Re: Populating multiple ptes at fault time

[Chris Snook]

Martin Bligh wrote:
> > Thanks, that was exactly what I was hoping to see.  I didn't see any
> > definitive statements against the patch set, other than a concern that
> > it could make things worse.  Was the upshot that no consensus was
> > reached about how to detect when its beneficial to preallocate anonymous
> > pages?
> >
> > Martin, in that thread you mentioned that you had tried pre-populating
> > file-backed mappings as well, but "Mmmm ... we tried doing this before
> > for filebacked pages by sniffing the
> > pagecache, but it crippled forky workloads (like kernel compile) with the
> > extra cost in zap_pte_range, etc. ".
> >
> > Could you describe, or have a pointer to, what you tried and how it
> > turned out?
>
> Don't have the patches still, but it was fairly simple - just faulted in
> the next 3 pages whenever we took a fault, if the pages were already
> in pagecache. I would have thought that was pretty lightweight and
> non-invasive, but turns out it slowed things down.
>
> > Did you end up populating so many (unused) ptes that
> > zap_pte_range needed to do lots more work?
>
> Yup, basically you're assuming good locality of reference, but it turns
> out that (as davej would say) "userspace sucks".

Well, *most* userspace sucks.  It might still be worthwhile to do this when 
userspace is using madvise().

-- Chris
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/