Transcript CSC420 สัปดาห์ที่ 2

บทที่ 5
Processor Management
(Thread and Deadlock)
ประเด็นที่ตอ้ งพิจารณา
• ถ้ าโปรแกรมทางานทีละคาสัง่ ตามลาดับ
– เมื่อสัง่ พิมพ์ออก spooler ก็ไม่สามารถป้อนเนื ้อหาใหม่ ไม่สามารถ save งาน
ไม่สามารถทาอย่างอื่นได้ ต้ องหยุดรอจนกว่าจะพิมพ์เสร็จ
– งานอื่นเช่นการสัง่ save หรื อป้อนเนื ้อหาใหม่ น่าจะทาไปพร้ อมกันได้ ไม่จาเป็ น
จะต้ องรอ
– ขณะรอ ถ้ าไม่มีงานอื่น CPU ก็วา่ ง สูญเสียไปโดยเปล่าประโยชน์
– น่าที่โปรแกรมเดียว ควรจะสามารถทางานหลายๆงานไปพร้ อมกันได้
• เกิดแนวคิดที่เรี ยกว่า thread
• ในแต่ละ process อาจมี thread เดียวหรื อหลาย thread ก็ได้
• Context switching ระหว่าง thread ใน process เดียวกัน จะ
เร็ วกว่า context switching ระหว่าง process เพราะ thread
ใช้ ข้อมูลร่วมกันเป็ นส่วนใหญ่
Thread Usage (1)
Figure 2-7. A word processor with three threads.
Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639
Thread Usage (2)
Figure 2-8. A multithreaded Web server.
Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639
Thread Usage (3)
Figure 2-9. A rough outline of the code for Fig. 2-8. (a) Dispatcher
thread. (b) Worker thread.
Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639
Thread Usage (4)
Figure 2-10. Three ways to construct a server.
Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639
Thread Scheduling (1)
Figure 2-43. (a) Possible scheduling of user-level threads with a
50-msec process quantum and threads that run 5 msec per
CPU burst.
Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639
Thread Scheduling (2)
Figure 2-43. (b) Possible scheduling of kernel-level threads with
the same characteristics as (a).
Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639
กิจกรรม
• ให้ นกั ศึกษาทัง้ 6 กลุม่ แบ่งออกเป็ น 3 กลุม่ ใหญ่ โดยให้ กลุม่ ที่นงั่ ติดกัน
2 กลุม่ รวมเป็ นกลุม่ ใหญ่กลุม่ เดียว
• ทากิจกรรมตามที่อาจารย์สงั่ ในห้ อง
• สรุปผลกิจกรรม
Four Conditions for Deadlock
• Deadlock preceded by simultaneous
occurrence of four conditions that operating
system could have recognized:
•
•
•
•
Mutual exclusion
Resource holding
No preemption
Circular wait
Understanding Operating
Systems
10
• Mutual exclusion -- the act of allowing only one process
to have access to a dedicated resource.
• Resource holding -- the act of holding a resource and
not releasing it; waiting for the other job to retreat.
• No preemption -- the lack of temporary reallocation of
resources; once a job gets a resource it can hold on to it
for as long as it needs.
• Circular wait -- each process involved in impasse is
waiting for another to voluntarily release the resource
so that at least one will be able to continue.
Understanding Operating
Systems
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A Lack of Process Synchronization
Causes Deadlock or Starvation
• Deadlock (“deadly embrace”) -- a system-wide tangle of
resource requests that begins when 2+ jobs are put on
hold.
– Each job is waiting for a vital resource to become available.
– Needed resources are held by other jobs also waiting to run but
can’t because they’re waiting for other unavailable resources.
– The jobs come to a standstill.
– The deadlock is complete if remainder of system comes to a
standstill as well.
• Resolved via external intervention.
Understanding Operating
Systems
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Seven Cases of Deadlocks
1.Deadlocks on file requests
2.Deadlocks in databases
3.Deadlocks in dedicated device allocation
4.Deadlocks in multiple device allocation
5.Deadlocks in spooling
6.Deadlocks in disk sharing
7.Deadlocks in a network
Understanding Operating
Systems
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Case 1 : Deadlocks on File
Requests
• If jobs can request and hold
files for duration of their
execution, deadlock can occur.
• Any other programs that
require F1 or F2 are put on hold
as long as this situation
continues.
• Deadlock remains until a
programs is withdrawn or
forcibly removed and its file is
released.
