05-Processos_e_threads

Prévia do material em texto

PROCESSES AND 
THREADS
Prof. Dr. Everton Cavalcante
https://www.docente.ufrn.br/everton.cavalcante
https://www.docente.ufrn.br/everton.cavalcante
GOALS
• To present the concepts of processes and threads from the 
perspective of concurrent programming
• To point out the differences between processes and threads
• To discuss the benefits and the problems 
from using threads
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THE CONCEPT OF PROGRAM
A program is a sequence of statements 
in source code or machine code
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PROCESS
It is a program in execution. A 
process is the unit of work in a 
modern computing system. 
Abraham Silberschatz, Peter Baer Galvin, Greg Gagne. 
Operating system concepts, 10th ed. United Kingdom: 
John Wiley & Sons, Inc., 2018.
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THE CONCEPT OF PROCESS (1/9)
A given program can be instantiated in one or more processes
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Program X
Process A
Process B
Process N
...
THE CONCEPT OF PROCESS (2/9)
Multiple chrome.exe processes in Task Manager
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https://superuser.com/questions/174652/multiple-chrome-exe-processes-in-task-manager
THE CONCEPT OF PROCESS (3/9)
Main events that cause the creation of a process
• System initialization
• Execution of system call to create processes by a running process (fork in 
Unix-based systems)
o Process hierarchy (parent process, child process)
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THE CONCEPT OF PROCESS (4/9)
•The execution of a process encompasses resource allocation 
(CPU time, memory, files, input/output devices, etc.) for its task 
to be executed
•Each process receives an identifier (PID) and an address space, 
being controlled by the operating system during its existence
•Concurrent processes compete for the available resources and 
are scheduled by the operating system
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THE CONCEPT OF PROCESS (5/9)
Loading a process into memory results in assigning 
it to an address space
• Every process has its own virtual address space, and the 
operating system maps it to the physical address space in 
memory
• The process’ address space comprises
o Code (text), the compiled program code
o Data with global variables allocated and initialized before execution
o Heap, used for dynamic memory allocation
o Stack with local variables
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THE CONCEPT OF PROCESS (6/9)
Events that cause a process to terminate
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Voluntary termination
• Normal termination when the 
process finishes its task
• Termination due to the 
occurrence of an error
Involuntary termination
• Fatal error, usually a 
programming one
• Cancellation by a request 
from other process to the 
operating system
THE CONCEPT OF PROCESS (7/9)
States of a process
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Andrew S. Tanembaum, Hebert Bos. Modern 
operating systems – 5th ed. USA: Pearson, 2023
THE CONCEPT OF PROCESS (8/9)
•Despite process representing an important building block in 
operating systems, operations involving processes (creation, 
termination, communications) are relatively costly
•Context switch performed by the operating system
o Storing the state (context) of the process
o Later restoration of the process’ context
o Transfer of control from one process to another
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THE CONCEPT OF PROCESS (9/9)
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Context switch between processes
THE CONCEPT OF THREAD (1/4)
A thread is an independent execution flow of a program within a 
process
• A thread is also known as a lightweight process
• Threads can be executed simultaneously, either concurrently or in parallel
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THE CONCEPT OF THREAD (2/4)
Traditional processes versus multithreaded processes
• A traditional process has at least a single thread that controls the 
resources held by the process
• When a process has more than one thread (i.e., it is multithreaded)
o more than one thread may be simultaneously active in the process
o threads share resources held by the process
o one of the threads starts running when the process is scheduled for execution
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THE CONCEPT OF THREAD (3/4)
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Traditional processes versus multithreaded processes
Process A Process B
single thread multithread
call(routine x)
detour
return
single execution flow
three concurrent 
execution flows
create(threads x and y)
Routine x
main thread
thread ythread x
THE CONCEPT OF THREAD (4/4)
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Traditional processes versus multithreaded processes
one process | one thread one process | multiple threads
multiple processes | 
one thread per process
multiple processes | 
multiple threads per process
PROCESSES VERSUS THREADS (1/3)
•Threads within a process are typically invisible from outside it
•A thread within a process is scheduled and runs independently 
from other threads
o Depending on the number of processors available in the system, 
different threads may run in different processors
o In single-processor computers, threads must have their execution 
interleaved by context switching
•Threads within a given process share the same address space 
and hence the same data
o Concurrent among threads may exist
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PROCESSES VERSUS THREADS (2/3)
•Communication among threads within a process takes place 
through shared memory
•Protected address space in processes
o Grouping resources as a process eases their management
