Download Mach Memory

The Duality of Memory and Communication in the
Implementation of a Multiprocessor Operating
System
Young, M. / Tevanian, A. / Rashid, R. / Golub, D. / Eppinger, J. / Chew, J. / Bolosky, W. / Black, D. / Baron, R.
Carnegie-Mellon University, Pittsburgh
ACM 1987
Presented by Mehmet Belgin
October, 7th 2004
MACH Operating System
 MACH is a Microkernel object oriented OS
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A ‘light weight’ kernel.
Basic abstractions over the hardware.
Design goals are multiprocessor support, binary
compatibility with Berkeley UNIX, Network transparency,
flexible memory management and IPC, High performance,
simple programmer interface.
 History
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Project for DARPA by Carnegie-Mellon University (19851994)
MacOS X
Based on Accent (by CMU)
MACH  NeXT  NeXTSTEP  MacOS X (Darwin)
Some principles are used in Windows NT/XP
GNU Hurd on MACH Microkernel
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Focus of the Paper
 External memory management facility of MACH
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Four basic abstractions are inherited from Accent:
Task and Thread : To control program execution
Port and Message : For InterProcess Communication
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MACH introduces a fifth one: ‘Memory Object’
The relationship (duality) between memory and
communication:
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Flexibility and efficiency
Multiprocessor support
Performance
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Inter-Process Communication (IPC)
Port: A communication channel (send/receive). May have any number of
senders, but only one receiver.
Message: Collection of data objects to be used in thread communications.
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Access to a port is granted by receiving a message containing a port
capability.
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Tasks allocate/deallocate their ports to perform communication.
•
Very clean and flexible interface, small number of abstractions.
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Analogy to TCP/IP communication:
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Instead of hosts, there are MACH tasks.
Instead of firewall rules, there are port rights.
Both provide reliability in transferring massages.
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Virtual Memory Management
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Each process has its own virtual address space.
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Maintenance of virtual address space is implemented by
keeping a linearly ordered page table for each process in
kernel.
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Only restriction: Regions (contiguous areas of address spaces)
must be aligned on system page boundaries.
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Operations on a task can be performed by Virtual memory
operation calls.
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MACH supports sharing of memory among tasks of ancestry
through inheritance.
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External Memory Management
 Special-purpose memory managers, instead of
kernel-supplied systems.
 MACH kernel acts like a cache manager
Mapped
areas
Address space
of a process
Memory
Objects
External
Memory
Managers
Destination
Ref: Distributed Operating Systems: Concepts and Design, by Pradeep, K. Sinha
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…External Memory Management
The interface between data manager and MACH
kernel consists of three parts:
 Calls by an application to map a memory object
into its address space
 Calls by the kernel to the data manager (generally
as a respond to requests of data manager)
 Calls by the data manager on the MACH kernel to
control the use of its memory object.
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Example: Consistent Network Shared Memory
Client A
(4) Client A is resumed
(2) Client A calls
vm_allocate_with_pager
to map X into its address
space
MACH Kernel
A
(1) Client A calls server
to acquire X
(5) Client B calls
server to acquire X
Client B
(8) Client B is resumed
Shared
Memory
Server
(6) Client B calls
vm_allocate_with_pager to
map X into its address space
(3) Kernel A calls (7) Kernel B calls
pager_init (…) pager_init (…)
MACH Kernel
B
Initialization: Clients map object X into their address spaces
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Example: Consistent Network Shared Memory
Client A
Client B
(4) Client A is
(1) Client A read faults resumed
(2) Kernel A calls
pager_data_request(…)
MACH Kernel
A
(5) Client B read faults
(8) Client B is
resumed
(6) Kernel B calls
Shared pager_data_request(…)
Memory
Server
Server calls
(3)Server(7)
calls
pager_data_provided(…)
pager_data_provided(…)
MACH Kernel
B
Clients read from the same shared memory data page
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Example: Consistent Network Shared Memory
Client A
(1) Client A
write faults on
data being read
by B
Client B
(5) Client A is resumed
(2) Kernel A calls
pager_data_unlock(…)
MACH Kernel
A
Shared
Memory
Server
(4) Server calls
pager_data_lock(…)
(3) Server calls
pager_flush_request(…)
MACH Kernel
B
Client A writes a page being read on another host (copy_on_write)
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External Memory Management Implementation
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Address Maps:
A doubly linked list to keep information of mappings from a range of addresses
to a region of a memory object. Two levels: Top and Sharing.
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Virtual Memory Object Structures:
A structure to represent the information (ports, size, etc.) about memory
objects to conveniently release associated pages when it is destroyed.
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Resident Memory Structures:
Each resident page corresponds to a page of physical memory. Records the
memory object, its offset and permissions. A hashed table is kept for a fast
lookup.
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Pageout Queues:
For page replacements. Three kind of pageout queues are linked trough the
resident page structures: active, inactive, free.
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Fault Handling: (!)
Validity and protection, page lookup (cache), copy-on-write, hardware
validation.
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Problems of External Memory Management
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Problems may occur if memory manager:
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does not return data
fails to free flushed data
floods the cache
changes data
backs its own data
Most of these are can be
prevented by various
mechanisms like using some
virtual memory calls, multiple
threads, backing up data to
safe memory.
Memory Failures: (Analogous to communication failures)
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Specifying a timeout period: May require recovery (memory backed by
‘Default Pager’).
Default Pager: Similar interface to other data managers, but a trusted
component.
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Benefits & Applications
 Multiprocessor
 UMA, NUMA and NORMA systems can be supported.
 Mach primitives enables a programmer to choose
from either shared memory and message passing
paradigms
 Emulating Operating Systems
 Generic Operating system calls can be implemented.
 Theoretically, two operating system emulations can
run on MACH kernel at the same time.
 Successful implementations for task migration,
database management (Camelot) and AI knowledge
bases (Agora) exists.
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Analysis of the Paper
 Good
 A significant contribution to the literature
 Two expressive examples
 Detailed information of implementation
 Bad
 Big amount of information, more than needed
(especially in multiprocessor issues section)
 No test results for performance evaluation
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Questions and Comments
THANK YOU!
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