posix threads: a first step toward parallel...
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POSIX Threads: a first step toward parallel programming
George Bosilcabosilca@icl.utk.edu
Process vs. Thread• A process is a collection of
virtual memory space, code, data, and system resources.
• A thread (lightweight process) is code that is to be serially executed within a process.
• A process can have several threads.
Threads executing the same block of code maintain separate stacks. Each thread in a process shares that
process's global variables and resources.
Possible to create more efficient applications ?
ProcessState:register,SP,PC,…
CodeSegment
DataSegment
Heap
Stack
Process
Code
Data
Process
State
Stack
ThreadLocalStorage
ThreadState
Stack
ThreadLocalStorage
Thread
HardwareThreads
• Hardwarecontrolswitchingbetweenthreadstohidelatencies(memoryaccesses/operations)– Differentswitchingpolicies:cachemiss,aftereachoperation
– Hardwaremaintainindependentstateforeachthread(registers)
– Possibletoswitchingthreadsinonecycle(TERAmachine)
Terminology• LightweightProcess(LWP):orkernelthread• X-to-Ymodel:themappingbetweentheLWPandtheuserthreads
(1:X– Unix&co.,X:1– Userlevelthreads,X:Y – Windows7).• ContentionScope:howthreadscompeteforsystemresources• Thread-safeaprogramthatprotectstheshareddataforitsthreads
(mutualexclusion)• Reentrantcode:aprogramthatcanhavemorethanonethread
executingconcurrently.• Async-safemeansthatafunctionisreentrantwhilehandlinga
signal(i.e.canbecalledfromasignalhandler).• Concurrencyvs.Parallelism- Theyarenotthesame!Parallelism
impliessimultaneousrunningofcode(whichisnotpossible,inthestrictsense,onuniprocessormachines)whileconcurrencyimpliesthatmanytaskscanruninanyorderandpossiblyinparallel.
Threadvs.Process• Threadssharetheaddressspaceoftheprocessthatcreatedit;
processeshavetheirownaddressspace.• Threadshavedirectaccesstothedatasegmentofitsprocess;
processeshavetheirowncopyofthedatasegmentoftheparentprocess.
• Threadscandirectlycommunicatewithotherthreadsofitsprocess;processesmustuseinterprocess communicationtocommunicatewithsiblingprocesses.
• Threadshavealmostnooverhead;processeshaveconsiderableoverhead.
• Newthreadsareeasilycreated;newprocessesrequireduplicationoftheparentprocess.
• Threadscanexerciseconsiderablecontroloverthreadsofthesameprocess;processescanonlyexercisecontroloverchildprocesses.
• Changestothemainthread(cancellation,prioritychange,etc.)mayaffectthebehavioroftheotherthreadsoftheprocess;changestotheparentprocessdoesnotaffectchildprocesses.
2.2.2TheClassicalThreadModel in ModernOperatingSystems3e byTanenbaum
Process vs. Thread• Multithreaded applications must avoid two
threading problems: deadlocks and races.• A deadlock occurs when each thread is waiting
for the other to do something.• A race condition occurs when one thread
finishes before another on which it depends, causing the former to use a bogus value because the latter has not yet supplied a valid one.
The key is synchronization
• Ordering memory accesses– Flush / memory barrier– Volatile (poor-man solution in C)
• Synchronization = gaining access to a shared resource.• Synchronization REQUIRE cooperation.
x = 1;barrier();y = 5;x = 2;
Thread1
y = 1;barrier();while(x==1);printf( “%d\n”, y );
Thread2
POSIX Thread
• What’s POSIX ?– Widely used UNIX specification– Most of the UNIX flavor operating systems
POSIX is the Portable Operating System Interface, the open operating interface standard accepted world-wide. It is produced by IEEE and recognized by ISO and ANSI.
