DragonFly BSD
DragonFly kernel List (threaded) for 2003-07
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Re: just curious

From: David Leimbach <leimy2k@xxxxxxx>
Date: Mon, 21 Jul 2003 13:12:41 -0500


>:>     or mmapping() it.  mmap()ing it would result in the data being shared,
>:>     but with mmap() one can also make things copy-on-write (MAP_PRIVATE), and
>:>     so forth.  We already have the VM object layering model in place in the
>:>     kernel to make it all possible so it would not require any special effort.
>:So you'll be able to use the normal UNIX system calls to access 
>:arbitrary VM objects? So if I have a VM object that represents a buffer 
>:in another user process, it'll just look like a file to me? What happens 
>:if I overrun the end of the buffer, ENOSPC?
>:That's got some amazingly cool possibilities.
>    Yes.  Compare it against the Mach 'I'm going to map the data into your
>    address space' model.  There will be some cases where having the kernel
>    map the address space is more efficient (saving two system calls),
>    but most of the time there is no need or reason to map the data and in
>    those cases passing a descriptor is far more efficient.

I should note this is exactly the sort of thing I am looking for... I want 
access to the other end's data... I don't care if its read-only or not...

After all, a send can be seen as nothing more than a remote read permission
from the sender's side to a region of memory... at which point a receive is just
a copy of read only memory from the other process.

Even if I cannot remotely "POKE" into someone elses address space [like a Portals Put,
http://www.sandiaportals.org , or a MPI_Put]  I can still have send/receive semantics
but without all those damned copies laying around [in the kernel... a mmaped file etc.] :).

That would be cool in itself.

>    Not to mention the space abstraction... passing a descriptor allows you
>    to abstract I/O operations of any size.  You could handle ten someones
>    each trying to write a 2GB buffer to your userland VFS.  And people are
>    always forgetting atomicy.  There is no expectation of atomicy with
>    memory mappings but there is one for I/O operations like read() or write().

queuing == atomic ordering of ops?

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