三星+計算存儲的建模分析-Modeling Analytics for Computational Storage

ModelingAnalyticsfor

ComputationalStorage

VeronicaLagrange

MemorySolutionsLab

SamsungSemiconductor,Inc.

SanJose,U.S.A.

veronica.l@

Harry(Huan)Li

MemorySolutionsLab

SamsungSemiconductor,Inc.

SanJose,U.S.A.

harry.li@

AnahitaShayesteh

MemorySolutionsLab

SamsungSemiconductor,Inc.

SanJose,U.S.A.

Abstract

Nextgenerationflashstoragewillbearmedwithasubstantialamountofcomputingpower.Inthis

paper,weinvestigateopportunitiestoutilizethiscomputationalcapabilitytooptimizeOnlineAnalytical

Processing(OLAP)applications.WehavedirectedouranalysisattheperformanceofasubsetofTPC-DS

queriesusingApacheHadoopclustersandtwodatabaseengines,ApacheSPARK-SQLandPresto1.We

modeltheexpectedspeed-upachievedbyoffloadingafewoperationsthatareexecutedfirstwithin

mostSQLplans.Offloadingtheseoperationsrequiresminimalcooperationfromthedatabaseengine,

andnochangestotheexistingplan.Weshowthatthespeed-upachievedvariessignificantlyamong

queriesandbetweenengines,andthatthequeriesbenefitingthemostareI/Oheavywithhighselectivity

ofthe“needleinthehaystack”variety.Ourmaincontributionisestimatingthespeed-upanticipated

frompushingtheexecutionofafewkeySQLbuildingblocks(scan,filter,andprojectoperations)to

computationalstoragewhenusingreadoptimized,columnarApacheParquetformatfiles2.

CCSConcepts

?Computingmethodologies→Modelingandsimulation→Modeldevelopmentandanalysis→Modelverificationandvalidation;

?Hardware→Communicationhardware,interfacesandstorage→Externalstorage;

?Informationsystems→Datamanagementsystems→Databasemanagementsystemengines→Databasequeryprocessing→Queryplanning;

?Informationsystems→Datamanagementsystems→Databasemanagementsystemengines→Onlineanalyticalprocessingengines;

1PrestoisaregisteredtrademarkofFacebook,Inc.

2AnearlierversionofthisreportwillappearintheProceedingsofICPE2020.

ModelingAnalyticsforComputationalStorage2

Keywords

ColumnarDatabase,Parquet,SQL,SmartStorage,acceleration,offloading,TPC-DS,Spark,Presto,OLAP

1Introduction

Currentdevelopmentsin“bigdata”storagesolutionsgeartowardsmovingdataprocessingcloserto

wherethedataresides,reducingunnecessarymovementandspeedingupdataprocessingconsiderably.

Computationalstorageisanemergingtrendwhereacomparativelylargeamountofdataprocessing

occursinsidethestoragelayer.Examplesofnewdevicesexposingflashstorageinternalcomputing

powerincludeSamsung’sSmartSSD[1],NGDSystems[2],andScaleFlux[3].Thisnewfunctionalitysignals

performanceimprovementopportunitiesforI/Oheavyworkloadscontainingoperationsamenableto

beingcompletednearthestoragesource.Oneofthemostcriticaltypesofdatabaseanalytics–OLAP–

wellexemplifiesthistypeofopportunity.ItistypicallyveryI/Ointensiveandcontainsquiteafewbuilding

blocksthatmaybeseamlesslymovedto,orexecutedby,acomputationalstoragedevice.

Offloadingisnotanewconcept.Networkprocessors,GPUsandrecentlymachinelearningspecialized

processorsarewidelyusedtoacceleratespecificcomputekernelswhilefreeingCPUresources.Wewill

showthattheoffloadingofmanymoretime-consumingoperationsfromthehostCPUtostorageimproves

bothworkloadperformanceandsystemefficiency.Theimmediatebenefit,ofcourse,isasizeabledecrease

inI/Ovolume.ThisreductioninI/Oleadstolesshostresourceutilization,whichnotonlyimproves

performanceofindividualqueries,butalsoincreasesservercapacity.Besidesdatabaseoperations,other

frequentoperationsthatcanbeexecutednearthestoragedeviceincludeencryptionandcompression.

