Deadlocks and Memory Management
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Questions and Answers

What is the main objective of deadlock avoidance algorithms?

  • To ensure high resource utilization at all times.
  • To prevent resource allocation leading to an unsafe state. (correct)
  • To eliminate the need for resource sharing among processes.
  • To allow processes to hold multiple resources simultaneously.

Which condition must be eliminated to prevent deadlock?

  • Resource sharing
  • Process termination
  • Circular wait (correct)
  • Memory allocation

What is the role of the memory manager in an operating system?

  • To store data permanently on the disk.
  • To prevent processes from using more memory than needed.
  • To compile programs into executable code.
  • To allocate and deallocate memory resources. (correct)

Dynamic loading improves program performance by:

<p>Loading only the required routines into memory when called. (B)</p> Signup and view all the answers

How does paging work in memory management?

<p>By retrieving data from secondary storage in fixed-size blocks called pages. (C)</p> Signup and view all the answers

What does the term 'swapping' refer to in memory management?

<p>The temporary exchange of data between RAM and a secondary storage. (B)</p> Signup and view all the answers

Which condition is defined by a process holding at least one resource while waiting for others?

<p>Hold and wait (C)</p> Signup and view all the answers

What is the consequence of deadlock prevention strategies on resource utilization?

<p>They can lead to higher idle resources due to delayed allocations. (A)</p> Signup and view all the answers

What is the primary condition for deadlock that involves one or more resources being non-sharable?

<p>Mutual Exclusion (A)</p> Signup and view all the answers

Which deadlock condition describes a situation where a process is holding a resource while waiting for another?

<p>Hold and Wait (A)</p> Signup and view all the answers

In a resource-allocation graph, what do directed arcs from a process to a resource signify?

<p>The process is waiting for the resource (A)</p> Signup and view all the answers

What does the Least Recently Used (LRU) algorithm primarily focus on when selecting a page for replacement?

<p>The page with the longest time since it was last used (D)</p> Signup and view all the answers

What indicates that deadlock cannot occur in a resource-allocation graph?

<p>No Cycles present in the graph (C)</p> Signup and view all the answers

What are the two status bits typically used in the Not Recently Used Page Replacement Algorithm?

<p>R and M (C)</p> Signup and view all the answers

What occurs if a cycle exists in a resource-allocation graph and all resource types in that cycle have only one instance?

<p>Deadlock is certain (A)</p> Signup and view all the answers

How can deadlock detection be performed when there is a single instance of each resource?

<p>By running a cycle detection algorithm on the resource allocation graph (B)</p> Signup and view all the answers

In the Second-Chance Page Replacement Algorithm, what happens if all pages are referenced when trying to select a page for replacement?

<p>The algorithm reverts to FIFO (A)</p> Signup and view all the answers

How does the Clock Page Replacement Algorithm operate in terms of page replacement?

<p>It uses a circular list and checks pages in sequence (C)</p> Signup and view all the answers

Which condition of deadlock implies that a resource cannot be forcibly taken from a process?

<p>No Pre-emption (C)</p> Signup and view all the answers

What conclusion can be drawn when multiple instances of resources are present in a cycle of a resource-allocation graph?

<p>Deadlock is possible, but not guaranteed (D)</p> Signup and view all the answers

What defines the working set of a process in the context of page replacement algorithms?

<p>The subset of pages actively used during execution (A)</p> Signup and view all the answers

What is the primary consequence of thrashing in paging systems?

<p>Frequent page faults and slower execution (D)</p> Signup and view all the answers

What is the key difference between local and global allocation in paging systems?

<p>Local allocation can lead to thrashing even if there is sufficient memory (C)</p> Signup and view all the answers

Which of the following is an inefficiency of the Second-Chance Page Replacement Algorithm?

<p>It may constantly move pages around unnecessarily (D)</p> Signup and view all the answers

What confirms the presence of a deadlock in resource management?

<p>When there is a circular wait among processes for resources (A)</p> Signup and view all the answers

Which approach to resolving a deadlock is considered more conservative?

<p>Terminate processes one by one until the deadlock is broken (B)</p> Signup and view all the answers

Which factor is NOT important when selecting a victim for resource preemption?

<p>The type of operating system (B)</p> Signup and view all the answers

What can cause a process to starve in a deadlock resolution context?

<p>Continuously being preempted without gaining resources (D)</p> Signup and view all the answers

What is the purpose of deadlock avoidance strategies?

