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Date:	Thu, 13 Jul 2006 18:42:14 +0200
From:	Corinna Vinschen <corinna-cygwin AT cygwin DOT com>
To:	cygwin AT cygwin DOT com
Subject:	Re: 1.5.21s mmap error
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On Jul 13 10:07, Brian Ford wrote:
> On Thu, 13 Jul 2006, Corinna Vinschen wrote:
> > Shared anonymous maps seem to be possible (and would probably make sense
> > to minimze the swap space footprint), but are somewhat tricky since it's
> > not quite clear what happens to memory which is commited in one process
> > and then accessed in another.  This would require some extensive testing
> > which I'm not willing to do yet.
> 
> Huh, what does a shared anonymous (/dev/zero) mapping even mean?  You
> can't modify /dev/zero, so the only thing that makes sense to me is that
> changes made would be visible by the whole process tree containing the
> mapping?  I'm not aware of any OS that supports this.

Huh?  Say "shared memory" multiple times...  think again... and?

> > File-backed mappings are always ignoring the MAP_NORESERVE flag and are
> > using committed memory, since uncommitted file maps are not supported on
> > Windows.
> 
> That's too bad :-(.  Does the above mean that a private file-backed
> mapping consumes the full mapping size of swap space just in case all the
> pages are dirtied?  Obviously, the file is the backing store until the
> page is written, so there is no extra swap usage then.

Isn't that what !MAP_NORESERVE usually means?  To re-quote from your
Solaris quote:

  Without this flag, the creation of a writable MAP_PRIVATE mapping
  reserves swap  space  equal  to the  size  of the mapping.

It might be possible to get MAP_NORESERVE working for files on NT.
I see how it could work, but further mmap enhacements are pretty
low on my overlong TODO list.

> BTW, If you haven't already, you might consider transparently changing
> /dev/zero mappings to anonymous ones so MAP_NORESERVE is possible there.

Did you try it lately, say, in the last 5 1/2 years?


Corinna

-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Project Co-Leader          cygwin AT cygwin DOT com
Red Hat

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