Databaseanalyticsworkloadsareespeciallyread-intensive.ItisnotuncommonforI/Oreadstotake90%

ormoreofthetotalexecutiontime.OffloadingsomeofthattostoragereducesI/Obandwidthalong

withotherhostresourceusage,andmayimproveperformanceconsiderably.Furthermore,SSDshavean

internalbandwidththatismuchhigherthanthatwhichisexposedtothehostcomputerthroughexisting

channels(SAS,SATA,PCI-E,etc.)[4],whichmeansthatcomputationalstoragehasalargeamountof

untappedpotentialtoexploit.

Thispaperdiscussestheexpectedperformancebenefitsofoffloadingsomeimportantbasicdatabase

operations–namelyScan,FilterandProject–tocomputationalstorage.Weevaluatetheperformance

estimatemodelusingTPC-DSworkloadandtwodatabaseenginesrunningonHadoopclusters:SPARK-

SQLandPresto.

Thispaperisorganizedasfollows:aftercoveringpreviouscomputationalstoragedatabaseoffloading

work,weexplaintheOLAPworkloadselection,andtheconfigurationofourtwoclusters.InSection

IVwediveintoTPC-DScharacteristicsandexaminetheoverallperformancefromrunningonthetwo

Hadoopclusters,whichhavebeenthefocusofourexperimentation.InSectionV,weexplainourmodeling

methodologies,andinSectionVIwedescribeandanalyzeresultsfromthatmodeling.Specifically,we

showhowasubstantialspeed-upfromcomputationalstorageoptimizationcandependonmultiple

factors.Finally,webrieflydiscussotherSQLbuildingblocksamenabletocomputationalstorage

pushdowns,andconclude.

ModelingAnalyticsforComputationalStorage3

2PreviousWork

MostpreviousworkonpushingSQLfunctionsdowntocomputationalstorageconcentrateonspecific

functionsofaspecificDatabaseEngine.Summarizer[5]modifiestheexistingNVMecommandinterface

toimplementfouroperations:initializevariablesorsetqueries;readdataandexecutecomputation;read

dataandfilter–theselectioncase;andthetransferoftheoutputresultstothehost.Fromtheircase

studies,using3TPC-Hqueriesandaverysmallscalefactor(100MB–0.1SF),wedeterminedthatthey

coulddosimilarityjoinsaswell.Theycomparedifferentdegreesofcomputationoffloadingforthesethree

queries.Theauthorsshowthatsomewhatcomplexcomputationscanbecarriedoutnearstorage,and

brieflydiscussthedataintegrationproblem:howtocombinedatafromdifferentformatsandsources.

Theyconcentrateononespecificintegrationproblem:similarityjoin,anddescribetheheuristicstheyuse.

Leftunansweredisthebiggerissueonhowtointegratetrulydistinctformats.

YourSQL[6]isbasedonMariaDB.YourSQLallowsforcomplexqueryoperationstobeoffloadedtoasmart

SSDintheformofanISCtask.Thatpaperspendsthebulkofitstimetalkingaboutoptimizerheuristics.

Oneveryinterestingobservation,fromtheauthors’performanceanalysisisthatwhileyourtypicalSQL

application–OLTPorOLAP–cannotexhaustanNVMebandwidth,itsnear-storageimplementationcan.

Biscuit[7]iswhatYourSQLusestoenableitscomputationalstorageoperations.Itprovidestheuser

applicationwithC++APIs.Theuser’sSSD-sideC++programwithBiscuitAPIs,calledanSSDlet,isloaded

inthedevice.Ahost-sideprograminvokesandcoordinatesexecutionoftheSSDlettasksusinglibsisc;

communicationisdonebylinkinginputandoutputportstospecifictasks.Heretheyalsoclaimthatthe

APIsusedtoaccessfilesarenearlyidenticaltostandardlibraries.

ExtraV[8]isIBM’seffortatcomputationalstorageforgraphprocessingbasedontheirCAPRI[9].This

paperdescribesanFPGAprototypethatexecutescommongraphtraversalfunctionsnearthedevice.It

workslikevirtualmemoryforgraphapplications,asitprovidesthehostwiththeillusionthattheentire

graphlivesinmemory,whileitisactuallypartlystoredandcompressedinanSSD.Theauthorshavestated

thatgraphprocessingismostlydoneinmemory,eitherinsingleserversorclusters,andthatitcannotbe

doneefficientlywhengraphsgrowbeyondtheavailablememory.

