
> On Sun, Oct 9, 2011 at 9:55 PM, Andi Kleen <andi@firstfloor.org> wrote:
> > From: Andi Kleen <ak@linux.intel.com>
> >
> > Add a threshold to avoid freeing pages back too early to the OS.
> > This avoid virtual memory map fragmentation.
> >
> > Based on a idea from Honza
> 
> Less than 20% looks high.  Shouldn't ggc-free-min be enough to
> avoid fragmentation?

It depends on the working set. If there's more to garbage collect
than max(ggc-free-min, threshold*total) a host without MADV_DONTNEED
will get holes. And ggc-free-min isn't very much on a large 
build.

So it seems safer to me to have a threshold which adjusts for large
working sets. What value do you prefer instead of 20%? (or just 0)

> 
> This will hide gc bugs with always-collect (ggc-checking), so
> the parameter(s) need to be adjusted for that case to always
> give pages back.  The current values should probably be printed
> where the two existing ones are printed as well (with -v).

Will fix.
-Andi