Understanding Operating
Systems
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Case 2 : Deadlocks in Databases
1. P1 accesses R1 and locks it.
• Deadlock can occur if 2 processes
access & lock records in database.
2. P2 accesses R2 and locks it.
• 3 different levels of locking :
3. P1 requests R2, which is locked by
P2.
4. P2 requests R1, which is locked by
P1.
– entire database for duration of
request
– a subsection of the database
– individual record until process is
completed.
• If don’t use locks, can lead to a
race condition.
Understanding Operating
Systems
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Case 3: Deadlocks in Dedicated
Device Allocation
1. P1 requests tape drive 1 and gets
it.
2. P2 requests tape drive 2 and gets
it.
3. P1 requests tape drive 2 but is
blocked.
• Deadlock can occur when
there is a limited number of
dedicated devices.
• E.g., printers, plotters or
tape drives.
4. P2 requests tape drive 1 but is
blocked.
Understanding Operating
Systems
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Case 4 : Deadlocks in Multiple
Device Allocation
• Deadlocks can
happen when
several processes
request, and hold
on to, dedicated
devices while other
processes act in a
similar manner.
Understanding Operating
Systems
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Case 5 : Deadlocks in Spooling
• Most systems have transformed dedicated
devices such as a printer into a sharable device by
installing a high-speed device, a disk, between it
and the CPU.
• Disk accepts output from several users and acts
as a temporary storage area for all output until
printer is ready to accept it (spooling).
• If printer needs all of a job's output before it will
begin printing, but spooling system fills available
disk space with only partially completed output,
then a deadlock can occur.
Understanding Operating
Systems
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Case 6 : Deadlocks in Disk Sharing
P1
at
P2
Read records
cylinder 20
Write to file at
cylinder 310
I/O
Channe
l
Disk
control
unit
• Disks are designed to be shared, so it’s not
uncommon for 2 processes access different areas of
same disk.
• Without controls to regulate use of disk drive,
competing processes could send conflicting
commands and deadlock the system.
Understanding Operating
Systems
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Case 7: Deadlocks in a Network
A network that’s
congested (or filled
large % of its I/O
buffer space) can
become deadlocked
if it doesn’t have
protocols to control
flow of messages
through network.
Understanding Operating
Systems
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Modeling Deadlocks Using
Directed Graphs (Holt, 1972)
• Processes represented by circles.
• Resources represented by squares.
• Solid line from a resource to a process means that
process is holding that resource.
• Solid line from a process to a resource means that
process is waiting for that resource.
• Direction of arrow indicates flow.
• If there’s a cycle in the graph then there’s a deadlock
involving the processes and the resources in the cycle.
Understanding Operating
Systems
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Directed Graph Examples
R1
P
1
Figure 5.7 (a)
R1
P
2
P
1
R2
R1
P
1
Figure 5.7 (c)
Figure 5.7 (b)
Understanding Operating
Systems
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Figure 5.8
R1
R1
R2
R2
R3
R3
P
1
P
1
P
2
P
2
P
3
P
3
Figure 5.9
R1
R2
R3
R1
R2
R3
P
1
P
2
P
3
P
1
P
2
P
3
Understanding Operating
Systems
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P
1



P
3
Figure 5.11 (a)
P
2
P
1



P
3
Figure 5.11 (b)
P
2
Understanding Operating
Systems
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Strategies for Handling Deadlocks
• Prevent one of the four conditions from
occurring.
• Avoid the deadlock if it becomes probable.
• Detect the deadlock when it occurs and
recover from it gracefully.
Understanding Operating
Systems
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Prevention of Deadlock
• To prevent a deadlock OS must eliminate 1 out of 4
necessary conditions.
– Same condition can’t be eliminated from every resource.
• Mutual exclusion is necessary in any computer system
because some resources (memory, CPU, dedicated
devices) must be exclusively allocated to 1 user at a
time.
– Might be able to use spooling for some devices.
– May trade 1 type of deadlock (Case 3) for another (Case 5).
Understanding Operating
Systems
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Prevention of Resource Holding
or No Preemption
• Resource holding can be avoided by forcing each job to
request, at creation time, every resource it will need to
run to completion.
– Significantly decreases degree of multiprogramming.
– Peripheral devices would be idle because allocated to a job
even though they wouldn't be used all the time.
• No preemption could be bypassed by allowing OS to
deallocate resources from jobs.
– OK if state of job can be easily saved and restored.
– Bad if preempt dedicated I/O device or files during
modification.
Understanding Operating
Systems
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Prevention of Circular Wait
• Circular wait can be bypassed if OS prevents formation
of a circle.