PG-Strom[10]isanacceleratorforPostgreSQLthatoffloadspartoftheSQLworkloadtoaGPU.Supports

JoinsandAggregates.However,bythetimeofthatpublication[10]alldatafedtotheGPUcamefrom

mainmemory(notstorage).

NeteezawasthefirstsuccessfulproducttouseFPGAsascomputationalstoragecomputingaccelerators

foranalyticsdataengines.Itdoesnotrequireanysoftwareinstallationortuning.Justplugandplay.

NeteezadatabaseengineisbasedonPostgres[11],andimplementsfourfunctionsinitsFPGAengine:

Compress,Project,RestrictandVisibility.Francisco[12]claimsthatNeteeza’senginedecompresses

dataatwirespeed.ProjectandRestrictoperationsfilteroutcolumnsandrows,respectively,basedon

theparametersintheSELECTandWHEREclausesofaquery.TheNeteezaVisibilityengineisfocusedon

databaseintegrity,andtherein,filtersoutrowsthatshouldnotbeseenbythequery,suchasanyrows

beinginsertedbyatransactionthathasnotyetcommitted.

ModelingAnalyticsforComputationalStorage4

ComputationalstoragehasalsoattractedinterestbeyondSQLanddatabaseapplications.Forexample,

REGISTOR[24]isanFPGAplatformapplyingregexsearch,on-the-fly,toanyfilebeingtransferredfroman

SSDtothehost;INSIDER[25],alsoanFPGA-baseddrivecontroller,exposesavirtualfilesystemwithem-

beddedprogrammability,allowingprogrammerstopushdownoperationscustomizedtotheapplication’s

specificneeds.

3WorkloadandSetup

Here,weexplaintheTPC-DSbenchmark,aswellasthetwoclusterconfigurationsusedintheexperi-

mentsdescribed.Moreover,wedescribethetwodatabaseengines(SPARK-SQLandPresto),andexplain

therationalebehindusingtheParquetfileformattooffloadSQLoperationstocomputationalstorage.

3.1TPC-DS

“TheTPCBenchmarkDS(TPC-DS)isadecisionsupportbenchmarkthatmodelsseveralgenerallyappli-

cableaspectsofadecisionsupportsystem”[13].

TPC-DScontains24tables,organizedasasnowflakeschema.Itcontains6verylargeFACTtables,and

manysmallDIMENSIONtables.FurthermoreTPC-DSiscomprisedof99queries,eachonerepresenting

adifferentbusinessquestion.So,eventhoughthisisanartificialbenchmark,ittriestomirrorreal-life

applications.Schemaisscalable,withthesmallestbeing1GBandthelargest100TB.The1GBdatasetis

usedforQAonly.PerformanceismeasuredinQueriesperHour@ScaleFactor(QphDS@SF),andmust

includemultipletests(pertainingtopower,throughput,anddatamaintenance).Inthisstudy,wecon-

siderasubsetofthepowertest.ForamoredetailedexplanationoftheTPC-DSbenchmark,wereferthe

readerto[14].

TPC-DShasbeenaroundsince2007,butdidnotcatchupuntilrecentlyandafteramajorre-write,with

thefirstpublishedofficialreportdatedMarch2018(Cisco)[15].AsofJanuary2020,thereareonlysix

officialreportspublished.Nonetheless,subsetsofTPC-DSareheavilyusedinformallybytheindustryto

demonstrateupandcomingtrends[16][17].TPC-DSisoneofmanyTransactionProcessingPerformance

Council(TPC)benchmarks[18],andassuchcoversenoughgeneralOLAPcasestobeusefultopracti-

tioners.

BecauseFACTtablesareordersofmagnitudelargerthanDIMENSIONtables,wewillgravitatetowards

queriesthatare“FACTtableScanheavy,”asopposedtoqueriesthatare“DIMENSIONtableScanheavy.”

