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OS07: MX Challenges

Based on Chapter 4 of [Hai19]

(Usage hints for this presentation)

Computer Structures and Operating Systems 2021
Dr. Jens Lechtenbörger (License Information)

1 Introduction

1.1 OS Plan

OS course plan, summer 2021

1.2 Today’s Core Question

  • Am I fine if I lock all shared resources before use?

(Short answer: No. Issues such as priority inversion, deadlocks, starvation may arise.)

1.3 Learning Objectives

  • Explain priority inversion and counter measures
  • Explain and apply deadlock prevention and detection
  • Explain convoys and starvation as MX challenges

1.4 Previously on OS …

1.4.1 Threads, again

1.5 Different Learning Styles

  • In previous years, some students reported that Section 4.8.1 (pp. 135 – 137) of [Hai19] on Priority Inversion is quite easy to understand, while they perceived that section in this presentation to be confusing.
  • Note that [Hai19] discusses Priority Inversion resulting from locks/mutexes with blocking, while the slides also contain a variant with spinlocks.

Table of Contents

2 Priority Inversion

2.1 Priority Inversion with Spinlocks

  • Example; single CPU core (visualization on next slide)
    1. Thread T0 with low priority enters CS
    2. T0 preempted by OS for T1 with high priority
      • E.g., an important event occurs, to be handled by T1
      • Note that T0 is still inside CS, holds lock
    3. T1 tries to enter same CS and spins on lock held by T0
  • This is a variant of priority inversion
    • High-priority thread T1 cannot continue due to actions by low-priority thread
      • If just one CPU core exists: Low-priority thread T0 cannot continue
      • If multiple cores exist: Low-priority thread T0 runs although thread with higher priority does not make progress

2.1.1 Single Core

2.1.2 Two Cores

2.2 Priority Inversion with Blocking

  • (Visualization on next slide)
  • T0 with low, TM with medium, T1 with high priority
    1. T0 in CS
    2. An important event occurs, OS preempts T0 for T1
      • T1 attempts entry into same CS, T1 gets blocked
      • T0 could continue if no higher priority thread existed
    3. Another, less important, event occurs, to be handled by TM
      • Based on priority, OS favors TM over T0
      • TM runs instead of more highly prioritized T1Priority inversion
        • (TM does unrelated work, without need to enter the CS)
      • T0 cannot leave CS as long as TM exists
  • With long running or many threads of medium priority, T1 (and important event) need to wait for a long time

2.2.1 Blocking CS

2.3 Priority Inversion Example

  • Mars Pathfinder, 1997; see Wikipedia for details
    • Robotic spacecraft named Pathfinder

      Sojourner Rover

      Sojourner Rover” by NASA under Public domain; from Wikimedia Commons

      • With rover named Sojourner (shown to right)
    • A “low-cost” mission at $280 million
  • Bug (priority inversion) caused repeated resets
    • “found in preflight testing but was deemed a glitch and therefore given a low priority as it only occurred in certain unanticipated heavy-load conditions”
  • Priority inversion had been known for a long time

2.4 Priority Inversion Solutions

  • Priority Ceiling (PC)
    • Every resource has priority (new concept; so far only threads had priorities)
      • (Highest priority that “normal” threads can have) + 1
    • Accessing thread runs with that priority
  • In both cases: Restore old priority after access

2.5 JiTT and Exercise Task

  • This task is a variant of Exercise 4.10 of [Hai19]. Solve part (1) as self-study in Learnweb, part (2) as exercise task.

