Subject: Re: [patch 3/9] Guest page hinting: volatile page cache.
From: Martin Schwidefsky <schwidefsky@de.ibm.com>
Reply-To: schwidefsky@de.ibm.com
To: Dave Hansen <haveblue@us.ibm.com>
Cc: Andy Whitcroft <apw@shadowen.org>, linux-kernel@vger.kernel.org,
       virtualization@lists.osdl.org, akpm@osdl.org, nickpiggin@yahoo.com.au,
       frankeh@watson.ibm.com
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Organization: IBM Corporation
Date: Fri, 01 Sep 2006 19:02:30 +0200
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On Fri, 2006-09-01 at 09:29 -0700, Dave Hansen wrote:
> > 3) The page-has-a-writable-mapping (PG_writable) bit is set when the
> > first writable pte for a page is established. The page needs to have a
> > different state if a writable pte exists compared to a read-only page.
> > The alternative without the page bit would be to do the state change
> > every time a writable pte is established or to search all ptes of a
> > given page. Both have performance implications.  
> 
> What are the performance implications?  Do they completely erase any
> performance gains that these patches might have given in the first
> place?  Has there been any evaluation of these other two alternatives?
> As I understand it, carrying out this performance analysis would be very
> difficult for most of the kernel community to perform.

It seemed obvious to me that anything else than checking a bit is way to
expensive. I never implemented nor measured any of the alternatives. The
alternative to do the state change every time a writable pte is
established can be implemented without too much trouble. Perhaps I will
give it a try next week.

> Keeping a nice count of the number of writable PTEs sounds like
> something that might be generally useful.  Could we split
> page->_mapcount to keep track of r/o and r/w ptes separately?  Or,
> perhaps a single bit in it can be utilized to replace PG_writable,
> instead.

Yes, that would be really useful for the writable ptes. But I have the
feeling that the actual implementation of it will be tricky.

-- 
blue skies,
  Martin.

Martin Schwidefsky
Linux for zSeries Development & Services
IBM Deutschland Entwicklung GmbH

"Reality continues to ruin my life." - Calvin.


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