• Havender’s solution (1968) is based on a numbering
system for resources such as: printer = 1, disk = 2,
tape= 3.
– Forces each job to request its resources in ascending order.
– Any “number one” devices required by job requested first;
any “number two” devices requested next …
• Require that jobs anticipate order in which they will
request resources.
– A best order is difficult to determine.
Understanding Operating
Systems
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Avoidance
• Even if OS can’t remove 1 conditions for deadlock, it can
avoid one if system knows ahead of time sequence of
requests associated with each of the active processes.
• Dijkstra’s Bankers Algorithm (1965) used to regulate
resources allocation to avoid deadlock.
• Safe state -- if there exists a safe sequence of all processes
where they can all get the resources needed.
• Unsafe state -- doesn’t necessarily lead to deadlock, but it
does indicate that system is an excellent candidate for one.
Understanding Operating
Systems
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Banker’s Algorithm
• Based on a bank with a fixed amount of capital that
operates on the following principles:
 No customer will be granted a loan exceeding bank’s total
capital.
 All customers will be given a maximum credit limit when
opening an account.
 No customer will be allowed to borrow over the limit.
 The sum of all loans won’t exceed the bank’s total capital.
• OS (bank) must be sure never to satisfy a request that
moves it from a safe state to an unsafe one.
• Job with smallest number of remaining resources < =
number of available resources
Understanding Operating
Systems
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Problems with Banker’s Algorithm
1. As they enter system, jobs must state in advance the maximum number of
resources needed.
2. Number of total resources for each class must remain constant.
3. Number of jobs must remain fixed.
4. Overhead cost incurred by running the avoidance algorithm can be quite
high.
5. Resources aren’t well utilized because the algorithm assumes the worst
case.
6. Scheduling suffers as a result of the poor utilization and jobs are kept
waiting for resource allocation.
Understanding Operating
Systems
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Detection
• Use directed graphs to show circular wait
which indicates a deadlock.
• Algorithm used to detect circularity can be
executed whenever it is appropriate.
Understanding Operating
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Reducing Directed Resource Graphs
1. Find a process that is currently using a resource and
not waiting for one. Remove this process from graph
and return resource to “available list.”
2. Find a process that’s waiting only for resource classes
that aren’t fully allocated.
– Process isn’t contributing to deadlock since eventually gets
resource it’s waiting for, finish its work, and return
resource to “available list.”
3. Go back to Step 1 and continue the loop until all lines
connecting resources to processes have been removed.
Understanding Operating
Systems
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R1
P
2
P
3
R1
P
2
P
3
P
1
R2
R3
P
1
R2
R3
Figure 5.12 (a)
Figure 5.12 (b)
R1
P
2
P
3
R1
P
2
P
3
P
1
R2
R3
P
1
R2
R3
Figure 5.12 (c)
Understanding Operating
Systems
Figure 5.12 (d)
34
Recovery
• Once a deadlock has been detected it must be
untangled and system returned to normal as quickly as
possible.
• There are several recovery algorithms, all requiring at
least one victim, an expendable job, which, when
removed from deadlock, frees system.
1. Terminate every job that’s active in system and restart
them from beginning.
2. Terminate only the jobs involved in deadlock and ask
their users to resubmit them.
Understanding Operating
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Recovery Algorithms - 2
3. Terminate jobs involved in deadlock one at a time,
checking to see if deadlock is eliminated after each
removal, until it has been resolved.
4. Have job keep record (snapshot) of its progress so it can
be interrupted and then continued without starting
again from the beginning of its execution.
5. Select a non-deadlocked job, preempt resources it’s
holding, and allocate them to a deadlocked process so it
can resume execution, thus breaking the deadlock
6. Stop new jobs from entering system, which allows nondeadlocked jobs to run to completion so they’ll release
their resources (no victim).
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Select Victim with Least-Negative
Effect
• Priority of job under consideration—highpriority jobs are usually untouched.
• CPU time used by job—jobs close to
completion are usually left alone.
• Number of other jobs that would be affected
if this job were selected as the victim.
• Programs working with databases deserve
special treatment.
Understanding Operating
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Starvation
• Starvation -- result of conservative allocation of
resources where a single job is prevented from
execution because it’s kept waiting for resources
that never become available.
• “The dining philosophers” Dijkstra (1968).
• Avoid starvation via algorithm designed to detect
starving jobs which tracks how long each job has
been waiting for resources (aging).
Understanding Operating
Systems
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