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Date:      Wed, 8 Dec 1999 14:51:17 -0800 (PST)
From:      Alfred Perlstein <bright@wintelcom.net>
To:        Matthew Dillon <dillon@apollo.backplane.com>
Cc:        freebsd-hackers@FreeBSD.ORG
Subject:   Re: Getting a new MAP_ flag into mmap() prior to 4.x freeze
Message-ID:  <Pine.BSF.4.21.9912081447110.4557-100000@fw.wintelcom.net>
In-Reply-To: <199912081721.JAA37897@apollo.backplane.com>

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On Wed, 8 Dec 1999, Matthew Dillon wrote:

>     I would like to add a new MAP_ flag to mmap() prior to the 4.x freeze.
>     The purpose of the flag is to prevent the syncer from syncing the 
>     file underlying the map.  The VM system will still page to the file as
>     needed and, of course, the data will remain consistent with the file.
> 
>     This will allow normal files to be used to back (for example) shared 
>     memory without incuring the extra overhead that sync causes every 30 
>     seconds.  Currently it is not possible to use normal files in this
>     fashion (verses using MAP_ANON, for which sharability is limited, or
>     SysV shared memory which often has unexpected limitations) and still 
>     have an efficient system.
> 
>     The flag will be called MAP_NOSYNC
> 
>     Operationally, the syncer will not sync the file while any mmap()'s
>     exist with that flag set.  Once such flagged maps go away, the syncer 
>     will be able to sync the file (if it still exists).
> 
>     Comments are welcome.

I'd like to see this happen, go for it! :)

Don't forget how getnewbuf refils the buffers though, it will need to
somehow to communicate to the syncer to disregard MAP_NOSYNC during a
shortage... ? :)

-Alfred



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