Pthread APIPrefix Use
pthread_ Threadmanagement(create/destroy/cancel/join/exit)
pthread_attr_ Threadattributes
pthread_mutex_ Mutexes
pthread_mutexattr_ Mutexes attributes
pthread_cond_ Conditionvariables
pthread_condattr_ Conditionattributes
pthread_key_ Thread-specificdatakey(TLS)
pthread_rwlock_ Read/writelocks
pthread_barrier_ Synchronizationbarriers
ThreadManagement(create)
Attributes:Detachedorjoinablestate,Schedulinginheritance,Schedulingpolicy,Schedulingparameters,Schedulingcontentionscope,Stacksize,Stackaddress,Stackguard(overflow)size
int pthread_create (pthread_t *restrictthread,[OUT]threadidconst pthread_attr_t *restrictattr,[IN]attributesvoid*(*start_routine)(void*),[IN]threadfunctionvoid*restrictarg)[IN]argumentforthreadfunction
int pthread_exit (void*value_ptr)[OUT]Returntocaller/joinerint pthread_cancel (pthread_t thread)[IN]threadtobecancelledint pthread_attr_init (pthread_attr_t *attr)[OUT]attributestobeinitializedint pthread_attr_set*(pthread_attr_t *restrictattr,*)[IN]setattributed(state,stack)int pthread_attr_destroy (pthread_attr_t *attr)[IN]attributedtobedestroyed
Questions:- Oncecreatedwhatwillbethestatusofthethreadandhowitwillbescheduled
bytheOS?(usesched_setscheduler)- Whereitwillberun?(usesched_setaffinity orHWLOC)
ThreadManagement(join)
Joinblocksthecallingthreaduntilthetargetthreadterminates,andreturnsit’sthread_exit argument.Itisimpossible tojoinathreadinadetachedstate.Itisalsoimpossibletoreattachit
intpthread_join (pthread_t thread,void**value_ptr)[OUT]threadreturnvalue
int pthread_detach (pthread_t thread)int pthread_attr_setdetachstate (pthread_attr_t *attr, [IN/OUT]attribute
int detachstate)[IN]statetobesetint pthread_attr_getdetachstate (const pthread_attr_t *attr,
int *detachstate)[OUT]detachstatevalue
ThreadManagement(state)
ThePOSIXstandarddoesnotdictatethesizeofathread'sstack!
int pthread_attr_getstacksize (const pthread_attr_t *restrictattr,size_t *restrictstacksize)
int pthread_attr_setstacksize (pthread_attr_t *attr,size_t stacksize)int pthread_attr_getstackaddr (const pthread_attr_t *restrictattr,
void**restrictstackaddr)int pthread_attr_setstackaddr (pthread_attr_t *attr,void*stackaddr)pthread_t pthread_self (void)int pthread_equal (pthread_t t1,pthread_t t2)
Threading- example• NumericalsolutiontoLaplace’sequation
( )nji
nji
ni
nji
nji UUUUU 1,1,1,11
, 41
+−+−+ +++=
i,j+1
i,j-1
i+1,ji-1,j
for j = 1 to jmaxfor i = 1 to imaxUnew(i,j) = 0.25 * ( U(i-1,j) + U(i+1,j)
+ U(i,j-1) + U(i,j+1))end for
end for
Threading- example• Theapproachtomakeitparallelisbypartitioningthedata
Threading- example• Theapproachtomakeitparallelisbypartitioningthedata
Overlapping the data boundaries allow computation without communication for each superstep
On the communication step each processor update the corresponding columns on the remote processors.
Threading- examplefor j = 1 to jmaxfor i = 1 to imaxunew(i,j) = 0.25 * ( U(i-1,j) + U(i+1,j)
+ U(i,j-1) + U(i,j+1))end for
end for
MemoryConsistencyModelsMemoryLevel Size Response
CPUregisters ≈100B 0.5ns(1 cycles)
L1Cache 64KB– 1M 1ns(fewcycles)
L2Cache ≈1-30 MB 10ns(tensofcycles)
MainMemory ≈? GB 150ns(hundreds ofcycles)
HardDisk ≈? TB 10ms(thousandsofcycles)
NetworkStorage ≈ ?PB 100msto 1s(muchmore)
• Definingaconsistentmemorymodelsisdifficultandnotnecessarilyrequiredforcorrectness– Weakerdefinitionsthatareeasiertoimplementandgoodenoughtoimplementpredictableanddeterministicapplications
Strict(atomic)consistency• Definition:anyreadtoamemorylocationXreturnsthevaluestoredbythemostrecentwriteoperationtoX– themostrecentcoversallcomputingunitsinthesystem
x =1[W(x)1]P1
?x… [R(x)?]P2
?x… [R(x)?]
