Zwaenepoel, Willy

Infoscience - École polytechnique fédérale de Lausanne

P2P, DSM, and Other Products from the Complexity FactoryWilly ZwaenepoelEPFLImpact of ResearchNot so great? Many research ideas have lost out? Many non-research developments won outImpact of ResearchNot so great? Many research ideas have lost out? Many non-research developments won outWhy is that?? We make things too complex? Note: not: things are too complexImpact of ResearchNot so great? Many research ideas have lost out? Many non-research developments won outWhy is that?? We make things too complex? Not: things are too complexWhy?? Publishing/reviewing pushes us to complexityApologies, Caveats and ExcusesTalk is rather polemic in nature… things are said a little crasslyNow a dean – intellectual life prohibited? “There was once a dean who was so dumb, that other deans actually started noticing it”P2PPeer-to-peerNo (central) serverEasier to operate, maintain, scale, make more reliable …Started as an applicationProposed as an infrastructure for a large number of applicationsResearch on P2PConcentrated largely on DHTsLog(n) accessChord, Pastry, …Applications: backup, streaming, …The Problem with P2PVery little application other than illegal file sharingReality CheckIf we have learned anything about distributed computing over the last 25 years, it is that anything distributed is harder than anything centralizedReasons for DistributionYou cannot handle it in one place? Performance – controlled replication? Availability – controlled replicationGeographical distribution? Google!Illegality – P2P? From Napster to Gnutella, Kazaa, …? “Raw” traffic numbers are high? Much of it static? Could be handled by conventional replication (?)Difficulties for P2PHard to find anythingHard to make anything secure? Open invitation to attack? Actively used by RIAA (pollution attacks)Hard to write anythingAdvantages for P2P ResearchComplex to find anythingComplex to make anything secureComplex to write anythingAdvantages for P2P ResearchComplex to find anythingComplex to make anything secureComplex to write anythingComplexity begets papersP2P = Paper-to-PaperThere are ApplicationsLarge file multicastCan be handled by very simple techniques? BitTorrentIt should worry us that these come fromnon-research corners of the world!DSMDistributed shared memoryParallel computing on clustersDistributed memories abstracted as a single shared memoryEasier to write programsUsually by page faultingTreadMarks (ParallelTools)Reality CheckClusters are only suitable for coarse-grained parallel computationA fortiori true for DSMProblems with Fine-Grained DSMExpensive synchronizationExpensive fine-grained data sharing? Smaller than a pageFalse sharing (can be solved)True sharingAdvantages for DSM ResearchComplex fine-grain synchronizationComplex fine-grain data sharing? Compiler, language, runtime, …Complexity begets papers …TreadMarks(Almost) every paper or grant for research on fine-grain DSM was accepted(Almost) every paper or grant for research on coarse-grained DSM was rejectedIt turns out that for real applications a page is not large enough!Coarse-grain ApplicationsLarge (independent) units of computationLarge chunks of data? 1 page = 4k? Not very large at all? Page faulting brings in one page at a time? Message passing brings in whole data segment at a time (> page)Can be and was done with DSM? Increase page size (!!)? Compiler supportCompetition is Message PassingMPI (Message Passing Interface)Low abstractionNo room for complexity fabricationAs a result more successfulIt should worry us that MPI did not come from distributed systems research but fromlinear algebra!Server PerformanceAt the beginning of the Internet boom, server performance was badly laggingMultithreaded or multiprocess servers? Context switching? LockingTwo types of solutions? Exokernel? Event-driven serversEvent-Driven ServersEvents? Incoming request, i/o completion, …Single thread, event loop Event handler per event? Straight code (no blocking)? At end: nonblocking or asynchronous i/ocreate (hand-made) continuationAdvantagesNo multithreading? No context switching? No locking (at least on uniprocessor)Control over order of event handling? Not bound by OS schedulerFlashMost popular event-driven Web serverCombined multithreaded / event-drivenMany follow-onsiMimic NetworkingReality CheckIt’s too complexMaybe Ph.D.s can figure it outYour average industry programmer cannotActually, most Ph.D.s can’t eitherMany (expensive) bugsHow the Problem was SolvedLinux O(1) thread schedulerLinux futex? User-level locking? No overhead if no contentionBenefits of event-driven remainBut too small to warrant complexityHow the Problem was SolvedThe main servers are all process-based or thread-based (Apache, MySQL)It should worry us that these servers didnot come out of research!Painful Observations (1)Most of the strong research trends have not found much applicationNon-research designs have won outHas to do with this fabricated complexityPainful Observations (2)Has to do with publishing/reviewing? Simple papers tend to get rejected? Complex papers tend to get inYour Average Review FormNoveltyExcitementWritingConfidenceSome Questions to Add?Does the added functionality justify the increase in complexity?Does the performance improvement justifythe increase in complexity?Could this system be maintained by an above-average programmer in industry?Does this paper simplify a known solution to a worthwhile problem?Some Likely Review Comments« Incremental »« Engineering »« Nothing new »« Boring »It IS PossibleVirtual machinesProvide simple solutions to real problems? Server consolidation? MigrationVirtual MachinesVirtual machine monitorVMM provides a number of VMs? IBM VM? VMWare? XenOpen-sourceParavirtualization (VM ~ machine)ProvenanceDISCO: a very complex OS for SMPsVMWare: ? Simplified to Linux/Windows on one machine? Precise virtualization on x86 very complexXen? Paravirtualization to improve performance and decrease complexityVMM less complexGuest OS (slightly) more complexPerformance better (?)The Way of All TechnologyAll technology? Becomes more complex on the inside? Becomes less complex on the outsideExample: car, Windows (?!)Not sure it fully applies to software? Most complex systems ever built? Rare example of discrete complex system? Maybe we are over the limit alreadyNonethelessSuccess = interfaces defined early?Very successful systems? Apache, MySQL, MPI, VMWare, Xen? Interfaces stable (few iterations)? Internal complexity grewLess successful systems? DSM, event-driven? Interfaces unstable, complexifiedStandardization (!?)I am afraid some of it is necessaryFind a way through publishing systemOther People’s AdviceLampson: « Keep it simple »? True, but somewhat impracticalEinstein: « Everything should be as simple as possible, but no more thanthat »? Implement functionality at the right interface? Keep interfaces stableLessonsBrute force often (not always) worksOur publishing and reviewing system pushes us in the opposite directionMore LessonsIt is the interface, stupidThe implementation can be complexThe interface has to be simple and stableNYT, June 26, 2006Thank you

P2P, DSM, and Other Products from the Complexity Factory

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P2P, DSM, and Other Products from the Complexity Factory

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Infoscience - École polytechnique fédérale de Lausanne