Reading Circle

1. HybridMPI:EfficientMessagePassingfor
Multi‑coreSystems
ReadingCircleinTauraLaboratory
B4TakuyaFukuoka
February20,2019
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2. Aboutthispaper
Author:AndrewFriedley,GregBronevetsky,TorstenHoefler,and
AndrewLumsdaine.
ThispaperispublishedinSC'13.
ItintroducesHybridMPI(HMPI),whichadoptsprocess‑based
approachthatenablessharedmemorycommunication.
Theimplementationispublishedin
https://code.google.com/archive/p/hmpi.
Thefirstauthor'sdoctoraldissertionexplainsmoredetailsabout
thisimplementation.
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3. Introduction
MPIisdesignedforsystemswheresingle‑corecomputenodes
wereconnectedbyaninter‑nodenetwork
ThechallengeofMPIimplementationsisprovideefficient
abstractionforsharedmemory
ThispaperproposedHybridMPI(HMPI)thatisoptimizedfor
intra‑nodesharedmemorycommunication
HMPIadoptsprocess‑basedapproachinsteadofthread‑based
approach
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4. Background
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5. Traditionalprocess‑basedMPI
Noapplication‑visiblememoryisshared
Withinthesamesharedmemorynode,two‑copyprotocolis
adopted.
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6. Two‑copyprotocol
fromAndrew'sdoctoraldissertion
Thesendercopiesoneblockintothesharedregion,thenthe
receivercopiesittothereceivebuffer
Topiplinethetwocopiesbybreakingthemessageintoblocks 6

7. Thread‑basedMPI
AnyMPIusingmultiplethreadsmustprotectallinter‑node
communicationusingalockanditcausespoorperformance
Nothread‑basedMPIhasbeenwidelyadoptedinpractice
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8. ImplementationofHMPI
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9. HybridMPI(HMPI)
LayeringHMPIontopofanyexistingMPIlibrary
Thisarchitectureassuresportabilityandtransparency
Basically,HMPIadoptssingle‑copymessagepassingtechnique
insteadoftwo‑copyprotocol(explainlater)
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10. SharedMemoryHeapAllocator(1)
HMPIoverridesthesystem'sdefaultmemoryallocator(malloc)to
allocatememoryfromaspeciallycraftedsharedmemorypool
Therefore,thereisnoneedofinstallationofkernelextentions,
modificationofsystemlibraries,oradministrativepermissions
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11. SharedMemoryHeapAllocator(2)
fromAndrew'sdoctoraldissertion
Eachprocessmapstheentireregionatthesameaddress,then
allocatesitsmemoryonlyfromitsportion
Thisapproacheliminatestheneedforanysynchronization
betweenprocesses 11

12. MessageMatching
SharedQueuecanbeaccessedwithlockbyallprocessesinone
node
ThesenderinsertsmessageintoSharedQueue
EachprocesshasitsownPrivateQueue
ThereceiverdrainsSharedQueueandaddincomingsendsto
PrivateQueue 12

13. Single‑copyProtocol(1)
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14. Single‑copyProtocol(2)
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15. Single‑copyProtocol(3)
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16. CommunicationProtocol
Withthesharedmemoryheapallocator,youneedtocopyonly
onceinintra‑nodecommunication
Therearemoreeffectiveprotocolsforsmallmessagesizeand
largemessagesizerespectively
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17. ImmediateTransferProtocol(1)
fromAndrew'sdoctoraldissertion
Forsmallmessagesize,HMPIadoptimmediatetransferprotocol
Themessagedataislocatedwiththemessagematching
information
Thisprotocolneedsthatmessageshouldbecopiedtwice,itcan
preventthesecondcachemiss 17

18. ImmediateTransferProtocol(2)
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19. ImmediateTransferProtocolResult
Choosethethresholdof256bytesand
ifthemessagesizeisunderit,youuse
immediatetransferprotocol
Whenthemessagesizeisunderthis
threshold,theimmediateprotocol
performbetterthanMemcpy(single‑
copyprotocol)
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20. SynergisticTransferProtocol(1)
fromAndrew'sdoctoraldissertion
Forlargemessages,HMPIadoptsynergistictransferprotocolsince
bandwidthismostimportant
Notonlythereceiverbutalsothesendercopiesblocksfromsend
buffertoreceivebuffer
Useacounterandanatomicoperationforcooperation 20

21. SynergisticTransferProtocol(2)
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22. SynergisticTransferProtocolResult
Forthemessagegreaterthantwice
theblocksize,synergistictransfer
protocolwasadopted
Synergistictransferprotocolperform
betterthanMemcpy(single‑copy
protocol)intermsofbandwidth
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23. Evaluation
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24. CommunicationAnalysis(1)
ThisexperimentisconductedtoverifylesscachemissinHMPI
Readdata_buf,andcommunicateandthenreaddata_bufagain
Twokindsofexperimentsareconductedchangingwhatbufferto
useincommunication
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25. CommunicationAnalysis(2)
Icannotunderstandwhycommunicationbuffersizeisassignedas
verticalaxis...(inthecaseofCommonBuffer,communication
bufferisnotused)
ThepalecolormeanslesscachemissesinHMPI
Forsmallerdatabuffer,HMPIcauseslesscachemisses 25

26. ApplicationAnalysis(1)
MiniMDisbasedonmoleculardynamic
(MD)codeanditperformsneighbor
exchangetwiceperworkiteration
ComparedwithoriginalMPI,HMPI
achievescommunicationspeedupand
applicationspeedup
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27. ApplicationAnalysis(2)
LuleshsimulatestheSedovblastwave
problemonauniform3Dmesh
decomposedspatially
Ineachtimestep,multipleexchanges
withupto27neighborsareperformed
ComparedwithoriginalMPI,HMPI
achievescommunicationspeedupand
applicationspeedup
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28. ApplicationAnalysis(3)
ComparedwithoriginalMPI,HMPI
achieveslessL2andL3cachemissesin
MiniMDandLulesh
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29. Conclusion
ThispaperproposesHMPIwhichenablessharedmemory
communicationwithprocess‑basedapproach
HMPIsharesmemoryusingSharedMemoryHeapAllocatorand
improvesperformancewithimmediatetransferprotocoland
synergistictransferprotocol
HMPIcauseslesscashemissesthanoriginalMPI
HMPIperformsbetterinminiMDandLuleshthanoriginalMPI
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30. MyThought
ThispaperiswrittenwellandIwasabletounderstandeasilywhy
theproposedmethodisuseful
HMPIisexpectedtouseinMPIeverywheremodel,butIam
wonderingthatthemodelisbetterintermsofproductivity(code
complexity)orperformanceinthefirstplace
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