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From: timk@cs.qmw.ac.uk (Tim Kindberg)
Newsgroups: comp.os.mach
Subject: message passing & migration
Keywords: mach message passing reconfiguration migration
Message-ID: <2991@redstar.cs.qmw.ac.uk>
Date: 15 Mar 91 14:05:27 GMT
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I have some questions about Mach facilties, and would be very grateful to hear
from anyone with some answers:

1  Message passing.  I know about the use of copy-on-write for intra-machine
message passing, but I never could find any mention of using
copy-on-reference for large messages sent over a network. There's an obvious
problem of dependency upon some other machine for fetching pages after the
message has been received.
a. Is it the case that this feature hasn't been
implemented - for this and perhaps other reasons - and are there any plans to
include it?  I note that the Chorus developers seem to have decided against
including a similar feature.

b. Given that copy-on-reference isn't used, how/when is buffering done for
large messages sent asynchronously across the network?

c.  Could somebody clarify when I can/should/must use out-of-line data as
opposed to in-line data?  Out-of-line data is useful for scatter/gather, but
does it have to be used for "large" amounts of data, and is out-of-line
data always sent copy-on-write (intramachine), and therefore must be
page-aligned?  What, in practice, do programmers (mostly) do?

d.  Can (part of) a region created in receiving a message be sent in a further
message?

2  Sending port rights. Is there anyone out there sending port receive rights
in messages?  I would very much like to hear about the application context in
which you chose to do this, and how you found using this mechanism.

3  Migration.  The Accent work on process migration doesn't seem to have been
carried over to Mach.  Are there any plans in this direction?  (Accent
migration also used copy-on-reference, with a residual dependency problem
similar to that referred to in 1 above).
--

Tim Kindberg

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