Section 1
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Many-to-Many Multithreading Model
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Cards (71)
Section 1
(50 cards)
Many-to-Many Multithreading Model
Allows many user level threads to be mapped to many kernel threads. Allows the operating system to create a sufficient number of kernel threads to handle user threads
Describe the reason why the Operating System has Zombie processes instead of directly freeing them.
Zombie processes stay in memory to store their exit state information for their parent process to extract with wait in the future, when the parent is ready for it.
In a system with non-preemptive scheduling, the CPU scheduler is invoked when:
A process switches from the running state to the waiting state or the process terminates
fork() and threads
creates new process with only one thread
Many to One Multithreading Model
Many user-level threads mapped to single kernel thread One thread blocking causes all to block. Multiple threads may not run in parallel on muticore system because only one may be in kernel at a time
Atomic operation means
that it completes in its entirety without worrying about interruption by any other potentially conflict-causing process.
Arbitrary Speed
No assumption can be made about the relative speed of different processes (though all processes have a non-zero speed).
t/f In Symmetric Multiprocessing, all the other CPUs are coordinated by a Master CPU.
. False
A race condition exists when
the order of the threads change the final result. Solution: Make sure that at most one thread is in its critical section at a time. These critical sections must be mutually exclusive in time.
One to One Multithreading Model
Each user-level thread maps to one kernel thread. If one user-level thread blocks, the scheduler will just switch to another thread in the same process. Most used: Windows and Linux
Processes have
their own address space: • Text section (text segment) contains the executable code • Data section (data segment) contains the global variables • Stack contains temporary data (local variables, return addresses..) • A process may contain a heap, which contains memory that is dynamically allocated at run- time.
On a single-processor system, which of the following would appear in a correct solution to the critical-section problem for user-level processes?
A process that does not execute code related to the critical section should not prevent others from entering the critical section
User threads
supported above the kernel and are implemented by a thread library at the user level. Since the kernel is not involved, thread switching may be very fast and thread scheduler may be implemented in the user space very efficiently. The implementation of blocking system calls is highly problematic : all the threads in the process risk being blocked!
Describe the key steps that occur when an Interrupt is generated.
1. Interrupt is Issued 2. Finish the current instruction 3. Switch to Kernel Mode 4. Save the Process State 5. The Interrupt Service Routine (ISR) is executed 6. Restore the Process State 7. Switch to User Mode
critical section
code segment in which the shared data is accessed.
If a process P executes the wait operation on condition variable C within a monitor:
It is immediately suspended
what were batch systems
Operators batch together jobs to run as a group. • First (rudimentary) operating system • Main memory allocated to the kernel and a single user job at a time • Jobs with similar needs were often grouped. • No user interaction; programs ran until they finished. "Uniprogramming"
Which of the following causes a Process to leave the Running state?
Timer expires
Progress:
No process executing a code segment unrelated to a given critical section can block another process trying to enter the same critical section
What is a real-time system and what is the difference between hard and soft real-time?
A real-time system is one that has well-defined, rigid time constraints. Hard real-time has critical tasks that must be completed on time. Soft real-time makes not absolute time guarantees, but does it by best-effort.
Assume that in Linux, a process P creates a child process Q. Process Q loads a program that will require many hours to run. Immediately after the Q is created, P calls exit(0) and exits without ever calling wait. Which of the following best describes one of the results of this scenario?
Q will be adopted by init
Von Neumann Cycle of Instruction Execution
Fetches the next instruction from memory • Decodes it to determine its type and operands • Executes it
Task parallelism
Distributing threads across cores, each thread performing unique operation
Data parallelism
Distributes subsets of the same data across multiple cores, same operation on each core
Multiple threads share and dont share:
Share: The address space, Open files Don't Share: Registers, Stack
What does multiprogramming refer to?
Multiprogramming allows multiple processes/jobs to be in memory at a time, ready to run. This also requires a scheduler that can bring jobs in to memory and a scheduler that can switch between ready jobs
Kernel threads
supported directly by the OS. The kernel performs thread: Creation, Scheduling, Management All calls that might block a thread are implemented as system calls. When a thread blocks, the kernel may choose another thread from the same process, or a thread from a different process.
Two-level Multithreading Model
Similar to Many to Many, except that it allows a user thread to be directly bound to kernel thread
Which of these hardware concepts stores the address of the next instruction to be executed?
Program Counter
busy waiting
If the critical section is being used, waiting processes loop continuously at the entry point
Select the best description of what happens when the following code is executed on the parent and child processes. pid_t id = fork ();
id is set to the child's PID for the parent and 0 for the child.
Asymmetric Multiprocessing
"Boss/worker" structure • The kernel runs on a particular processor
t/f After handling an Interrupt, the OS will set the dual mode bit to 0 before returning to the process
false
Describe the function of a short-term scheduler
The short-term scheduler selects the next Process to run on the CPU from the Ready Processes.
Amdahl's Law
Identifies performance gains from adding additional cores to an application that has both serial and parallel components 1 / ( s + ( 1 - s) / n )
Which mode is switched to when an Interrupt is received?
Kernel Mode
Bounded Waiting:
No process should have to wait forever to enter a critical section.
What does the Process Control Block save
Context of process being switch out (ie General-purpose Registers, Stack Pointer, Program Status Word)
What does the Interrupt Service Routine (Interrupt Service Vector) store?
The address of each procedure to handle the interrupt.
If a process P executes the signal operation on semaphore S:
it will move another process blocked on S to the ready state
Which of the following is stored in the Process Control Block during a context switch?
Program Status Word
Dual Mode
User mode • Execution done on behalf of a user. Dual Mode bit is 1 Kernel mode Execution done on behalf of operating system. Dual Mode bit is 0
What can cause an interrupt?
I/O Device Instruction, System Call, Timer Triggers, Page Fault
Mutual Exclusion
No two processes may be simultaneously inside the same critical section
Shared Memory v Message Passing
Shared Memory is faster but is prone to race conditions
what is a time sharing system
multiprogramming, but also allowed User Interaction • Each user is provided with an on-line terminal. • Response time for each user should be short. • All active users must have a fair share of the CPU time
Symmetric Multiprocessing
All processors are peers • Kernel routines can execute on different CPUs, in parallel • Tightly-Coupled, running Independently
TestAndSet (TAS) instruction
tests and modifies the content of a memory word atomically. 1) Read a Variable from Memory in to a Register 2) Sets that Variable to a new Non-Zero Value (eg. 1) Both of these steps happen in hardware in a single operation.
cac
The new program replaces the entire process. • "All threads other than the calling thread are destroyed during an exec()"
Describe the function of a medium-term scheduler. Mention how it will improve system performance
The medium-term scheduler selects a process to remove from main memory, decreasing the degree of multiprogramming. The selected processes for removal are ones in a waiting state when more resources are required. It will free resources (memory) and allow the long-term scheduler to add more processes to run on the CPU, which will increase resource utilization.
Section 2
(21 cards)