From: Rob Landley <rob@landley.net>
Organization: Boundaries Unlimited
To: user-mode-linux-devel@lists.sourceforge.net
Subject: Re: [uml-devel] Re: [Lhms-devel] [PATCH 0/7] Fragmentation Avoidance V19
Date: Fri, 4 Nov 2005 09:50:58 -0600
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Cc: Blaisorblade <blaisorblade@yahoo.it>, Jeff Dike <jdike@addtoit.com>,
       Nick Piggin <nickpiggin@yahoo.com.au>,
       Yasunori Goto <y-goto@jp.fujitsu.com>,
       Dave Hansen <haveblue@us.ibm.com>, Ingo Molnar <mingo@elte.hu>,
       Mel Gorman <mel@csn.ul.ie>, "Martin J. Bligh" <mbligh@mbligh.org>,
       Andrew Morton <akpm@osdl.org>, kravetz@us.ibm.com,
       linux-mm <linux-mm@kvack.org>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
       lhms <lhms-devel@lists.sourceforge.net>
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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
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