<p>To anticipate deadlocks before they occur (C)</p> Signup and view all the answers

What is the safest approach when rolling back a preempted process?

<p>Abort the process and restart from the beginning (B)</p> Signup and view all the answers

Which of the following indicates that a system is in a safe state?

<p>Resources are allocated without leading to deadlock (A)</p> Signup and view all the answers

What is a potential consequence of continuously preempting a process?

<p>Reduction in process priority over time (D)</p> Signup and view all the answers

What is the primary purpose of swapping in a timesharing system?

<p>To manage active processes that exceed main memory capacity. (B)</p> Signup and view all the answers

Which of the following best describes monoprogramming without swapping or paging?

<p>Only one process is loaded at a time into memory. (A)</p> Signup and view all the answers

How does static memory partitioning differ from dynamic memory partitioning?

<p>Static partitioning maintains fixed sizes regardless of system needs. (A)</p> Signup and view all the answers

What does the Page Fault Frequency (PFF) algorithm indicate?

<p>It indicates when to increase or decrease page allocation. (B)</p> Signup and view all the answers

What is the function of virtual memory in operating systems?

<p>It allows processes to exceed the physical memory limits. (A)</p> Signup and view all the answers

What happens to a program's partition after it finishes execution in a fixed partitioning scheme?

<p>It becomes available for the next program in the queue. (C)</p> Signup and view all the answers

What is a primary indicator of thrashing in a computer system?

<p>The combined working sets exceeding memory capacity (A)</p> Signup and view all the answers

How does internal fragmentation occur in page allocation?

<p>When entire pages are allocated but unused space is left. (D)</p> Signup and view all the answers

When is dynamic partitioning most beneficial in memory management?

<p>When a system frequently runs processes of unpredictable sizes. (A)</p> Signup and view all the answers

What describes the CPU's function in a multiprogramming environment?

<p>It passes control rapidly among programs in memory. (A)</p> Signup and view all the answers

Which of the following is a benefit of using shared pages in large multiprogramming systems?

<p>It allows multiple users to efficiently run the same program. (D)</p> Signup and view all the answers

Which operating system is an example of using monoprogramming?

<p>MS-DOS (B)</p> Signup and view all the answers

What effect does page size have on memory wastage?

<p>Smaller page sizes generally reduce internal fragmentation. (D)</p> Signup and view all the answers

What is the role of cleaning policies in paging systems?

<p>To manage how pages are swapped out to disk. (A)</p> Signup and view all the answers

What is the advantage of mapped files in a virtual address space?

<p>They simplify file access as a memory array. (D)</p> Signup and view all the answers

Why might large libraries be shared among many processes?

<p>To avoid the overhead of static library binding. (C)</p> Signup and view all the answers

Flashcards

Deadlock

A state where multiple processes are stuck, each holding a resource needed by another process to complete.

Mutual Exclusion

One or more resources cannot be shared, meaning only one process can access them at any given time.

Hold and Wait

A process holds at least one resource while waiting for another resource to become available.

No Pre-emption

A resource cannot be taken away from a process unless the process releases it voluntarily.

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Circular Wait

A set of processes are waiting for each other in a circular fashion, where each process is blocked for a resource held by another process in the loop.

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Resource-Allocation Graph

A graphical representation used to visualize resource allocation and potential deadlocks.

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Deadlock Detection

Deadlock detection identifies processes and resources involved in a deadlock.

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Deadlock Recovery

The process of recovering from a deadlock by breaking the deadlock cycle and allowing processes to continue.

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Deadlock Avoidance

A method of preventing deadlocks by denying resource allocation if it could lead to an unsafe state. This approach may result in lower resource utilization due to delayed allocation.

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Deadlock Conditions

A set of conditions that must all be met for a deadlock to occur. These conditions are: Mutual Exclusion, Hold and Wait, No preemption, and Circular Wait.

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Deadlock Prevention

A method of preventing deadlocks by eliminating one or more of the four deadlock conditions.

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Memory Manager

The part of the operating system responsible for managing memory. It allocates and deallocates memory for processes, keeps track of memory usage, and facilitates swapping between main memory and disk.

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Process Termination (All)

Terminating all processes involved in a deadlock to immediately resolve the issue, but potentially wasting resources.

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Process Termination (Incremental)

Terminating processes one by one in a deadlock until the deadlock is broken. It's more conservative but requires checking for deadlock after each step.

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Resource Preemption

Taking resources away from processes to resolve a deadlock, but requires careful selection and handling pre-empted resources.