P1
P2
W(x)1
R(x)1 R(x)1
P1
P2
W(x)1
R(x)0 R(x)1
P1
P2
W(x)1
R(x)0 R(x)1
SequentialConsistency• Definition:theresultofanyexecutionisthesameasifthereadsandwritesoccurredinsomeorder,andtheoperationsofeachindividualprocessorappearinthissequenceintheorderspecifiedbyitsprogram– Lamport ordering– expandingfromthesetsofreadsandwritesthat actually happenedtothesetsthat could havehappened,wecanreasonmoreeffectivelyabouttheprogram
x =1[W(x)1]P1
?x… [R(x)?]P2
?x… [R(x)?]
P1
P2
W(x)1
R(x)0 R(x)1
SequentialConsistencyx =1[W(x)1]
P1?x… [R(x)?]
P2
?x… [R(x)?]
x =2[W(x)2] ?x… [R(x)?]
?x… [R(x)?]
P3 P4
P1 W(x)1
P2 R(x)1 R(x)2
P3 W(x)2
P4 R(x)1 R(x)2
P1 W(x)1
P2 R(x)1 R(x)2
P3 W(x)2
P4 R(x)1 R(x)2
P1 W(x)1
P2 R(x)1 R(x)2
P3 W(x)2
P4 R(x)2 R(x)1
CachecoherencyisNOTsequentialconsistencybecauseseq.consistencyrequiresaglobally consistencyviewofmemoryoperationswhilecachecoherencyonlyrequiresthemlocally
CacheCoherenceP1 W(x)1
P2 R(x)0 R(x)2
P3 W(x)2
P4 R(y)0 R(y)1
W(y)2
W(y)1
R(y)0
R(x)1
R(y)1Can’thappenwithasnoopy-cacheschemebutitcanwithadirectory-basedcache
Typesofmemoryaccesses:• SharedAccess:wecanhavesharedaccesstovariables vs. privateaccess.But
thequestionswe'reconsideringareonlyrelevantforsharedaccesses.• Competing vs. Non-Competing:Ifwehavetwoaccessesfromdifferent
processors,andatleastoneisawrite,theyarecompetingaccesses.Theyareconsideredascompetingaccessesbecausetheresultdependsonwhichaccessoccursfirst(iftherearetwoaccesses,butthey'rebothreads,itdoesn'tmatterwhichisfirst).
• Synchronizing vs. Non-Synchronizing:Ordinarycompetingaccesses,suchasvariableaccesses,arenon-synchronizingaccesses.Accessesusedinsynchronizingtheprocessesare(ofcourse)synchronizingaccesses.
• Acquire vs. Release:Finally,wecandividesynchronizationaccessesintoaccessestoacquirelocks,andaccessestoreleaselocks.
WeakConsistency• Weakconsistencyresultsifweonlyconsidercompeting
accessesasbeingdividedintosynchronizingandnon-synchronizingaccesses,andrequirethefollowingproperties:– Accessestosynchronizationvariablesaresequentiallyconsistent.– Noaccesstoasynchronizationvariableisallowedtobeperformed
untilallpreviouswriteshavecompletedeverywhere.– Nodataaccess(readorwrite)isallowedtobeperformeduntilall
previousaccessestosynchronizationvariableshavebeenperformed.
P2 R(x)0 R(x)2 S R(x)2P3 R(x)1 R(x)2S
P1 W(x)1 W(x)2 S
Releaseconsistency• WeakConsistency(viasynchronization)requiresthatwhena
synchronizationoccurs,allprocessorsgloballyupdatememory–eachlocalchangemustbepropagatedtoallprocessorswithacopyofthesharedvariable,andeachprocessorneedtoobtainallchangesfromtheothers
• Releaseconsistencyconsiderfinergrainlocksofmemoryregions,andonlypropagatesthelockedmemory(asneeded)– Beforeanordinaryaccesstoasharedvariableisperformed,all
previousacquiresdonebytheprocessmusthavecompletedsuccessfully.