ModelingAnalyticsforComputationalStorage5

3.2TestConfiguration

Twoclustersareusedinthispaper,andsincetheyareconfiguredtorunSPARK-SQLandPresto,werefer

tothemsimplyastheSPARK-SQLclusterandthePrestocluster.Eachhaseightdatanodeswithdiffer-

enthardware.ThedetailedconfigurationislistedinTableI.BothenginesuseApacheHiveMetadata,

andtheParquetfileformat.

SPARK-SQLisApacheSpark’shigh-leveltoolforstructureddataprocessing[19].Itisanin-memory,

distributed,RDBMSthatunderstandsSQLandaDatasetAPI(availableinJavaandScala).User

applicationsinterfacewithSPARK-SQLviaacommand-linemodule,JDBCorODBC.SPARK-SQLalso

supportsreadingandwritingdatastoredinanexistingApacheHiveinstallation.

Spark-SQL

Presto

DataNode

Hardware

CPU

Intel(R)Xeon(R)

Gold6152CPU@

2.10GHz

Intel(R)Xeon(R)

CPUE5-2699v4@

2.20GHz

Memory

256GB

256GBto1024GB

LocalStorage

2xNVMeSSD3.2TB

3xNVMeSSD1.6TB

SoftwareStack

OS

LinuxKernel4.13.0

LinuxKernel4.x.x

SPARK-SQL/Presto

2.3.0

0.205

Hadoop

2.7.3

2.9.0

Hive

1.2.1000

1.2.2

HDFSReplication

1

TPC-DS

ScaleFactor

10000

StorageFormat

Paquet

Table1-ClusterConfiguration

PrestoisadistributedSQLqueryenginedesignedtoquerylargedatasetsdistributedoveroneormore

heterogeneousdatasources[20].PrestoprovidesaCLIinterface,andqueryprocessing(parser,planner,

scheduler),butwillusedataandmetadataprovidedbyothersoftwarecomponents(ApacheHBase,

ApacheHive,MySQL,etc.).Prestointeractswiththeseothercomponentsviaconnectors,andthisisits

claimtofameasitispossibletocombinemultiple,differentdatasourcesintoonequeryseamlessly.

ThereisnoneedforveryexpensiveETL(Export-Transform-Load)datasetsinordertoanalyzethem.

Similartoclassicmassivelyparallelprocessing(MPP)DBMS[21],Prestoisadistributedsystemthatruns

onacluster.PrestoclientsubmitsSQLstatementstoamasterdaemoncoordinator.Usingmetadata

ModelingAnalyticsforComputationalStorage6

fromconnectors,thecoordinatorparsesthequery,generatestheplan,andthenschedulesandcoordi-

nateshowitisexecutedbytheworkers.Workersgetdatafromconnectors,executeassignedtasks,and

deliverresultstotheclient.Allprocessinghappensinmemory,anddataispipelinedacrossthenetwork

betweendifferentstages.

ParquetisanopensourcecolumnarfileformatthatwasdesignedtobeusedwithOLAPsystems[22].

TheParquetfileformatisREADoptimized,asinsertsorupdatescanbeexpensiveoperations.Itwas

inspiredbythe“Dremel”paper[23],andisextensivelyusedintheHadoopecosystem.Furthermore,each

Parquetdiskfilecontainsthetable’sschema.Thisfeatureresolvestheissueofthedevicebeingaware

ofthetablemetadata,arequirementforanycomputationalstorageprocessing.Furthermore,existing

Parquetreadersarecapableofprojectingandfilteringcertaindatatypesusingstatisticsprovidedin

metadata.Implementingsomefunctionalityinacomputationalstoragedeviceiscomplementaryandin

additiontotheexistingpushdowncapabilitiesofParquet.

TPC-DSqueriesaredownloadedfromtheTPCwebsiteandresultswereverifiedagainstsampleoutput

fromtheTPC.Allqueriesrunsequentiallyasasingletestjob.Beforeeachquery,thememorycacheis

cleared.Inaddition,

?SPARK-SQLisrestartedbeforeeveryquery

?Prestoisrestartedbeforethejob

4TPC-DSCharacterization

Inthissection,wediscussthemanystages(orfragments)oftheexecutionplangeneratedbythequery

optimizer.Next,weshowSPARK-SQLandPrestoqueryruntimeresultsforTPC-DS.Welistthemsideby

sidetoshowthattheybehavedifferentlyinordertoillustrateandexplainthedifferentspeed-upsthat

onemightseeforthesamequeryexecutedwithdifferentengines.Next,weexaminetheconceptofScan

Ratio,andhowweuseittocharacterizeandrankqueries.

