Scheduling. Scheduling. EDA421/DIT171 - Parallel and Distributed Real-Time Systems, Chalmers/GU, 2011/2012 Lecture #4 Updated March 16, 2012

Transcription

1 EDA421/DIT171 - Parallel and Disribued Real-Time Sysems, Chalmers/GU, 2011/2012 Lecure #4 Updaed March 16, 2012 Aemps o mee applicaion consrains should be done in a proacive way hrough scheduling. Schedule = resources + operaions on a ime line is used in many disciplines: (a.k.a. operaions research ) Producion pipelines ( Ford s auomoive assembly line ) Acors: workers + car pars Goal: generae schedules ha maximizes sysem hroughpu (cars per ime uni) Technique: job- and flow-shop scheduling Real-ime sysems Acors: processors, daa srucures, I/O hardware + asks Goal: generae schedules ha mee iming consrains (deadlines, periods, jier) Technique: prioriy-based ask scheduling is used in many disciplines: (a.k.a. operaions research ) Classroom scheduling Acors: classrooms, eachers, projecors + courses Goal: generae periodic schedules wihin 7-week blocks Technique: branch-and-bound algorihms Airline crew scheduling Acors: aircraf, saff + roues Goal: generae periodic schedules ha minimizes he number of aircraf and saff used and fulfill union regulaions for saff Technique: advanced branch-and-bound algorihms A scheduling algorihm generaes a schedule for a given se of asks and a cerain ype of run-ime sysem. The scheduling algorihm is implemened by a scheduler ha decides in which order he asks should be execued. Observe ha he scheduler selecs which ask should be execued nex, while he dispacher sars he execuion of he seleced ask. ask arrival scheduling dispaching preempion execuion ask erminaion 1

2 EDA421/DIT171 - Parallel and Disribued Real-Time Sysems, Chalmers/GU, 2011/2012 Lecure #4 Updaed March 16, 2012 A schedule is said o be feasible if i fulfills all applicaion consrains for a given se of asks. A se of asks is said o be schedulable if here exiss a leas one scheduling algorihm ha can generae a feasible schedule. A scheduling algorihm is said o be opimal wih respec o schedulabiliy if i can always find a feasible schedule whenever any oher scheduling algorihm can do so. A scheduling algorihm is said o be opimal wih respec o a performance meric if i can always find a schedule ha maximizes/minimizes ha meric value. consrains Examples of scheduling consrains: No processor sharing: A processor can only execue one ask a a ime This is a realisic assumpion for any processor ype being used in pracice Noe: in case of muli-core processors, each core is viewed as a separae processor No dynamic ask parallelism: A ask can only execue on one processor a a ime This is a realisic assumpion for any programming model being used in pracice consrains Examples of scheduling consrains: Non-preempive scheduling: Once sared, a ask canno be preemped by anoher ask This assumpion is no so common in prioriy-based scheduling Greedy scheduling: Once sared, a ask canno be preemped by a lower-prioriy ask This assumpion applies for all run-ime sysems used in pracice No ask migraion: A ask can only execue on one given processor, or canno change processor once i has sared is execuion This is a realisic assumpion for disribued sysems, and is also enforced for some muli-core processor designs (e.g. AUTOSAR) 2

3 EDA421/DIT171 - Parallel and Disribued Real-Time Sysems, Chalmers/GU, 2011/2012 Lecure #4 Updaed March 16, 2012 consrains consrains Non-preempive scheduling: Advanages: Muual exclusion can be auomaically guaraneed Resuls from WCET analysis correspond well wih real WCET behavior Disadvanages: Negaive effec on schedulabiliy decision akes effec only afer a ask has compleed is execuion Once a ask sars execuing, all oher asks on he same processor will be blocked unil execuion is complee Preempive scheduling: Advanages: Schedulabiliy is no negaively affeced decisions can ake effec as soon as he sysem sae changes (even in he middle of ask execuion) The capaciies of ask prioriies can be used in full Disadvanages: Muual exclusion has o be guaraneed by e.g. semaphores (or similar consrucs) WCET analysis is more complicaed since cache and pipeline conens will be affeced by a ask swich Program securiy may be compromised (hrough so-called cover channels) if full preempion is allowed consrains consrains Greedy scheduling: Example: radiional saic-prioriy scheduling (RM, DM) Once a ask sars execuing, lower-prioriy asks canno grab he processor unil execuion is complee Advanages: Scheduler relaively simple o implemen Suppored by all run-ime sysems used in pracice Disadvanages: Schedulabiliy is negaively affeced: Lower-prioriy asks can sarve and hence miss heir deadlines Fair scheduling: Example: p-fair scheduling (Baruah e al. 1995) Alhough a ask has sared execuing, lower-prioriy asks receive a guaraneed ime quanum per ime uni for execuion All asks hence make some kind of progress per ime uni Advanages: Schedulabiliy can be maximized on a muliprocessor sysem (assuming ha ask swich cos is negligible) Disadvanages: No suppored by run-ime sysems used in pracice Poor schedulabiliy when ask swich cos is non-negligible Fairness implies significanly more ask swiches han greediness 3