– Beforeareleaseisallowedtobeperformed,allpreviousreadsandwritesdonebytheprocessmusthavecompleted.
– Theacquireandreleaseaccessesmustbesequentiallyconsistent.
Mutual exclusion
• Simple lock primitive with 2 states: lock and unlock
• Only one thread can lock the mutex.• Several politics: FIFO, random,
recursive
lock
unlock…
…Thread 1
lock
unlock…
…Thread 2
lock
unlock…
…Thread 3
Active threads
Mutual exclusion
• Simple lock primitive with 2 states: lock and unlock
• Only one thread can lock the mutex.• Several politics: FIFO, random,
recursive
lock
unlock…
…Thread 1
lock
unlock…
…Thread 3
Active threads
lock
unlock…
…Thread 2
Mutual exclusion
• Simple lock primitive with 2 states: lock and unlock
• Only one thread can lock the mutex.• Several politics: FIFO, random,
recursive
lock
unlock…
…Thread 1
lock
unlock…
…Thread 3
Active threads
lock
unlock…
…Thread 2
Mutual exclusion
• Simple lock primitive with 2 states: lock and unlock
• Only one thread can lock the mutex.• Several politics: FIFO, random,
recursive
lock
unlock…
…Thread 1
lock
unlock…
…Thread 3
Active threads
lock
unlock…
…Thread 2
Mutual exclusion
• Simple lock primitive with 2 states: lock and unlock
• Only one thread can lock the mutex.• Several politics: FIFO, random,
recursive
lock
unlock…
…Thread 1
lock
unlock…
…Thread 3
Active threads
lock
unlock…
…Thread 2
Mutual exclusion
• Simple lock primitive with 2 states: lock and unlock
• Only one thread can lock the mutex.• Several politics: FIFO, random,
recursive
lock
unlock…
…Thread 1
lock
unlock…
…Thread 3
Active threads
lock
unlock…
…Thread 2
Mutual exclusion• Spin vs. sleep ?• What’s the desired lock grain ?
– Fine grain – spin mutex– Coarse grain – sleep mutex
• Spin mutex: use CPU cycles and increase the memory bandwidth, but when the mutex is unlock the thread continue his execution immediately.
Shared/Exclusive Locks• ReadWrite Mutual exclusion • Extension used by the reader/writer model• 4 states: write_lock, write_unlock, read_lock and
read_unlock.• multiple threads may hold a shared lock
simultaneously, but only one thread may hold an exclusive lock.
• if one thread holds an exclusive lock, no threads may hold a shared lock.
Shared/Exclusive Locks
rw_lock
rw_unlock…
…Writer 1
rw_lock
rw_unlock…
…Writer 2
rd_lock
rd_unlock…
…Reader 1
rd_lock
rd_unlock…
…Reader 2
rw_lock
rw_unlock…
…Writer 1
rw_lock
rw_unlock…
…Writer 2
rd_lock
rd_unlock…
…Reader 1
rd_lock
rd_unlock…
…Reader 2
Active threadSleeping thread
Legend
Step 1
Step 2
Shared/Exclusive Locks
Active threadSleeping thread
Legend
rw_lock
rw_unlock…
…Writer 1
rw_lock
rw_unlock…
…Writer 2
rd_lock
rd_unlock…
…Reader 1
rd_lock
rd_unlock…
…Reader 2
rw_lock
rw_unlock…
…Writer 1
rw_lock
rw_unlock…
…Writer 2
rd_lock
rd_unlock…
…Reader 1
rd_lock
rd_unlock…
…Reader 2
Step 3
Step 4
Shared/Exclusive Locks
Active threadSleeping thread
Legend
rw_lock
rw_unlock…
…Writer 1
rw_lock
rw_unlock…
…Writer 2
rd_lock
rd_unlock…
…Reader 1
rd_lock
rd_unlock…
…Reader 2
rw_lock
rw_unlock…
…Writer 1
rw_lock
rw_unlock…
…Writer 2
rd_lock
rd_unlock…
…Reader 1
rd_lock
rd_unlock…
…Reader 2
Step 5
Step 6
… …
…
Shared/Exclusive Locks
Active threadSleeping thread
Legend
rw_lock
rw_unlock…
…Writer 1
rw_lock
rw_unlock…
…Writer 2