4.1TypicalTPC-DSqueryplan

SQLqueryplansarecomposedofbasicbuildingblocks.Theyformanexecutiontree.Eachbuildingblock

typicallyfocusesononespecificoperation,andisscheduledbyaSQLengine.Howthesebuildingblocks

areassembleddictatesqueryperformance.Mainbuildingblocksinclude:Scan,Filter,Project,Aggregate,

Sort,Join,Merge,Union.Figure1(A)illustratesatypicalquerysequence,withbuildingblocksbeingexe-

cutedfromtoptobottom.Figure1(B)isthebuildingblocksequencecreatedbytheSPARK-SQLplanner

forTPC-DSQuery44.

ModelingAnalyticsforComputationalStorage7

(A)GenericSQLqueryplan(B)SPARK-SQLplanforQuery44

Figure1-SQLqueryplans

Thefunctionalityofsomebuildingblocksincludes:

?Scan:Readdatabasecontentfromstoragetocomputehostmemoryandapplyanyneededtransformations

?Filter:Filtertablerowsinmemorywithgivingcriteria

?Project:Selecttablecolumnsinmemory

?Join:Combinetwotablesbasedongivencriteria

ModelingAnalyticsforComputationalStorage8

QueryRuntime(sec.)

Figure2-TPC-DSruntimequerycomparison

4.2Performanceofallqueries

Figure2showstheruntimeforallTPC-DSqueriesforSPARK-SQLandPresto.With10TBdataset,SPARK-

SQLcompletes91andPrestocompletes61queries.Bothdatabaseenginesstoreallintermediateresults

inmemory,andthequeriesthatfailedincurredan“out-of-memory”error.Thequeryruntimehasawide

rangefromlessthanaminutetomanyhours.Wehavenotmatchedourclusterhardwareconfigurations

forSPARK-SQLandPresto,asitisnotourgoaltocomparetheperformancebetweenthem.Thepointof

thispaperistoshowcaseasubsetofthemanydifferentsystemparametersinfluencingthepotentially

substantialspeed-upaffordedbycomputationalstoragedevices.Wedemonstratethateventhough

computationalstoragecanprovideimpressivespeed-ups,thebenefitsvarysignificantlydependingon

manyotherparameterssuchastablesize,selectivity,queryplan,etc.

Inthefollowingsections,wewillselectfivequeriesfromeachclusterbasedonsystemcharacterization

ofthequeriesandpotentialoffloadbenefits,andprovidefurtheranalysisofeach.

4.3ScanRatio

ScanRatioisdefinedasthetotalCPUtimespentonadatabaseScanoperation,dividedbytotalCPU

timeconsumedbythequery.TheCPUtimeisreportedbyqueryplannerfromdatabaseengine.Thistime

isnotthewallclocktimeandshouldnotbeconfusedwithqueryruntime.

ForTPC-DSqueries,theScanRatiorangesfromnear0%to~93%onSPARK-SQLanduptonearly100%

forPresto.InFigure3,queriesaresortedbytheirScanRatio,fromlefttoright.Q9,withthehighestScan

ModelingAnalyticsforComputationalStorage9

Ratio,isfurthesttotheright.NoticethatmostCPUintensivequerieshaveasmallScanRatio,butnot

all.Somecomplexqueries,suchasQ44,arebothcomputeandI/Ointensive.

HighScanRatiodoesnotnecessarymeanthequeryreadsmoredatafromstorage,itonlyindicatesthat

timespentonI/Oishigherrelativetootherqueryoperations.Forexample,Query45hasatotaldisk

readof~1.3TB,itsScanRatioisonly2.99%.ButforQuery9,whichhasthehighestScanRatioof~93%,

totaldiskreadisonly~105GB.Althoughthetotalqueryruntimedifferenceisnotlarge(Q9,212.36sec,

Q45,176.02sec.),theCPUcyclesspentonnon-I/OoperationscausedtheScanRatiotobelowerforQ45.