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Selecting a Victim (Resource Preemption)

Choosing which process(es) to take resources from during resource preemption. Factors include priority, resource type, and completion progress.

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Rollback (Resource Preemption)

Returning a pre-empted process to a safe state before resource allocation. Easier said than done! Aborting and restarting is often the safest approach.

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Starvation (Resource Preemption)

A situation where a process is repeatedly preempted and never gets its resources due to resource preemption strategies. To prevent this, increase priority with each preemption.

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Swapping

A technique where processes are temporarily moved to disk and brought back to main memory as needed, allowing more programs to run than available physical memory allows.

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Monoprogramming

A system where one process occupies the entire main memory at a time, effectively running one program after another.

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Multiprogramming

A system where multiple processes share the main memory, allowing for faster context switching between programs.

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Fixed Partitioning

A memory partitioning method where the main memory is divided into fixed-size sections, each dedicated to a single program.

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Virtual Memory

A memory management scheme that allows programs to use more memory than physically available by temporarily storing parts of programs on disk and bringing them back to main memory when needed.

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Dynamic Partitioning

A type of memory partitioning where the size and number of partitions are determined during program execution.

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Fixed Partitioning

Dividing the memory into fixed-size partitions, each capable of holding a single program.

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Contiguous Allocation

A technique where the operating system allocates contiguous memory blocks for each process.

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Least Recently Used (LRU)

A page replacement algorithm that selects the page that has not been used for the longest time in memory.

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Not Recently Used (NRU)

Two status bits, 'R' and 'M', associated with each page to track usage, allowing the operating system to monitor page activity.

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Second-Chance Page Replacement Algorithm

An algorithm that attempts to find an old page not referenced recently. If all pages are referenced, it becomes FIFO.

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Clock Page Replacement Algorithm

A circular list where each page frame represents a 'clock' tick. When a page fault occurs: If 'R' bit = 0, it's evicted; if 'R' bit = 1, it's cleared and the pointer moves to the next page.

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Working Set

The set of pages a process actively uses during its execution.

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Thrashing

A situation where a process is constantly requesting pages from disk due to insufficient memory for its working set, leading to very slow performance.

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Local Page Allocation

A strategy where each process manages its own page frames independently.

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Global Page Allocation

A strategy where a single pool of page frames is used for all processes, and the operating system allocates them as needed.

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What is the PFF algorithm?

A memory management algorithm that measures the frequency of page faults to determine whether to increase or decrease the memory allocation for a process. It helps identify if a process requires more memory or less memory without specifying which page to replace on a fault.

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What is thrashing?

A phenomenon that occurs when the combined memory requirements of processes exceed the available memory capacity, resulting in excessive page swapping between memory and disk, leading to slow performance.

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What is internal fragmentation?

The wasted space within a page due to the allocation of a fixed-size page when the actual data size is smaller than the page size.

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What is demand paging?

A memory allocation technique where a process can request the allocation of a memory page only when it needs it instead of allocating the entire memory at once.

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What is page sharing?

A technique where processes can share the same physical memory, allowing efficient use of memory resources, especially when multiple processes use the same code.

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What is mapped file?

A technique that allows processes to access file data directly in memory by mapping a file into the process's virtual address space, enabling efficient file access like accessing memory arrays.

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What is a cleaning policy?

A memory management mechanism that governs how pages are loaded and replaced in memory, ensuring sufficient available frames for page faults.

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Why is it important to have enough free page frames?

Keeping enough free page frames available in memory to handle page faults efficiently. This avoids the need to write pages to disk before loading new ones, ensuring smooth operation.

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Study Notes

Deadlocks and Memory Management

  • Deadlocks occur when a set of processes are blocked, each holding a resource and waiting to acquire a resource held by another process.
  • Processes in operating systems request, use, and release resources in a specific way.

Deadlock Conditions

  • Mutual Exclusion: Resources are non-sharable, only one process can use them at a time.
  • Hold and Wait: A process is holding at least one resource and waiting for more resources.
  • No Pre-emption: A resource cannot be taken away from a process unless it releases it.
  • Circular Wait: A set of processes are waiting for each other in a circular fashion.

Deadlock Modeling

  • Resource-Allocation Graph: A visual representation of resource allocation, which helps understand deadlocks.
    • Resource categories (square nodes) represent specific instances of a resource (e.g., printers).
    • Dots inside resource nodes indicate resource instances
    • Processes (circles) request and hold resources.
    • Request edges (directed arcs) show processes requesting resources.
    • Assignment edges (directed arcs) show resources allocated to processes.
  • Cycle Detection:
    • No cycles mean no deadlock.
    • Cycles exist if deadlock might occur (but not guaranteed), especially if multiple instances of resources in the cycle exist.