•Threads receive an amount of CPU time (time slice or quantum) 
at which they perform their task
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PROCESSES VERSUS THREADS (3/3)
•All threads have access to 
globally shared data
•Threads also have their own 
private data
•The responsible for synchronizing 
access to shared data is the 
programmer
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THE LIFECYCLE OF THREADS (1/4)
A thread starts its lifecycle at the 
state and remains in that state 
until the program starts it, putting 
it at the state running to perform 
its task
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new
running
wait time wait terminated
program starts 
the thread
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THE LIFECYCLE OF THREADS (2/4)
A thread may go to a wait state 
while waiting for another thread 
to perform a task. Once at that 
state, the thread returns to the 
state running only when another 
thread signals the waiting thread 
to resume its execution.
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new
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program starts 
the thread
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THE LIFECYCLE OF THREADS (3/4)
A thread may go to a 
synchronized (time) wait for a 
given time interval. A thread in 
that state returns to the state 
running when the time interval 
expires or when the event is 
waiting to happen.
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new
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program starts 
the thread
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THE LIFECYCLE OF THREADS (4/4)
A running thread goes to the state 
terminated when it completes its 
task, otherwise it is finished.
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new
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program starts 
the thread
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THREAD PRIORITY AND SCHEDULING
•Each thread has a priority that supports the operating system in 
determining the order in which threads should be scheduled
• Informally, threads with higher priority are more important in a 
program and hence they must be assigned to the CPU time 
before threads with lower priority
o Thread priority may not guarantee the order in which threads are 
executed
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The need for using a process is clear 
since a process is a program in 
execution.
So why divide the execution flow of a 
process into threads?
WHY THREADS?
BENEFITS FROM USING THREADS (1/4)
Simplification of concurrent programming
• Reduction of the problem’s complexity: 
divide and conquer
• Representation of the tasks to be performed 
as distinct lines of execution control
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BENEFITS FROM USING THREADS (2/4)
Throughput (processing capacity)
• Gains in terms of performance since multithreaded systems can better 
exploit the machine’s processing capabilities
o Example: a thread waits for the response from an input/output request while another 
thread executes another task
• Threads allow performing operations in background
• When compared to the cost of creating and managing processes,a thread 
can be created with much less overhead at the operating system level
• Managing threads requires fewer system resources than managing 
processes due to resource sharing
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BENEFITS FROM USING THREADS (3/4)
Servers
• Server programs in client-server applications may receive multiple requests 
simultaneously
• If the server program has a single thread, then requests are handled 
sequentially
o In case of many requests, such a behavior makes clients to wait
• Multithreaded servers can handle requests faster
o Multiple requests are handled simultaneously
o Independent scheduling of each request
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BENEFITS FROM USING THREADS (4/4)
An example: loading videos on the Internet
• Users may not want to wait for loading all the video to start playing it
• It is possible to use multiple threads: a thread loads the video, 
another plays it
o Tasks executed concurrently
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BENEFITS FROM USING THREADS (?) 31
WHY THREADS MAY BE NOT GOOD (1/5)
Comparison between programming with multiple threads and 
events (single-threaded)
Keynote handouts
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Keynote handouts
WHY THREADS MAY BE NOT GOOD (2/5)
Problems with using threads
• Challenges to programming
o The programmer is responsible for ensuring mutual exclusion
o Possibility of deadlocks
• Challenges to debugging
o Challenges to reproducing execution conditions
o Non-determinism
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WHY THREADS MAY BE NOT GOOD (3/5)
Problems with using threads
• Challenges to synchronization
• Challenges to obtaining an actually good performance
o Explicit mutual exclusion mechanisms reduce the 
potential of concurrency
o Mutual exclusion mechanisms increase complexity and 
may reduce performance
o Not every program can benefit from using threads
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WHY THREADS MAY BE NOT GOOD (4/5)
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Md. Aminul Haque Chowdhury, S. M. Saif Shams, Ki-Hyung Kim, Seung-wha Yoo. An empirical study of I/O bounded processes and threads in operating 
system. Proceedings of the Third International Conference on Covergence and Hybrid Information Technology. USA: IEEE, 2008, pp. 712-717
WHY THREADS MAY BE NOT GOOD (5/5)
Problems with using threads
• Breaking the idea of abstraction and modularity
o It is not possible to design modules in an independent way
• Relatively low portability
o Different implementations
o Dependency on the programming language
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That is the question!