    • Suppose a computer with only one processor runs a program that creates three threads, which are assigned high, medium, and low fixed priorities. (Assume that no other threads are competing for the same processor.) The threads of high and medium priority are currently blocked, waiting for different events, while the thread with low priority is runnable. Some threads share a single mutex (to protect shared resources that are not shown). Pseudocode for each of the threads is shown subsequently.
      1. Suppose that the mutex does not provide priority inheritance. How soon would you expect the program to terminate? Why?
      2. Suppose that the mutex provides priority inheritance. How soon would you expect the program to terminate? Why?
    High-priority thread:
      “initially blocked; unblocked to handle event after 300 milliseconds”
      perform lock() on mutex
      run for 1 millisecond on CPU
      perform unlock() on mutex
      terminate execution of the whole program
    
    Medium-priority thread:
      “initially blocked; unblocked to handle event after 300 milliseconds”
      run for 800 milliseconds on CPU
    
    Low-priority thread:
      “initially runnable”
      perform lock() on mutex
      perform I/O operation which leads to blocking for 500 milliseconds
      run for 1 millisecond on CPU
      perform unlock() on mutex
    

3 Deadlocks

3.1 Deadlock

3.2 Deadlock Example

  • Money transfers between bank accounts
    • Transfer from myAccount to yourAccount by thread 1; transfer in other direction by thread 2
  • Race conditions on account balances
  • Need mutex per account
    • Lock both accounts involved in transfer. What order?
  • “Natural” lock order: First, lock source account; then, lock destination account
    • Thread 1 locks myAccount, while thread 2 locks yourAccount
      • Each thread gets blocked once it attempts to acquire the second lock
        • Neither can continue
      • Deadlock

3.3 Defining Conditions for Deadlocks

Deadlock if and only if (1) – (4) hold [CES71]:

  1. Mutual exclusion
    • Exclusive resource usage
  2. Hold and wait
    • Threads hold some resources while waiting for others
  3. No preemption
    • OS does not forcibly remove allocated resources
  4. Circular wait
    • Circular chain of threads such that each thread holds resources that are requested by next thread in chain

3.4 Resource Allocation Graphs

  • Representation and visualization of resource allocation as directed graph
    • (Necessary prior knowledge: directed graphs and cycles in such graphs)
    • Nodes
      • Threads (squares on next slide)
      • Resources (circles on next slide)
    • Edges
      • From thread T to resource R if T is waiting for R
      • From resource R to thread T if R is allocated to T
    • Example on next slide
  • Fact: System in deadlock if and only if graph contains cycle

3.5 Resource Allocation Graph Example

Figure 4.22 of cite:Hai17

Figure 4.22 of [Hai17]

Figure by Max Hailperin under CC BY-SA 3.0; converted from GitHub

(Note: Choice of shapes is arbitrary; just for visualization purposes)

4 Deadlock Strategies

4.1 Deadlock Strategies

  • (Ostrich algorithm)
  • Deadlock Prevention
  • Deadlock Avoidance
  • Deadlock Detection

These strategies are covered in subsequent slides.

4.2 Ostrich “Algorithm”

  • A joke about missing deadlock handling
    • “Implemented” in most systems
      • Pretend nothing special is happening
      • (E.g., Java VMs act like ostriches)
    • Reasoning
      • Proper deadlock handling is complex
      • Deadlocks are rare, result from buggy programs

(Refresh HTML presentation for other drawings)

4.3 Deadlock Prevention

  • Prevent a defining condition for deadlocks from becoming true
  • Practical options

    • Prevent condition (2), “hold and wait”: Request all necessary resources at once
      • Only possible in special cases, e.g., conservative/static 2PL in DBMS
      • Threads either have no incoming or no outgoing edges in resource allocation graph → Cycles cannot occur
    • Prevent condition (4), “circular wait”: Number resources, request resources according to linear resource ordering
      • Consider resources Rh and Rk with h < k
        • Threads that need both resources must lock Rh first
        • Threads that already requested Rk do not request Rh afterwards
      • Requests for resources in ascending order → Cycles cannot occur

4.3.1 Linear Resource Ordering Example

  • Money transfers between bank accounts revisited
  • Locks acquired in order of account numbers
    • A programming contract, not known by OS
    • Suppose myAccount has number 42, yourAccount is 4711
      • Both threads try to lock myAccount first (as 42 < 4711)
        • Only one succeeds, can also lock yourAccount
        • The other thread gets blocked
    • No deadlock
  • (See Fig 4.21 in [Hai19] for an example of linear ordering in the context of the Linux scheduler)

