Deadlock problem and solution
Speaker: Mrak
Date: 2012-12-12
1
Outline
•
•
•
•
•
•
Introduction
Deadlock Prevention
Deadlock Avoidance
Deadlock Detection & Recovery
Conclusion
Reference
2
Deadlock introduction
• Deadlock DEF:
– 系統中有processes呈現一種互相等待對方資源
釋放出來的情況(circular waiting)
– 使的這些process皆
無法往下執行，使
的CPU utilization降
低。
Fig . 1
3
Deadlock introduction
• Mutual exclusion
– Fig. 1.1
• Hold&wait
• No preemption
• Circular waiting
– Fig. 2
4
Deadlock prevention
• Howto prevention-破除下列四點
1. Mutual exclusion
2. Hold&wait
3. No preemption
 priority
4. Circular waiting
 Unique resource ID
 Ascending
5
Deadlock prevention
• Circular waiting
– Example:
Case 持有
申請
1
R1
R3
OK
2
R3
R1
NO,release R3
3
R1,R5
R3
NO,release R5
6
Deadlock prevention
• Circular waiting
– PF :ascending (Fig. 3)
•
•
p1 < p2 <……< pn < p1
=> p1 < p1 --><--
7
Deadlock avoidance
• DEF
– 當某process提出資源申請後，OS必須持行一個
algorithm(Banker’s)以判斷定process提出的申請
後，系統是否除與安全的狀況(不會進入死結)
8
Deadlock avoidance-banker’s
• Example P1提出Request = (1,0,2) 是否核准
Setp1:check Request(P1)<= Need(P1)
Setp2:check Request(P1)<=Available
Setp3:Allocation+=Request
Need-=Request
Available-=Request
Setp4:run safety algorithm
(1,0,2) <= (1,2,2)
(1,0,2) <= (3,3,2)
(2,0,0) + (1,0,2) =(3,0,2)
(1,2,2) - (1,0,2) =(0,2,0)
(3,3,2) - (1,0,2) =(2,3,0)
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Deadlock avoidance-Safety algorithm
F F
F
Step1:work=available;
Finish =
Step2:可找到P1滿足 (i)Finish[1]=false
(ii)Need1 < work go step3
(0,2,0) < (2,3,0)
Step3:Finish[1]=True 且work+=Allocation1
if Finish中還有false goto step 3
else goto step4
Step4:check Finish 皆為True 傳回safe state
Sate sequence = p1 , p3 , p4 , p0 , p2
F
F
10
Deadlock Detection & Recovery
• Detection DEF:
– 每X秒偵測deadlock是否存在，若存在必須恢復正常
Step1: work=Available Finish[i] = if Allocation ≠ 0 set false
(0,0,0)
if Allocation = 0 set true
Step2: 如果找到Pi滿足 (i) Finish[i] = False (ii) Requesti <=work then goto step 3
else goto step 4
Step3:set Finish[i] = true work += Allocationi then goto step 2
Step4: check Finish Array if all is true , No Deadlock
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Deadlock Detection & Recovery
• Recovery DEF:
– Case 1 : kill all process in Deadlock
– Case 2 : kill one process each time,then Detection
12
Conclusion
• Deadlock Prevention與Deadlock Avoidance的
resources utilization 保證系統不會有
deadlock
• Deadlock Detection & Recovery
resources utilization
但系統可能進入
deadlock
13
reference
• https://docs.google.com/a/mail1.ncnu.edu.tw/viewer?a=v
&q=cache:tE9tdn2zpIJ:sjchen.im.nuu.edu.tw/OS/97Spring/Ch_6.pdf+&
hl=zh-TW&pid=bl&srcid=ADGEESixtvIiw8J9u-2S89vjdnumwFUUphLv6k6Z7vT3dflJV1GxvKKTBpqF6C7V4SC1qV_Hb5WDNVAaArSi9z4Ne4pmoESx5siFAv_edb7WFDgX
GX-s4njkpON-u1zcQDqIFWyjn&sig=AHIEtbTNMCCDDCa0kjMpauyT9mndv7ZxWA [聯合
大學]
• http://kramli.staff.ui.ac.id/files/2011/02/Chapter-06-OSConcurrency-Deadlock-and-Starvation.pdf [Manatee
Community College]
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Q&A
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