AhighScanRatioindicatesthataqueryisastrongcandidateforcomputationalstorageoptimization,

sinceitsI/Ooperationsarelikelytobeinitscriticalpath,whilealowScanRatioindicatesthatopera-

tionsotherthanI/Oarethebottleneck.

5OffloadingModel

Here,weexplainhowweselectedeachplanstagetobeoffloadedtocomputationalstorage,followed

byadetaileddescriptionofthemodelmethodologyusedwithbothdatabaseengines.Noticethatthe

methodsaresomewhatdifferent,whichwechosetodoinordertocovermoreaspectsoftheoffloading

process.

ModelingAnalyticsforComputationalStorage10

Figure3-TPC-DSScanRatioandCPUutilization

5.1Offloadingcomponents(orkernels)

Inthissection,weexploitopportunitiestooffloadoperationsfromhosttocomputationalstorage.In

ordertoexecuteaquery,dataflowsfromtheleavesoftheplantotheroot.Usuallytheleavescontain

someformofSCANoperation:tablerowsandcolumnsarereadin(usuallyfromdisk,unlessthisdata

waspreviouslycached).TheSCANoperationusuallyincludessomesortofdatatransformation,from

theformatondisktotheoneunderstoodbythedatabaseengine.Onceatableisscanned(orsometimes

whilethetableisbeingscanned),rowsmaybefilteredorprojected.Nextplanstepsmaycontainaggre-

gates,sorts,joins,windowfunctions,orotheradvanceddatatransformations.Operationsneartheleaves

willgenerallybe“easier”topushdowntocomputationalstorage.BasicSCANs,FILTERs,andPROJEC-

TIONsmayhappenwithvirtuallynochangetothedatabaseenginequeryplan.Moreaggressivepush

downoptimizationsarepossible,butrequirethecooperationofthedatabaseengine,andre-factoringof

thequeryplan.

Forexample,inFigure1(B),weobservethispatterninbothFACTtableandDIMENSIONtableI/O.By

combining“Scan,”“Filter”and“Project”intoanewbuildingblock,wecanestimatetheperformance

benefitofoffloadingthisnewbuildingblock(“Scan/Filter”)tocomputationalstorage.Regardless,with

“Scan/Filter”offloading,theSPARK-SQLplanforQuery44stilllooksthesame.

ModelingAnalyticsforComputationalStorage11

5.2SPARK-SQLmodelmethodology

TheperformanceestimatemodelforSPARK-SQLisbasedonhowthedatabaseengineplanisexecuted

–instageswithdependencies.Weassumethereisnoresourcelimitationonthenumberofstagesthat

canbeexecutedconcurrently.

Forexample,Figure4showsagenericquerythatinvolves3tables,1DIMENSIONtableand2FACT

tables.Stage-0readsthecontentoftheDIMENSIONtable,whilereadingFACTtableshappensinStage-1

andStage-2.Then,Stage-3and4sorttheresultsfromStage-1and2.Theresultsaresubsequently

passedtoStage-5forthefinalJoinoperation.

Figure4-QueryStageScheduling

First3stages(0,1and2)includeScan/Filter/ProjectoperationsasmarkedwithlightdotshadeinFigure

4.Thetimespentontheoperationsare1,5and8secondsrespectively,andcouldbeoffloadedtocomputationalstorage.Theoffloadedexecutiontimeiscalculatedas:

?Reserve1secondforoffloading-relatedhandshaking.Thereservedtimeisanarbitrarynumber.

?AssumesthattheFilterruntimeonthedeviceisatwirespeedandcanbeomitted.Thisisanopti-misticassumptionthatprovidesanupperboundforouranalysis.TheactualFilterruntimedependsoncompute/IOcapabilitiesofthedevice,andcanbefurtherimprovedwithpre-processinginthedevice.

?Time-of-resultdatatransferbetweenthedeviceandthehostascalculatedbasedonthedeviceReadbandwidthspecification;inthispaper,3GB/sechasbeenused.

ModelingAnalyticsforComputationalStorage12

Withtheseassumptions,theexampleexecutiontimecanbereducedfrom18secondsto12seconds

(seeFigure5).

