Combining Parallel and Sequential Workloads on a Network of Workstations

CombiningParallelandSequentialWorkloadsonaNetworkofWorkstationsRemzi Arpaci, Amin Vahdat, Thomas Anderson, andDavid PattersonJune 29, 1994AbstractThis pap er examines the plausibil ity of using a network of workstations (NOW) for a mixture of parallel and sequentialjobs.Through trace-driven simulation,our study identi es a numb er of results that should b e of interest to NOW systemdesigners.First, it is not sucient to use workstation resources to provide a sup ercomputer only by night.Next, parallelprograms can cause a signi cantnumb er of lengthy delays to interactiveusers.Finally,simple schedulingtechniquescanidentifyavailableworkstationsandminimizeuserdelayswhileprovidingparallelprogramp erformancecomparableto adedicated massively parallel pro cessor.If these scheduling p olicies are employed, parallel programmers and interactive userscan p eacefully co exist on a NOW.1Intro ductionRapid improvement in workstation p erformance has resulted in widespread interest in using networks of worksta-tions (NOWs) for parallel pro cessing [Gelernter 1985, Kronenb erg et al. 1986, Carriero & Gelernter 1989, Sunderam1990, Blumrich et al.1994]. Becauseof the economies of scale, building parallel computers from mass-pro ducedhardwareandsoftwcomp onentscanb emorecoste ectivethanbuilding thesystemfrom scratch.Thoughmassively parallel pro cessor (MPP) interconnects to day have b etter p erformance than lo cal area networks (LANs),emerging standards such as ATM [Biagioni et al.1993] and the repackaging of MPP interconnects as LANs [Cohenet al.1993] makes NOWs more attractive, even for communication intensive parallel programs.Todate,NOWshaveb eenusedonlyforbatch-styleprogrammingonadedicatedclusterofworkstations,although idle machines representa substantial untapp edresource.As a consequence,parallel programmers andinteractive users often use logically similar but physically distinct computing platforms, leaving b oth systems idlemuch of the time.Two reasons account for this distinction in computing platforms.First, scheduling sequentialandparalleljobsonasingleplatform isdicultb ecauseprogramsmustb eco-scheduled[Ousterhout1982,Guptaetal.1991,Feitelson&Rudolph1992]toachieveacceptablecommunicationp erformance.Co-schedulingensuresthataparallel jobsimultaneously runsoneachof itsassignedpro cessors,allowing thejob'scommunication requeststob eservicedwithoutacontextswitch.Second,theinteractiveresp onsetimesthathave made p ersonal computing p opular may b e compromised if computing resourcesare shared with demandingapplications.As an example, many users of SRC's distributed load balancing system,dp, were found p erio dicallytapping their keyb oards to keep their workstation from b eing harvested by the system.This pap erexamines the plausibility of using a NOW for a mixture of parallel and sequential jobs.The goalof ourresearchis to build a NOW that runsparallel programs with thesame p erformance as a dedicatedMPPand runs sequential programs with the same p erformance as a dedicated unipro cessor.To ensure that interactiveresp onsetimeispreserved,paralleljobsarerunonlyonidleworkstations.Tosimplify theimplementation ofco-scheduling, parallel jobs are scheduled only when there are enough idle idle workstations to run the entire job.Thus, a 32-no de parallel job will not run unless there are at least 32 idle workstations in the cluster.As a resultof these two basic assumptions, active sequential users are not sloed by parallel programs, and parallel programscan b e co-scheduled to achieve acceptable p erformance.Currently,NOWsdonotsimultaneouslyexecutesequentialandparallelworkloadsb ecauseofanumb erlogistical diculties. First, the metric used to identify idle workstations must b e accurate, otherwise b oth sequentialand parallel jobs are signi cantly slowed.Next, users must wait for the eviction of parallel jobs and the restorationof their workstation state up on return to their machine: these delays can b e quite lengthy and b othersome.Finally,1