About the author Edsger Wybe Dijkstra (1930-2002) A pioneer in the - - PowerPoint PPT Presentation
The Structure of the THE-Multiprogramming System Edsger W . Dijkstra Technological University, Eindhoven, The Netherlands Communications of the ACM, 11(5):341--346, 1968 Presented by: Amin Almassian CS510 - Concepts of Operating
Edsger W . Dijkstra Technological University, Eindhoven, The Netherlands Communications of the ACM, 11(5):341--346, 1968
Presented by: Amin Almassian CS510 - Concepts of Operating Systems, Fall 2013 Portland State University
Edsger Wybe Dijkstra (1930-2002)
A pioneer in the area of distributed computing.
His foundational work:
Concurrency primitives (such as the semaphore),
Concurrency problems (such as mutual exclusion and deadlock),
Reasoning about concurrent systems, and self-stabilization
Winner of ACM's A.M. Turing Award in 1972
The Edsger W . Dijkstra Prize in Distributed Computing is named for him
Graphs “shortest path” algorithm: shortest route between two cities in the
complexities.”
“In 1955 when I decided not to become a physicist, to become a programmer instead. At the time programming didn't look like doing science, it was just a mixture of being ingenious and being accurate.” [1]
Not intended as a multi-access system To process smoothly a continuous flow of user programs Making economic use of peripheral devices Reduction of turn-around time for programs of short duration Automatic control of backing store to be combined with economic
use of the central processor
Feasibility to use the machine for multiple apps economically
Core memory: 2.5µsec, 27 bits; 32K Storage: drum of 512K words, 1024 words per track, 40 msec An indirect addressing mechanism well suited for stack
implementation
A sound system for commanding peripherals and controlling of
interrupts
low capacity channels
3 paper tape readers at 1000char/see; 3 paper tape punches at 150char/sec; 2 teleprinters (a plotter, a line printer)
Memory units (pages): core pages and drum pages
Information units: segments (a segment fits in a page)
Segment variable: Segment variable value tells if the segment is empty or not
Segment identifier gives fast access to a segment variable
If the segment is not empty, the value denotes which page(s) the segment can be found
Consequences:
A core page can be dumped onto a free drum page (the one with the minimum latency time) to free up the core page
Drum page occupation does not have to be consecutive.
A society of sequential processes (the concept of process abstraction)
logical meaning for a process:
The time succession of various states,
Not the actual speed of execution
Mutual synchronization
Allows for cooperation between sequential processes
Processor switches from process to process,
Temporal delaying the progress of the processes (blocking)
L5: Operator
Is in charge of allocation of the processor for processes Takes advantage of real-time clock interrupts to regain the control of the
processor
Provides an abstraction:
The number of processors actually shared is no longer relevant.
The actual processor that had lost its identity having disappeared from the picture. Priority rule included Level 0: Process Allocation
Is in charge of memory storage and allocation
Consists of a sequential process synchronized with the drum interrupt and sequential processes of higher levels
Provides a level of abstraction:
Higher levels identify information in terms of segments
the actual storage pages that had lost their identity having disappeared from the picture.
At higher levels, the actual storage pages have lost their identity Level 1: Segment Controller
A mediator between the operator and any of the higher level
processes
When a key is pressed, the character along with an interrupt is sent to
the system
Sends output commands to printer Provides an abstraction level:
Processes share the same physical console
Above this level, each process thinks it has its own private console (the advantage of using mutual synchronization) Level 2: Message Interpreter
Is in charge of buffering of input streams and Un-buffering of output
streams by using sequential processes
The sequential processes associated with the peripherals are of a level
above the message interpreter, because they must be able to converse with the operator (e.g. in the case of detected malfunctioning).
Provides an Abstract level:
Abstracts the actual peripherals as “logical communication units” to higher levels Level 3: Buffering I/O
Level 4 : The independent user programs Level 5: the operator (not implemented by the author’s
team).
Level 4: User Program Level 5: Operator
Semaphores Initialized with the value of 0 or 1
P-operation decreases value of a semaphore by 1
If sem ≥ 0 process can continue
If sem< 0 process is stopped and is put on waiting list
V-operation increases value of a semaphore by 1
If sem >0 no effect
If sem ≤ 0 a process on the waiting list is removed and continues progressing once allocated to a processor
If there is more than one process on the waiting list it is undefined which process is removed
Semaphore [2]
begin semaphore mutex; mutex := 1; parbegin begin L1: P(mutex); critical section 1; V(mutex); remainder of cycle 1; go to L1 end; begin L2: P(mutex); critical section 2; V(mutex); remainder of cycle 2; go to L2 end parend End
Mutex: {-(n-1), …,-1, 0, 1}
Allows for straightforward extension to more than two parallel processes
Analogous to condition synchronization mechanisms Each sequential process has associated with a number of
private semaphores (range between -1 and 1)
Whenever a process reaches a stage where the permission for dynamic progress depends on current values of state variables, it follows the pattern:
P(mutex) ; "inspection and modification of state variables including a conditional V(private semaphore)"; //if wants to continue // V(private-semaphore) //If (private semaphore > 0) V (mutex); P(private semaphore)
Whenever a process reaches a stage where as a result of its progress possibly one (or more) blocked processes should now get permission to continue, it follows the pattern:
P (mutex) ; "modification and inspection of state variables including zero or more V-
V(mutex).
Unstable Situation
1. a process generate a finite number of tasks for other processes.
processes can only generate tasks for processes at lower levels of
the hierarchy so that circularity is excluded.
2. It is impossible that all processes have returned to their homing
position while there is still pending tasks. (This is proved via instability of the situation)
3. After the acceptance of an initial task all processes eventually
will be (again) in their homing position. (proof by induction on the level of hierarchy, starting at the lowest level)
Old concepts yet state-of-the-art (1968-2013)
Segments == Segments
Core page and drum pages == Paged virtual memory abstraction
Sequential Process abstraction == Process/Thread
Semaphores == Semaphores
hierarchical implementation == hierarchical implementation
Hierarchical structure has been a great advantage to verification and testing
Abstraction at each layer
Proof of logical soundness at each level of abstraction
Small components results in small test cases => Easier to test
[1] Thomas J. Misa and Philip L. Frana. 2010. An interview with Edsger W . Dijkstra. Commun. ACM 53, 8 (August 2010), 41-47. DOI=10.1145/1787234.1787249 http://doi.acm.org/ 10.1145/1787234.1787249
[2] Animations for Operating Systems, Sixth Edition by William Stallings, available at http:// williamstallings.com/OS/Animation/Animations.html