Figure5-SPARK-SQLOffloadModel

Asthemostfundamentalstepinbuildingtheestimatemodel,weneedtoknowthetimespentforScan/

Filter/ProjectoneachSPARK-SQLquerystage.Fortunately,withSPARK-SQLthelogfileprovidesthe

followingkeylogginginformation(Figure6):

?MeasWClock:TheStagewallclockruntime

?ThrTime:Totalexecutiontimeforthestagefromallexecutionthreads.Thisisnotwallclocktime

?ThrTime:TheexecutiontimebreakdownforScan,Filter,Project

Withtheaboveinformation,theestimatedtimespentonScan/Filter/Projectcanbecalculatedas

ThrTime

EstWClockTime=MeasWClockTime

ThrTime

InadditiontoScantime,wealsoconsiderthefollowing:

WClockTime-Thetimetoinitializecomputationalstorageforoffloading.Wealwaysas-

sumeonesecondfortheestimationcalculation.

WClockTime-Thetimerequiredtotransfertheresultsfromtheoffloadingdevice

backtothehost.ItiscalculatedbasedtheReadbandwidthofthecomputationaldevice.Inourmodel,

theFilteredresultisusuallylessthan0.5%oftheresultsthatareunfiltered.Itwouldtakeonlyafrac-

tionofasecondtoreadbacktothehost,thereforeweignoreditthistime.

WithParquetformat,weassumethatnoProjectoperationorProjecttimeisomitted.

Withtheaboveassumption,theestimatedstageruntimewithoffloadingforSPARK-SQLiscalculatedas:

EstWClockTime=WClockTime+EstWClockTime

ModelingAnalyticsforComputationalStorage13

Figure6-OneSPARK-SQLQueryStagewithStatistics

5.3PrestoModelmethodology

TomodelpushdownbenefitsofScan/Filter/Projectoperations,wecreateandpopulatesmallertables

wecall“modeltables.”These“modeltables”containonlytherowsandcolumnsthatwouldbeselected

byacomputationalstorageengineexecutingtheScan/Filter/Projectoperationsdefinedbythequery.

Werepeatthequeryusingthemodeltable,andcompareresultsagainstthesamequeryusingthe

originaltables–seeFigure7.ForPresto,bothoriginalandmodelqueriesgeneratethesamequeryplan.

SimilartoourSPARK-SQLmodel,theperformancedifferenceistheupperboundofthespeed-upthata

computationalstoragedevicewouldyield,becausethismodelassumesthatthestoragedevicewould

becapableoffilteringandprojectingrowsandcolumnsatwirespeed.However,ifwetakeintoconsid-

erationthehigherinternalflashstoragebandwidth[4],thisisarealisticapproximationoftheexpected

speed-up.

ModelingAnalyticsforComputationalStorage14

Figure7-PrestoOffloadModel.

6OffloadingEvaluation

Here,wedescribeindetailthequeryselectionprocess,andgiveahigh-levelviewoftheresultsobtained

bythemodelingofbothdatabaseengines.Furthermore,wepresentside-by-sideanalysisoftheexpect-

edspeed-upforafewselectedqueries.

ModelingAnalyticsforComputationalStorage15

6.1Thequeries

Inthisstudy,wepickedfivequeriesfromeachconfigurationfordeepanalysis.Thequerieswereselected

basedonwheretheyfallonthedifferentquadrantsoftheScanRatioversusaCPUutilizationchart(see

Figure8)tocoverawiderrangeofcharacteristics.BecausewefocusonoffloadingScan/Filter/Project,

wewantqueriesthatareI/OintensiveandshowhighselectivitywhenfilteringandprojectingFACT

tables.Thatis,welookforqueriesofthe“needleinthehaystack”variety.Threeofthequeries(Q9,Q44,

andQ75)arefoundinbothstudies,whiletheothertwoarefoundexclusivelyineitherSPARK-SQLor

Presto.Wechosethisapproachbecause,duetotheirdifferentarchitectureandoptimizer,interesting

queriesinoneenvironmentarenotnecessarilyinteresting,orpossible,intheother.Forexample,Presto

cannotexecuteQ4(out-of-memoryerror).

UsingthechartinFigure8,weselectedthefollowingfiveSPARK-SQLqueriesforanalysis:Queries9and

44havehighScan/Filterratio;Query4hashighCPUutilization;Query72hasthelongest