[TUHS] SCCS, TeamWare, BitKeeper, and Git

[Larry McVoy]

On Sun, Dec 15, 2024 at 03:48:40PM -0500, Douglas McIlroy wrote:
> Unix originally imitated the Multics file system, but not the universal
> memory. mmap(2) weakly imitates universal memory by allowing a process
> to nominally incorporate a portion of a file into the process address space
> at page-level granularity. However, an update is guaranteed to be visible
> to the file and other processes only upon specific request.

That's not true with Sun's mmap().  It's coherent across process boundaries
and it's in sync with read()/write().  This is because Sun got rid of the
buffer cache and _only_ did file IO to/from the page cache.  You mapped
the actual page into your address space, if one process wrote it, the
other process will see it.

> Does anyone know whether there are implementations of mmap that
> do transparent file sharing? It seems to me that should be possible by
> making the buffer cache share pages with mmapping processes.

SunOS 4.0 did what I believe you are asking.

http://mcvoy.com/lm/papers/SunOS.vm_arch.pdf
http://mcvoy.com/lm/papers/SunOS.vm_impl.pdf
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Larry McVoy           Retired to fishing          http://www.mcvoy.com/lm/boat