14 distrib proc mgmt deadlock

8/19/2019 14 Distrib Proc Mgmt Deadlock

1/19

CS-550 (M.Soneru): Distributed process management – Distributed deadlock [Sta0!"# [SaS" !

Distributed process management:

Distributed deadlock


2/19

CS-550 (M.Soneru): Distributed process management – Distributed deadlock [Sta0!"# [SaS" $

Distributed deadlock

% &roblem de'inition

– &ermanent blocking o' a set o' processes tat eiter compete 'or sstem

resources or communicate *it eac oter

– +o node as complete and up-to-date kno*ledge o' te entire distributed

sstem

– Message trans'ers bet*een processes take unpredictable delas

% ,pes o' distributed deadlock

– esource deadlock

% Set o' deadlocked processes# *ere eac process *aits 'or a resource eld b

anoter process (e.g.# data obect in a database# /1 resource on a ser2er)

– Communication deadlocks:% Set o' deadlocked processes# *ere eac process *aits to recei2e messages

(communication) 'rom oter processes in te set.


3/19

CS-550 (M.Soneru): Distributed process management – Distributed deadlock [Sta0!"# [SaS" 3

Distributed deadlock (cont.)

%Sstem state representation *it *ait-'or graps (46) – +odes are processes# P 1# P 2# etc.

– Directed edge 'rom P 1 to P 2 i' P 1 blocked and *aiting 'or P 2 to release a

resource

– Sstem is deadlocked i' tere is a directed ccle


4/19


Deadlock in resource allocation

% Conditions 'or deadlock in resource allocation

– Mutual e8clusion: ,e resource can be used b onl one process at a time

– 9old and *ait: process olds a resource *ile *aiting 'or oter resources – +o preemption: process cannot be preempted to 'ree up te resource

– Circular *ait: closed ccle o' processes is 'ormed# *ere eac processolds one or more resources needed b te ne8t process in te ccle

% Strategies

– &re2ent te 'ormation o' a circular *ait – Detect te potential or te actual occurrence o' a circular *ait

% ,pes o' algoritms

– Deadlock pre2ention

– Deadlock a2oidance

– Deadlock detection% Special issues in distributed sstems

– esources are distributed across man sites

– ,e control processes tat control access to resources do not a2ecomplete# up-to-date in'ormation on te global state o' te sstem


5/19


Deadlock in resource allocation: Deadlock andling strategies

(!) Deadlock pre2ention

!.a &re2ent te circular-*ait condition b de'ining a linear ordering o'resource tpes

% process can be assigned resources onl according to te linear

ordering

% Disad2antages

- esources cannot be re;uested in te order tat are needed

- esources *ill be longer tan necessar

!.b &re2ent te old-and-*ait condition b re;uiring te process to ac;uire all

needed resources be'ore starting e8ecution

% Disad2antages

– /ne''icient use o' resources

– educed concurrenc

– &rocess can become deadlocked during te initial resource ac;uisition

– uture needs o' a process cannot be al*as predicted


6/19

CS-550 (M.Soneru): Distributed process management – Distributed deadlock [Sta0!"# [SaS" <

Deadlock in resource allocation: Deadlock andling strategies

(!) Deadlock pre2ention (cont.)

!.c =se o' time-stamps

% >8ample: =se time-stamps 'or transactions to a database – eac transactionas te time-stamp o' its creation

% ,e circular *ait condition is a2oided b comparing time-stamps: strict

ordering o' transactions is obtained# te transaction *it an earlier time-

stamp al*as *ins

% ?4ait-die@ metodif [ e (,$) A e (,!) "

altB,$ (*ait)

else

killB,$ (die)

% ?4ound-*ait@ metod

if [ e (,$) A e (,!) "

killB,! (*ound)

else

altB,$ (*ait)


7/19CS-550 (M.Soneru): Distributed process management – Distributed deadlock [Sta0!"# [SaS" E

Deadlock in resource allocation (cont.)

