Deadlocks: Deadlocks Chapter 3 3.1. Resource
3.2. Introduction to deadlocks
3.3. The ostrich algorithm
3.4. Deadlock detection and recovery
3.5. Deadlock avoidance
3.6. Deadlock prevention
3.7. Other issues
Agenda: Agenda 3.1. Resource
3.2. Introduction to deadlocks
3.3. The ostrich algorithm
3.4. Deadlock detection and recovery
3.5. Deadlock avoidance
3.6. Deadlock prevention
3.7. Other issues
Resources: Resources Examples of computer resources
printers
tape drives
tables
Processes need access to resources in reasonable order
Suppose a process holds resource A and requests resource B
at same time another process holds B and requests A
both are blocked and remain so
Resources (1): Resources (1) Deadlocks occur when …
processes are granted exclusive access to devices
we refer to these devices generally as resources
Preemptable resources
can be taken away from a process with no ill effects
Nonpreemptable resources
will cause the process to fail if taken away
Resources (2): Resources (2) Sequence of events required to use a resource
request the resource
use the resource
release the resource
Must wait if request is denied
requesting process may be blocked
may fail with error code
Introduction to Deadlocks: Introduction to Deadlocks Formal definition : A set of processes is deadlocked if each process in the set is waiting for an event that only another process in the set can cause
Usually the event is release of a currently held resource
None of the processes can …
run
release resources
be awakened
Four Conditions for Deadlock: Four Conditions for Deadlock Mutual exclusion condition
each resource assigned to 1 process or is available
Hold and wait condition
process holding resources can request additional
No preemption condition
previously granted resources cannot forcibly taken away
Circular wait condition
must be a circular chain of 2 or more processes
each is waiting for resource held by next member of the chain
Agenda: Agenda 3.1. Resource
3.2. Introduction to deadlocks
3.3. The ostrich algorithm
3.4. Deadlock detection and recovery
3.5. Deadlock avoidance
3.6. Deadlock prevention
3.7. Other issues
Deadlock Modeling (2): Deadlock Modeling (2) Modeled with directed graphs
resource R assigned to process A
process B is requesting/waiting for resource S
process C and D are in deadlock over resources T and U
Deadlock Modeling (3): Deadlock Modeling (3) Strategies for dealing with Deadlocks
just ignore the problem altogether
detection and recovery
dynamic avoidance
careful resource allocation
prevention
negating one of the four necessary conditions
Deadlock Modeling (4): How deadlock occurs A B C Deadlock Modeling (4)
Deadlock Modeling (5): Deadlock Modeling (5) How deadlock can be avoided (o) (p) (q)
Agenda: Agenda 3.1. Resource
3.2. Introduction to deadlocks
3.3. The ostrich algorithm
3.4. Deadlock detection and recovery
3.5. Deadlock avoidance
3.6. Deadlock prevention
3.7. Other issues
The Ostrich Algorithm: The Ostrich Algorithm Pretend there is no problem
Reasonable if
deadlocks occur very rarely
cost of prevention is high
UNIX and Windows takes this approach
It is a trade off between
convenience
correctness
Agenda: Agenda 3.1. Resource
3.2. Introduction to deadlocks
3.3. The ostrich algorithm
3.4. Deadlock detection and recovery
3.5. Deadlock avoidance
3.6. Deadlock prevention
3.7. Other issues
Detection with One Resource of Each Type: Detection with One Resource of Each Type Note the resource ownership and requests
A cycle can be found within the graph, denoting deadlock
Detection with Multiple Resource of Each Type (1): Detection with Multiple Resource of Each Type (1) Data structures needed by deadlock detection algorithm
Detection with Muliple Resource of Each Type (2): Detection with Muliple Resource of Each Type (2) An example for the deadlock detection algorithm
Recovery from Deadlock (1): Recovery from Deadlock (1) Recovery through preemption
take a resource from some other process
depends on nature of the resource
Recovery through rollback
checkpoint a process periodically
use this saved state
restart the process if it is found deadlocked
Recovery from Deadlock (2): Recovery from Deadlock (2) Recovery through killing processes
crudest but simplest way to break a deadlock
kill one of the processes in the deadlock cycle
the other processes get its resources
choose process that can be rerun from the beginning
Agenda: Agenda 3.1. Resource
3.2. Introduction to deadlocks
3.3. The ostrich algorithm
3.4. Deadlock detection and recovery
3.5. Deadlock avoidance
3.6. Deadlock prevention
3.7. Other issues
Deadlock AvoidanceResource Trajectories: Deadlock Avoidance Resource Trajectories Two process resource trajectories
Safe and Unsafe States (1): Safe and Unsafe States (1) Demonstration that the state in (a) is safe (a) (b) (c) (d) (e)
Safe and Unsafe States (2): Safe and Unsafe States (2) Demonstration that the sate in b is not safe (a) (b) (c) (d)
The Banker's Algorithm for a Single Resource: The Banker's Algorithm for a Single Resource Three resource allocation states
safe
safe
unsafe (a) (b) (c)
Banker's Algorithm for Multiple Resources: Banker's Algorithm for Multiple Resources Example of banker's algorithm with multiple resources
Agenda: Agenda 3.1. Resource
3.2. Introduction to deadlocks
3.3. The ostrich algorithm
3.4. Deadlock detection and recovery
3.5. Deadlock avoidance
3.6. Deadlock prevention
3.7. Other issues
Deadlock PreventionAttacking the Mutual Exclusion Condition: Deadlock Prevention Attacking the Mutual Exclusion Condition Some devices (such as printer) can be spooled
only the printer daemon uses printer resource
thus deadlock for printer eliminated
Not all devices can be spooled
Principle:
avoid assigning resource when not absolutely necessary
as few processes as possible actually claim the resource
Attacking the Hold and Wait Condition: Attacking the Hold and Wait Condition Require processes to request resources before starting
a process never has to wait for what it needs
Problems
may not know required resources at start of run
also ties up resources other processes could be using
Variation:
process must give up all resources
then request all immediately needed
Attacking the No Preemption Condition: Attacking the No Preemption Condition This is not a viable option
Consider a process given the printer
halfway through its job
now forcibly take away printer
!!??
Attacking the Circular Wait Condition (1): Attacking the Circular Wait Condition (1) Normally ordered resources
A resource graph (a) (b)
Attacking Deadlock Condition: Attacking Deadlock Condition Summary of approaches to deadlock prevention
Agenda: Agenda 3.1. Resource
3.2. Introduction to deadlocks
3.3. The ostrich algorithm
3.4. Deadlock detection and recovery
3.5. Deadlock avoidance
3.6. Deadlock prevention
3.7. Other issues
Other IssuesTwo-Phase Locking: Other Issues Two-Phase Locking Phase One
process tries to lock all records it needs, one at a time
if needed record found locked, start over
(no real work done in phase one)
If phase one succeeds, it starts second phase,
performing updates
releasing locks
Note similarity to requesting all resources at once
Algorithm works where programmer can arrange
program can be stopped, restarted
Nonresource Deadlocks: Nonresource Deadlocks Possible for two processes to deadlock
each is waiting for the other to do some task
Can happen with semaphores
each process required to do a down() on two semaphores (mutex and another)
if done in wrong order, deadlock results
Starvation: Starvation Algorithm to allocate a resource
may be to give to shortest job first
Works great for multiple short jobs in a system
May cause long job to be postponed indefinitely
even though not blocked
Solution:
First-come, first-serve policy