TO USE, OR NOT TO USE THREADS?
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Pushing the Limits of Windows: Processes and Threads (Microsoft Tech Community)
https://techcommunity.microsoft.com/t5/windows-blog-archive/pushing-the-limits-of-windows-processes-and-threads/ba-p/723824
PROCESSES AND 
THREADS
Prof. Dr. Everton Cavalcante
https://www.docente.ufrn.br/everton.cavalcante
https://www.docente.ufrn.br/everton.cavalcante
	Slide 1: PROCESSES AND THREADS
	Slide 2: GOALS
	Slide 3: The concept of program
	Slide 4: PROCESS
	Slide 5: The concept of process (1/9)
	Slide 6: The concept of process (2/9)
	Slide 7: The concept of process (3/9)
	Slide 8: The concept of process (4/9)
	Slide 9: The concept of process (5/9)
	Slide 10: The concept of process (6/9)
	Slide 11: The concept of process (7/9)
	Slide 12: THE CONCEPT of PROCESS (8/9)
	Slide 13: THE CONCEPT OF PROCESS (9/9)
	Slide 14: THE CONCEPT of THREAD (1/4)
	Slide 15: THE CONCEPT of THREAD (2/4)
	Slide 16: THE CONCEPT of THREAD (3/4)
	Slide 17: THE CONCEPT of THREAD (4/4)
	Slide 18: PROCESSES VERSUS THREADS (1/3)
	Slide 19: PROCESSES VERSUS THREADS (2/3)
	Slide 20: PROCESSES VERSUS THREADS (3/3)
	Slide 21: THE LIFECYCLE OF THREADS (1/4)
	Slide 22: THE LIFECYCLE OF THREADS (2/4)
	Slide 23: THE LIFECYCLE OF THREADS (3/4)
	Slide 24: THE LIFECYCLE OF THREADS (4/4)
	Slide 25: THREAD PRIORITY AND SCHEDULING
	Slide 26: WHY THREADS?
	Slide 27: BENEFITS FROM USING THREADS (1/4)
	Slide 28: BENEFITS FROM USING THREADS (2/4)
	Slide 29: BENEFITS FROM USING THREADS (3/4)
	Slide 30: BENEFITS FROM USING THREADS (4/4)
	Slide 31: BENEFITS FROM USING THREADS (?)
	Slide 32: Why Threads MAY BE NOT GOOD (1/5)
	Slide 33: Why Threads MAY BE NOT GOOD (2/5)
	Slide 34: Why Threads MAY BE NOT GOOD (3/5)
	Slide 35: Why Threads MAY BE NOT GOOD (4/5)
	Slide 36: Why Threads MAY BE NOT GOOD (5/5)
	Slide 37: To use, or not to use threads?
	Slide 38
	Slide 39: PROCESSES AND THREADS