RunaheadRunaheadProcessorProcessorFinale Finale DoshiDoshiRavi PalakodetyRavi PalakodetyOutlineOutlineMotivationMotivationHigh Level DescriptionHigh Level DescriptionMicroarchitectureMicroarchitectureResultsResultsConclusionsConclusionsWhere We Left Off…Where We Left Off…Lab 3 Lab 3 ––Building a 4Building a 4--stage pipelined stage pipelined SMIPS processorSMIPS processorCritical Path Critical Path ––LoadLoad--ααFetch Fetch ÆÆDecode Decode ÆÆExecute Execute ÆÆReadDataCacheReadDataCacheÆÆWritebackWritebackData Cache Miss?Data Cache Miss?Stall until data returns from Main MemoryStall until data returns from Main MemoryA A BaaadBaaadExampleExampleLdLd--αα, , LdLd--ββ, Ld, Ld--γγ, , ……If latency = 100 cycles from main If latency = 100 cycles from main memory to cache, then:memory to cache, then:Initiate Initiate LdLd--ααrequestrequestStall for 100 cyclesStall for 100 cyclesInitiate Initiate LdLd--ββrequestrequestStall for 100 cyclesStall for 100 cyclesAnd so onAnd so on……Key InsightKey Insight““RunaheadRunahead” to see whether there are ” to see whether there are memory accesses in the near futurememory accesses in the near futureWith an instruction sequence With an instruction sequence LdLd--αα, , LdLd--ββInitiate memory request for Initiate memory request for LdLd--ααContinue executionContinue executionInitiate memory request for Initiate memory request for LdLd--ββOutlineOutlineMotivationMotivationHigh Level DescriptionHigh Level DescriptionMicroarchitectureMicroarchitectureResultsResultsConclusionsConclusionsDataCacheDataCacheMiss Occurs…Miss Occurs…Backup the register fileBackup the register fileKeep running instructionsKeep running instructionsUse Use INVINVas the result of any ops that:as the result of any ops that:Are Are DataCacheDataCachemissesmissesDepend on calculations involving Depend on calculations involving DataCacheDataCachemissesmissesData Returns..Data Returns..Cache is updated from Cache is updated from MainMemMainMem::Restore the register fileRestore the register fileRerun the original “offending” instructionRerun the original “offending” instructionFollow the RulesFollow the RulesDo NOTDo NOTUpdate the Update the DataCacheDataCachewhile in while in RunaheadRunaheadmodemodeInitiate Memory Requests that depend on Initiate Memory Requests that depend on INVINVaddressesaddressesBranch when predicate depends on Branch when predicate depends on INVINVdatadataInitiate Memory Requests that cause Initiate Memory Requests that cause collisions in collisions in DataCacheDataCacheOutlineOutlineMotivationMotivationHigh Level DescriptionHigh Level DescriptionMicroarchitectureMicroarchitectureResultsResultsConclusionsConclusionsProcessor SideProcessor SideCache SideCache SideExecution Execution --Enter Enter RunaheadRunaheadExecution Execution --In In RunaheadRunaheadExecution Execution --Exit Exit RunaheadRunaheadDesign ExplorationsDesign ExplorationsStore Cache OptimizationStore Cache OptimizationDecisions when to exit Decisions when to exit runaheadrunaheadStore CacheStore CacheLdLd--αα, St, St--ββ, Ld, Ld--ββRather than return Rather than return LdLd--ββas as INVINV, return , return the value that was just stored.the value that was just stored.Use 4Use 4--entry table, as in Branch Predictorentry table, as in Branch PredictorWhen to Exit When to Exit RunaheadRunahead??When the “offending” miss returns? ORWhen the “offending” miss returns? ORWhen all memory requests that are When all memory requests that are currently incurrently in--flight are processed?flight are processed?OutlineOutlineMotivationMotivationHigh Level DescriptionHigh Level DescriptionMicroarchitectureMicroarchitectureResultsResultsConclusionsConclusionsKey ParametersKey ParametersVary Latency of Main MemoryVary Latency of Main MemoryAs the latency increases, the impact of As the latency increases, the impact of runaheadrunaheadbecomes more significantbecomes more significantAt small latencies, the penalty for At small latencies, the penalty for entering/exiting entering/exiting runaheadrunaheadcan reduce can reduce performanceperformanceKey ParametersKey ParametersVary Size of Vary Size of FIFOsFIFOsAs the As the FIFOsFIFOsget larger, the processor is get larger, the processor is able to run further ahead and generate more able to run further ahead and generate more parallel memory requests.parallel memory requests.As the As the FIFOsFIFOsget larger, the penalty for get larger, the penalty for exiting exiting runaheadrunaheadbecomes more severe.becomes more severe.Testing StrategyTesting StrategyLatencies of 1, 20, and 100 cyclesLatencies of 1, 20, and 100 cyclesFifosFifosof length 2, 5, 8, 15of length 2, 5, 8, 15Standard benchmarks; focus on Standard benchmarks; focus on vvaddvvaddWe’ll focus on length 15 We’ll focus on length 15 fifosfifoshere since here since they allowed for the most extensive they allowed for the most extensive runaheadrunahead..ResultsResultsResultsResultsResultsResultsOutlineOutlineMotivationMotivationHigh Level DescriptionHigh Level DescriptionMicroarchitectureMicroarchitectureResultsResultsConclusionsConclusionsConclusionsConclusionsRunaheadRunaheadis good.is good.ConclusionsConclusionsRunaheadRunaheadis a cheap and simple way to is a cheap and simple way to improve IPS.improve IPS.The enter/exit The enter/exit runaheadrunaheadpenalty is small penalty is small enough that the IPS is always enough that the IPS is always comparable to the Lab 3 processor.comparable to the Lab 3 processor.The control structure is (fairly) The control structure is (fairly) straightforward, with most improvements straightforward, with most improvements done on the cache side.done on the cache side.ExtensionsExtensionsAggressive Branch PredictionAggressive Branch PredictionDon’t stall when branch predicate is Don’t stall when branch predicate is INVINVSave valid Save valid runaheadrunaheadcomputationscomputationsAggressive Aggressive
View Full Document
Unlocking...