rd_lock
rd_unlock…
…Reader 1
rd_lock
rd_unlock…
…Reader 2
Step 7… …
Condition Variable
• Block a thread while waiting for a condition• Condition_wait / condition_signal• Several thread can wait for the same
condition, they all get the signal
signal…
…Thread 1
Active threads Sleeping threads
conditionwait…
…Thread 2
wait…
…Thread 3
Condition Variable
• Block a thread while waiting for a condition• Condition_wait / condition_signal• Several thread can wait for the same
condition, they all get the signal
signal…
…Thread 1
Active threads
wait…
…Thread 3
…
…Thread 2
wait
Condition Variable
• Block a thread while waiting for a condition• Condition_wait / condition_signal• Several thread can wait for the same
condition, they all get the signal
signal…
…Thread 1
Active threads
wait…
…Thread 3
…
…Thread 2
wait
Condition Variable
• Block a thread while waiting for a condition• Condition_wait / condition_signal• Several thread can wait for the same
condition, they all get the signal
signal…
…Thread 1
Active threads
wait…
…Thread 3
…
…Thread 2
wait
Condition Variable
• Block a thread while waiting for a condition• Condition_wait / condition_signal• Several thread can wait for the same
condition, they all get the signal
signal…
…Thread 1
Active threads
wait…
…Thread 2
wait…
…Thread 3
Semaphores• simple counting mutexes• The semaphore can be hold by as many
threads as the initial value of the semaphore.
• When a thread get the semaphore it decrease the internal value by 1.
• When a thread release the semaphore it increase the internal value by 1.
Semaphores
get
release…
…Thread 1
Semaphore (2)get
release…
…Thread 2
get
release…
…Thread 3
get
release…
…Thread 1
Semaphore (1)get
release…
…Thread 2
get
release…
…Thread 3
Semaphores
get
release…
…Thread 1
Semaphore (0)get
release…
…Thread 2
get
release…
…Thread 3
get
release…
…Thread 1
Semaphore (0)get
release…
…Thread 2
get
release…
…Thread 3
Semaphores
get
release…
…Thread 1
Semaphore (1)get
release…
…Thread 2
get
release…
…Thread 1
Semaphore (1)get
release…
…Thread 2
get
release…
…Thread 3
get
release…
…Thread 3
Semaphores
get
release…
…Thread 1
Semaphore (1)get
release…
…Thread 2
get
release…
…Thread 1
Semaphore (2)get
release…
…Thread 2
get
release…
…Thread 3
get
release…
…Thread 3
Atomic instruction• Is any operation that a CPU can perform such
that all results will be made visible to each CPU at the same time and whose operation is safe from interference by other CPUs– TestAndSet– CompareAndSwap– DoubleCompareAndSwap– Atomic increment– Atomic decrement
Example:AProducer– ConsumerQueue
• Wehaveaboundedqueuewhereproducersstoretheiroutputandfromwhereconsumerstaketheirinput
• Protectthestructureagainstintensiveunnecessaryaccesses– Detectboundaryconditions:queueemptyandqueuefull
Boun
dedQue
ue
…producers
…consumers
Example:dot-product
• Dividethearraysbetweenparticipantstoload-balancethework– Eachwillthencomputeapartialsum
• Addallthepartialsumstogetherforthefinalresult(reduceoperation)
• Technicaldetails:costofmanagingthethreadsvs.costofthealgorithm?Howtominimizethemanagementcost?
Scalarproduct,innerproduct
=PN
n=1 ai.bi
a
b
a
b
Thr 1 Thr 2 Thr 3
Partialsum
Partialsum
Partialsum
+
+
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