($) Deadlock a2oidance

– Decision made dnamicall# be'ore allocating a resource# te resulting

global sstem state is cecked - i' sa'e# allo* allocation

– Disad2antages

% >2er site as to maintain global state o' sstem (e8tensi2e o2eread in

storage and communication)

% Di''erent sites ma determine (concurrentl) tat state is sa'e# but globalstate ma be unsa'e: 2eri'ication 'or sa'e global state b di''erent sites

must be mutuall e8clusi2e

% Farge o2eread to ceck 'or e2er allocation (distributed sstem ma

a2e large number o' processes and resources)

– Conclusion: Deadlock a2oidance impractical in distributed sstems


8/19

CS-550 (M.Soneru): Distributed process management – Distributed deadlock [Sta0!"# [SaS" G

Deadlock in resource allocation (cont.)

(3) Deadlock Detection

– &rinciple o' operation% Detection o' a ccle in 46 proceeds concurrentl *it normal

operation

– e;uirements 'or te deadlock detection and resolutionalgoritms

% Detection – ,e algoritm must detect all e8isting deadlock in 'inite time

– ,e algoritm sould not report non-e8istent (pantom) deadlock

% esolution (reco2er)

– ll e8isting *ait-'or dependencies in 46 must be remo2ed# i.e. roll-back

one or more processes tat are deadlocked and gi2e teir resources to oter blocked processes

– 1bser2ation% Deadlock detection is te most popular strateg 'or andling deadlocks

in distributed sstems


9/19

CS-550 (M.Soneru): Distributed process management – Distributed deadlock [Sta0!"# [SaS" H

Deadlock in resource allocation:lgoritms 'or distributed deadlock detection

3) Deadlock Detection (cont.)

– Control 'or distributed deadlock detection can be:a. CentraliIed

b. Distributed

c. 9ierarcical

a.! CentraliIed deadlock detection algoritms

% central control site constructs te global 46 and searces 'or ccles% Control site an maintain 46 continuousl (*it e2er assignment) or *en running

deadlock detection (and asking all sites 'or 46 updates)

% Disad2antages: single point o' 'ailure and congestion

a.$ ,e completel centraliIed algoritm % ll sites re;uest resources and release resources b sending corresponding messages to

control site

% Control site updates 46 'or eac re;uestrelease

% or e2er ne* re;uest edge added to 46# control site cecks 46 'or deadlock

% lternati2e: eac site maintain its 46 and update control site periodicall or on re;uest


10/19

CS-550 (M.Soneru): Distributed process management – Distributed deadlock [Sta0!"# [SaS" !0

Deadlock in resource allocation:lgoritms 'or distributed deadlock detection


b. 9ierarcical deadlock detection algoritms% Sites organiIed in a tree structure *it one site at te root o' te tree

% >ac node (e8cept 'or lea' nodes) as in'ormation about te dependent

nodes

% Deadlock is detected b te node tat is te common ancestor o' all

sites *ic a2e resource allocations in con'lict

% Deadlock is detected at te lo*est le2el


11/19

CS-550 (M.Soneru): Distributed process management – Distributed deadlock [Sta0!"# [SaS" !!

Deadlock in resource allocation:

lgoritms 'or distributed deadlock detection (cont.)


c. Distributed deadlock detection algoritms – &rinciples

% ll sites responsible 'or detecting a global deadlock

% 6lobal state grap distributed o2er man sites: se2eral o' tem participate in detection

% Detection initiated *en a process suspected to be deadlocked

% d2antages: +o single point o' 'ailure# no congestion% Disad2antages: Di''icult to implement (no sared memor)

– ,pes o' algoritms

% &at-pusing algoritms

– >ac node builds a 46 based on local in'o J in'o 'rom oter sites

– Detect and resol2es local deadlocks

– ,ransmits to oter sites deadlock in'o in 'orm o' (*aiting pat)