4.4 Deadlock Avoidance

  • Dynamic decision whether allocation may lead to deadlock
    • If a deadlock cannot be ruled out easily: Do not perform that allocation but block the requesting thread (or return error code or raise exception)
    • Consequently, deadlocks do never occur; they are avoided
  • Classical technique
    • Banker’s algorithm by Dijkstra
      • Deny incremental allocation if “unsafe” state would arise
      • Not used in practice
        • Resources and threads’ requirements need to be declared ahead of time

4.5 Deadlock Detection

  • Idea
    • Let deadlocks happen
    • Detect deadlocks, e.g., via cycle-check on resource allocation graph
      • Periodically or
      • After “unreasonably long” waiting time for lock or
      • Immediately when thread tries to acquire a locked mutex
    • Resolve deadlocks: typically, terminate some thread(s)
  • Prerequisite to build graph
    • Mutex records by which thread it is locked (if any)
    • OS records for what mutex a thread is waiting

5 Further Challenges

5.1 Convoy Problem

  • Suppose a central shared resource R exists
    • Frequently accessed by lots of threads, protected by mutex M
  • Preemption of thread T1 holding that mutex is likely
    • Other threads wind up in wait queue of mutex, the convoy
      • Thread switches without much progress
      • (See [BGM+79] for origin of “convoy” in context of database transactions)
  • Suppose T1 continues
    • T1 releases lock, which is reassigned to T2
    • During its time slice, T1 wants R again, but M is now held by T2
      • T1 gets blocked without much progress
  • The same happens to the other threads
    • The convoy persists for a long time

5.1.1 A Convoy Solution

  • Change mutex behavior
    • Proposed in [BGM+79]
    • Do no immediately reassign mutex upon unlock()
    • Instead, make all waiting threads runnable
      • Without reassigning mutex
    • (In addition, for performance reasons in case of failing locking attempt [BGM+79] suggests “to spin for a few instructions in the hope that the lock will become free”)
  • Effect: T1 can lock() M repeatedly during its time slice

5.2 Starvation

5.2.1 Dining Philosophers

  • MX problem proposed by Dijkstra
  • Philosophers sit in circle; eat and think repeatedly
    • Two forks required for eating
      • MX for forks

Figure 4.20 of cite:Hai17

Dining Philosophers

Figure 4.20 of [Hai17]” by Max Hailperin under CC BY-SA 3.0; converted from GitHub

5.2.2 Starving Philosophers

  • Starvation of P0

    Figure 4.20 of cite:Hai17

    Figure 4.20 of [Hai17]” by Max Hailperin under CC BY-SA 3.0; converted from GitHub

    • P1 and P3 or P2 and P4 eat in parallel
    • Then they wake the other pair
      • P1 wakes P2; P3 wakes P4
      • P2 wakes P1; P4 wakes P3
    • Iterate

5.3 JiTT: Questions and Feedback

  • What did you find difficult or confusing about the contents of the presentation? Please be as specific as possible. For example, you could describe your current understanding (which might allow us to identify misunderstandings), ask questions that allow us to help you, or suggest improvements (maybe on GitLab). Please use the session’s shared document or MoodleOverflow. Most questions turn out to be of general interest; please do not hesitate to ask and answer where others can benefit. If you created additional original content that might help others (e.g., a new exercise, an experiment, explanations concerning relationships with different courses, …), please share.

6 Conclusions

6.1 Summary

  • MX to avoid race conditions
  • Challenges
    • Priority inversion
    • Deadlocks
    • Convoys
    • Starvation

Bibliography

License Information

This document is part of an Open Educational Resource (OER) course on Operating Systems. Source code and source files are available on GitLab under free licenses.

Except where otherwise noted, the work “OS07: MX Challenges”, © 2017-2021 Jens Lechtenbörger, is published under the Creative Commons license CC BY-SA 4.0.

No warranties are given. The license may not give you all of the permissions necessary for your intended use.

In particular, trademark rights are not licensed under this license. Thus, rights concerning third party logos (e.g., on the title slide) and other (trade-) marks (e.g., “Creative Commons” itself) remain with their respective holders.