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From: vijay@nixdorf2.osf.org (Vijay Kumar)
Newsgroups: comp.os.mach
Subject: MACH questions..
Message-ID: <19089@paperboy.OSF.ORG>
Date: 19 Feb 91 21:03:07 GMT
Sender: news@OSF.ORG
Reply-To: vijay@nixdorf2.osf.org (Vijay Kumar)
Lines: 68


Hi,

	I have some questions concerning MACH.

	1. I am having difficulty in understanding the MACH
	object-oriented construction of distributed systems.
	The no-senders detection mechanism lets servers detect
	receive ports for which there are no senders. Of what
	use is such a detection mechanism?

	2. In the article that I was reading, the authors also say
	that the mechanism significantly reduces the number of
	ports used by object-oriented servers, because they do
	not need to create a notify port for every object port.
	Clients holding send rights for the object port can not
	forge no-senders notifications, because the server can
	distinguish messages sent to send rights. 
	This totally confuses me. Can someone throw some light
	on this topic?

	3. I have a very sketchy idea about netmsgservers
	How would a client initiate communication with a remote
	server and reliabaly transfer data? Various steps are
	involved before a client start communicating with a
	server, like a lookup operation with a nameserver or
	the netmsgserver itself provides some sort of lookup operation.
	Then what kind of lookup can a client perform (object name,
	portname, server name, ......????) What are proxy ports?

	4. Servers can abort computations on behalf of a
	dead client by requesting dead-name notifications for the
	send-once right reply port. In what way is this dead name
	notification of any importance to the server because it
	will receive the notification only after the computation
	completes and also the next thing that a server does
	(before it receives the dead-name notification) will
	be to send a reply on the send-once right reply port and
	it will know about the client's death at that time. The
	dead-name notification is of no use to the server unless
	the kernel pre-empts the server and the server does some
	sort of a stack-rewind operation after it realizes that
	the client is dead.

	5. The new IPC mechanism implemented in Mach 3.0 uses
	copy-on-write optimization. Basically, when a client
	task sends a message to a server, the physical pages
	which contain the client's message are mapped into
	the server's unused virtual address space. That way,
	a copy operation is saved and the kernel changes the
	protection of the shared pages so the a protection violation
	fault gets kicked of if either one of them modifies data.
	But later I also read that a send operation has two
	phases. First, the kernel copies the message into
	an internal buffer, making out-of-line memory regions
	copy-on-write. Now why do we need to make the memory
	regions copy-on-write after it is being copied anyway?
	I also thought we are avoiding a copy operation, but
	actually we are not..

	6. Do send and receive ports have two different names?
	Is the port name some thing like a file descriptor?


	Thanks in advance.

							Vijay
							(vijay@osf.org)