Deadlock Detection and Recovery

  • Single Instance Resources: Run an algorithm to check the resource allocation graph for cycles. Cycles indicate deadlock.
  • Process Termination:
    • Terminate all involved processes. This resolves the deadlock, but potentially terminates more than necessary.
    • Terminate processes one by one. More conservative approach, but requires deadlock detection after each step.

Process Termination Considerations

  • Process priority: Higher priority processes should be considered before lower priority processes.
  • Resources held: Processes with fewer resources tend to be better candidates.
  • Completion status: Processes closer to completion may be better candidates to terminate.
  • Interactive vs. batch: Interactive processes (user-based) should be terminated last, as they are vital for user interaction.

Resource Preemption

  • Victim Selection: Decide which processes to pre-empt resources form which processes.
  • Rollback: Ideally, the system reverts a preempted process to a safe state before resource reallocation.
  • Starvation: To avoid, a process's priority can be increased after each pre-emption.

Deadlock Avoidance

  • Resource Usage Information: Processes declare their maximum resource needs (usually in advance).
  • Safe State Determination: Determines if allocating a resource to a process will lead to an unsafe state that may cause deadlock.
  • Deadlock Avoidance Algorithms: Prevent allocation when a resource allocation may lead to an unsafe state. Low resource utilization is a trade-off.

Memory Management

  • Dynamic Loading/Linking: Necessary for efficient performance by loading program parts/modules only upon use and linking dependent programs as needed dynamically.

Memory Management Techniques

  • Paging and Swapping: Dividing memory into fixed-size blocks (pages) and storing them in a secondary storage until needed.
  • Swapping: The process of temporarily moving processes from main memory to secondary storage (disk) when sufficient memory isn't available, and bringing them back as needed.
  • Monoprogramming (without Swapping or Paging): Only one process runs in memory at a time.

Multiprogramming with Fixed Partitions

  • Static Partitions: Fixed-size memory partitions assigned to different programs.
  • Dynamic Partitions: Variable-size memory partitions assigned according to program needs, decided during runtime.

Virtual Memory

  • Enabling Access to Larger Memory: Computer systems can access more memory than physically installed.
  • Virtual Address Space: Virtual space is much larger than physical memory.
  • Paging: Dividing memory into fixed-size blocks for efficient swaps between physical and virtual memory.

Benefits of Virtual Memory

  • Increased Program Size: Allows large programs to fit into memory.
  • Increased Speed: Enables faster process swapping.
  • Enhanced CPU Utilization: More simultaneous processes, leading to improved CPU usage.
  • Reduced I/O: Reduces I/O operations, improving efficiency.

Demand Paging

  • Page Swapping: Pages are swapped into memory only when needed (on demand).
  • Virtual Address Translation: Mapping between the virtual address space and physical memory.
  • Page Table: The data structure used to translate between virtual and physical addresses.

Page Replacement Algorithms

  • FIFO (First In, First Out): Removes the oldest page. Belady's anomaly may lead to more page faults with increasing frames.
  • LRU (Least Recently Used): Removes the least recently used page.
  • NRU (Not Recently Used): Removes pages not referenced recently.
  • Second Chance: Combines FIFO and checking reference bit.

Other Paging Considerations

  • Page Size: Impacts internal fragmentation, optimal size depends on program requirements and storage efficiency.
  • Clock Algorithm: An improvement over FIFO, utilizing a circular list with a reference bit.
  • Working Set Algorithm: Selects for replacement based on active pages (working set) of a program.
  • Page Fault Frequency Algorithm: Adjusts page allocation per page fault.
  • Load Control: Ensures processes have sufficient memory based on usage to avoid thrashing.

Memory Protection

  • Used to prevent unauthorized access or modification of memory locations by different programs.
  • Page tables and associated mechanisms (e.g. virtual addresses) protect access of specific memory regions by individual processes.

Cleaning Policy

  • The paging daemon runs in the background and periodically releases frames by evicting pages with the page replacement algorithm.

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Deadlocks and Memory Management PDF

Description

This quiz covers the concepts of deadlocks in operating systems, including the conditions that create deadlocks, such as mutual exclusion and hold and wait. It also explores deadlock modeling using resource-allocation graphs to help visualize and understand the allocation of resources among processes.

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