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Case Study: Mach
David Ramsey
3-17-2003
CPSC550
Mach: Overview
Mach is a microkernel that provides the most elementary
services needed for an operating system. It is not a
complete operating system.
More advanced operating system functions are handled by
separate user-space server programs that run on top of and
communicate with Mach.
Mach's only functions are to keep the servers running and to
coordinate their access to raw hardware. At its most basic
level, it is an event handler, not an operating system as such.
Mach: History (1)
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1985: Beginning of Project Mach
A research project at the Carnegie Mellon University
School of Computer Science
1994: End of Project Mach at CMU (Mach 3.0)
All goals of the research project were accomplished by
then
Other organizations are continuing to work on Mach
today
1995-1996: Work on Project Mach at the University of
Utah (Flexmach, which became Mach 4)
Mach: History (2)
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During testing, BSD was ported to run on top of Mach so
that it could be tested as part of a running system
Its main purpose was just to get a running system
It consisted of the monolithic BSD kernel converted into
one user-space server program, instead of into one userspace server for each individual service
Due to this, Mach's performance on it was very poor
This was not the last attempt at porting BSD to Mach,
however
Mach: History (3)
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Some other projects based on Mach technology:
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GNU Hurd: Free Software Foundation (Mach 3.0 & 4)
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LITES: Helsinki University of Technology (Mach 3.0 &
4)
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Darwin: Apple (Mach 3.0)
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NeXTStep: NeXT (Mach 2.5)
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OSF/1 Unix: Open Software Foundation (Mach 2.5 & 3.0)
The last three of these successfully run Mach on top of BSD,
getting better performance by running the latter inside the
former (i.e, at kernel level instead of user level)
Mach: Goals
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Mach's goals are to cover all basic features commonly
found and/or desired in operating systems, and to be
compatible with BSD Unix
The basic features are:
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Direct communication between applications
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Memory protection
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Multiple processor support
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Multitasking
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Multithreading
Mach: Definitions
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Abstractions
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Messages: data sent from and received by tasks
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Ports: unicast, unidirectional message queues used by
tasks
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Port sets: sets of ports on which tasks wait until they
receive a response from any one port in the set
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Port rights: what tasks use in order to make use of
ports (send rights, send-once rights, and receive
rights)
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Threads: tasks contained inside other (multithreaded)
tasks
Mach: Features
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Portability, needed for Mach's elementary operating
system services to work on a wide variety of machines
with different capabilities
Transparent network operation, needed for the possibility
that the servers providing advanced services for it are
running on different machines (i.e, that the system is
distributed)
Distributed virtual memory, needed for Mach's method
of transferring out-of-line data between tasks
All features needed to implement Mach's stated goals
Mach: Structure (1)
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Abstractions
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The interprocess sommunication system primitives
operate only on the abstractions of messages, ports,
port sets, port rights, and tasks
Port sets
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Port sets are accessible only via the kernel-exported
interface
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Any given port can belong to only one port set
Send rights
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More than one task can have send rights to any given
Mach: Structure (2)
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Send-once rights
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Send-once rights only allow the tasks holding them to
send messages once, after which the rights are lost
Receive rights
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Only one task can hold receive rights for a given port
or port set
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A port receive right can be transferred by a message
from the task that holds it, but a port set receive right
cannot
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If no task has the receive right for a port, the
messages sent to the port are queued until one gets it
Mach: Structure (3)
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Tasks
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Any given task, when first created, starts with a
bootstrap port right, send rights to a kernel port, and
send and receive rights to an exception port
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A task uses its kernel port to invoke kernel services
(which include performing operations on itself,
creating new threads, and the like)
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On an exception associated with an exception port,
the kernel sends a message containing a description of
the exception to the port, the task tries to fix the
problem and indicates success or failure via a
Mach: Structure (4)
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Out-of-line data transfer
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Out-of-line data is transferred from one task to
another via messages
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The address of the data in the former's address space
is included in the message body
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When the latter receives the message, the data is put
in an unused region of its address space
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During the transmission, the former and latter share
the data pages, which are copied only upon their
being written to (copy-on-write); up to one address
space-worth of data can be transmitted at a time
Mach: How to Use It (1)
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One uses Mach mainly by sending messages to it and
receiving messages from it, subject to its constraints
Not all kernel interface calls will be described here, and
not in all detail, since there are over 100
Message call API:
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#include mach/port.h and mach/message.h
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mach_msg_return_t mach_msg
(mach_msg_header_t *msg, mach_msg_option_t
option, mach_msg_size_t send_size,
mach_msg_size_t rcv_size, mach_port_t
rcv_name, mach_msg_timeout_t timeout,
mach_port_t notify)
Mach: How to Use It (2)
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Mach message format:
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a fixed size message header (mach_msg_header_t)
followed by an optional number of data items
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each data item contains a type descriptor followed by
data (or a data address, in the case of out-of-line data
transfers)
Mach message data types:
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mach_msg_bits_t: unsigned int corresponding to the
flags associated with a message
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mach_msg_size_t: unsigned int corresponding to the
size of a message
Mach: How to Use It (3)
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Mach message data types (continued):
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mach_msg_id_t: integer_t used to indicate a function
or operation id number
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All messages start with the mach_msg_header_t
Data port types:
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mach_port_t: unsigned int corresponding to a port
name
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A port can be blocked from being used by marking it
as a 'dead name'
Etc.
Mach: Applications
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Operating system emulation
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Mach can intercept operating system calls and redirect
them to servers that can handle them, allowing
emulation of that operating system
Efficient sharing of hardware resources between
programs on one or more machines
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Mach's mathods of interfacing to servers at unknown,
possibly non-local, locations allow this
Mach: Significance
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It is a first-generation microkernel
It is designed for use in Unix-based or Unix-like
distributed operating systems
It can emulate systems by acting as a middleman, so to
speak, between the emulated layer and the real layer
It is good as a proof of concept (since second-generation
microkernels such as L4 perform better than it does and
are less complex)
Mach: Summary
Mach is a microkernel designed to be the core of a
distributed UNIX-like system. It communicates via sending
messages directly between different applications via ports,
and handles only the most basic operating system functions
on its own. More advanced functions are handled by
servers that it interfaces with. It transparently handles
multiple processors, networks, multitasking, multithreading,
and memory protection, and virtual memory. It can be used
to emulate other operating systems as well by intercepting
calls from the emulated system and sending them to its
servers to be handled. It is an interfacer and scheduler.
Sources (1)
The Evolution of Darwin
http://developer.apple.com/darwin/history.html
Lites Home Page
http://www.cs.hut.fi/~jvh/lites.html
The Mach 4 Project
http://www.cs.utah.edu/flux/mach4/html/Mach4-proj.html
The Mach Project Home Page
http://www-2.cs.cmu.edu/afs/cs/project/mach/public/www/mach.html
http://www-2.cs.cmu.edu/afs/cs/project/mach/public/www/status.html
Mach kernel - Wikipedia
http://www.wikipedia.org/wiki/Mach_kernel
Sources (2)
Coulouris, George, et al. Distributed Systems: Concepts and Design.
3rd ed. Essex: Pearson Education Limited, 2001. 700-706, 719-720
The GNU Mach Reference Manual: Inter Process Communication
http://www.gnu.org/software/hurd/gnumach-doc/mach_1.html
http://www.gnu.org/software/hurd/gnumach-doc/mach_4.html