% >dge-casing algoritms

– Special messages (probes) sent along edges o' 46 to detect a ccle

– 4en blocked process recei2es probe# resends it on its outgoing edges o' 46

– 4en a process recei2es a probe it initiated# declares deadlock


12/19

CS-550 (M.Soneru): Distributed process management – Distributed deadlock [Sta0!"# [SaS" !$

Deadlock in message communication

% Mutual 4aiting

– Deadlock conditions% >ac o' a group o' processes is *aiting 'or a message 'rom anoter member o'

te group

% ,ere are no messages in transit

– Concepts

% Dependence set (DS) o' process P i is te set o' all processes 'rom *ic P i ise8pecting a message

% P i can proceed *en an o' te e8pected messages arri2e

% Deadlock in a set S o' processes

– ll processes in S are stopped# *aiting 'or messages

– S contains te dependence set o' all processes in S – +o messages are in transit bet*een processes in S


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% Mutual 4aiting (cont.)

– esource deadlock 2s. message deadlock

% esource deadlock – deadlock e8ists i' tere is a ccle in te 46

– process P i is dependent on process P j i' P j olds a resource tat P i needs

% Message deadlock – ll successors P j o' a process P i in S are also in S

– >8ample

% ig. !7.!


14/19


Deadlock in message communication: Mutual *aiting (cont.)


15/19



% =na2ailabilit o' Message Ku''ers – Deadlock can occur in te allocation o' bu''ers 'or te storage o' messages

in transit (>8ample: packet-s*itcing data net*orks)

!. Direct store-and-'or*ard deadlock

% >8ample:

– ,*o packet s*itcing nodes# eac using a common bu''er pool 'rom *ic bu''ersare assigned to packets on demand

– Ku''er space 'or is 'illed *it packets destined 'or K – Ku''er space 'or K is 'illed *it packets destined 'or

– +eiter node can transmit or recei2e packets: deadlock

% Solution: =se separate bu''ers# one 'or eac link


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17/19

CS-550 (M.Soneru): Distributed process management – Distributed deadlock [Sta0!"# [SaS" !E

Deadlock in message communication (cont.)

% =na2ailabilit o' Message Ku''ers (cont.)

$. /ndirect store-and-'or*ard deadlock:

Solution% =se a structured bu''er pool#

ierarcicall organiIed# *it N+1 le2els

( N is te ma8imum number o' ops an an

net*ork pat)

% Common pool – class 0 is unrestricted:

an incoming packet can be stored tere

% Ku''ers at le2el k (*ere 0 < k ≤ N ) are

reser2ed 'or packets tat a2e tra2eled at

least k ops so 'ar

% =nder ea2 load# bu''ers *ill 'ill 'rom0

to N

% 4en bu''ers 'ill to le2el k # ne* incoming

packets tat passed k ops or less are

discarded

% +o direct or indirect store-and-'or*ard

deadlock can occur


18/19

CS-550 (M.Soneru): Distributed process management – Distributed deadlock [Sta0!"# [SaS" !G

Deadlock in message communication: (cont.)

% =na2ailabilit o' Message Ku''ers (cont.)

3. Deadlock in a distributed 1S using message passing 'or inter-process

communication

% non-blocking Send operation re;uires a bu''er to old outgoing messages

% >8ample !

% &rocess X as a bu''er o' siIe n

% 'ter sending n messages# bu''er is 'ull

% 4en sending message n+1# process X *ill block until su''icient bu''er is 'reed

% >8ample $

% &rocess X as a bu''er o' siIe n

% &rocess Y as a bu''er o' siIe m

% Kot bu''ers# n and m# become 'ull and te t*o processes are blocked: deadlock

% &ossible solutions% &re2ention: estimate ma8imum number o' messages in transit and allocate

corresponding number o' bu''ers

% Detection: detect deadlock and roll back one o' te processes

D dl k i i ti


19/19

CS-550 (M.Soneru): Distributed process management – Distributed deadlock [Sta0!"# [SaS" !H

Deadlock in message communication:

=na2ailabilit o' message bu''ers (cont.)

14 distrib proc mgmt deadlock

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