Waysofmeasures
Therearemanywaystoimproveparallelismincomputers.Intermsofthebasicidea,theycanbesummarizedintothefollowingthreeways:
(1)Timeoverlap.Adjacentprocessesarestaggeredintime,andeachpartofthesamesetofhardwareisusedalternately.
(2)Duplicateresources.Repeatedlysethardwareresourcestoimprovereliabilityandperformance.
(3)Resourcesharing.Allowmultipleuserstousethesamesetofresourcesinturninacertaintimesequencetoimproveresourceutilization.
Essence
TheessenceofconcurrencyisthataphysicalCPU(ormultiplephysicalCPUs)ismultiplexedbetweenseveralprograms.Concurrencyistoforcelimitedphysicalresources.Multi-usersharingtoimproveefficiency.
Oneofthekeystoimplementingconcurrencytechnologyishowtoswitchbetweenmultipleactivities(processes)inthesystem.
Parallelismreferstotheoccurrenceoftwoormoreeventsoractivitiesatthesametime.Inamulti-programenvironment,parallelismenablesmultipleprogramstobeexecutedsimultaneouslyondifferentCPUsatthesametime.
Levelclassification
Parallelismcanusuallybedividedinto4levels:joblevelorprogramlevel,tasklevelorprogramlevel,inter-instructionlevelandinstructioninternallevel.Thefirsttwolevelsarecoarse-grainedlevels,alsoknownasprocesslevels;thelattertwoarefine-grainedlevels,alsoknownasinstructionlevels.