4 EDA421/DIT171 - Parallel and Disribued Real-Time Sysems, Chalmers/GU, 2011/2012 Lecure #4 Updaed March 16, 2012 algorihms algorihms How much an oracle is he scheduling algorihm? Myopic scheduler: algorihm only knows abou currenly ready asks. decisions are only aken whenever a new ask insance arrives or a running ask insance erminaes. Clairvoyan scheduler: algorihm knows he fuure ; ha is, i knows in advance he arrival imes of he asks. On-line clairvoyan scheduling is difficul o realize in pracice. Predicions are always hard o make. In paricular abou he fuure. (Yogi Berra) When are schedules generaed? Saic scheduling: Schedule generaed off-line before he asks becomes ready, someimes even before he sysem is in mission. Schedule consiss of a ime able, conaining explici sar and compleion imes for each ask insance, ha conrols he order of execuion a run-ime. : Schedule generaed on-line as a side effec of asks being execued, ha is, when he sysem is in mission. Ready asks are sored in a queue and receive access o he processor and shared resources a run-ime using conflicresolving mechanisms. Saic scheduling Off-line schedule generaion: Simulae dynamic scheduling Record a run-ime behavior (linear ime complexiy) Apply a search heurisic (e.g., a branch-and-bound algorihm) Find a feasible schedule (if one exiss) by considering all possible execuion scenarios (NP-complee problem) On-line schedule generaion: Mechanism for resolving conflics Prioriies possibly combined wih ime quana Feasibiliy of schedule mus be checked off-line by making predicions on how he conflics are resolved a run-ime τ 3 τ 4 15 * i * i * i * i + 15 : : : : Example: Cyclic saic schedule wih a hyper-period (period LCM) of 15. τ 3 τ Example: Schedule generaed wih rae-monoonic prioriy assignmen 4

5 EDA421/DIT171 - Parallel and Disribued Real-Time Sysems, Chalmers/GU, 2011/2012 Lecure #4 Updaed March 16, 2012 Rae-monoonic scheduling (RM): Uses saic prioriies Prioriy is deermined by ask frequency (rae) Tasks wih higher raes (i.e., shorer periods) are assigned higher prioriies Theoreically well-esablished (for single-processor sysems) Sufficien schedulabiliy es can be performed in linear ime (under cerain simplifying assumpions) Exac schedulabiliy es is an NP-complee problem RM is opimal among all scheduling algorihms ha uses saic prioriies under he assumpion ha D i = T i for all asks (shown by C. L. Liu & J. W. Layland in 1973) Deadline-monoonic scheduling (DM): Uses saic prioriies Prioriy is deermined by ask deadline Tasks wih shorer (relaive) deadlines are assigned higher prioriies Noe: RM is a special case of DM, wih D i = T i Theoreically well-esablished (for single-processor sysems) Exac schedulabiliy es is an NP-complee problem DM is opimal among all scheduling algorihms ha uses saic prioriies under he assumpion ha D i T i for all asks (shown by J. Y.-T. Leung & J. Whiehead in 1982) Earlies-deadline-firs scheduling (EDF): Uses dynamic prioriies Prioriy is deermined by how criical he process is a a given ime insan The ask whose absolue deadline is closes in ime receives he highes prioriy Theoreically well-esablished (for single-processor sysems) Exac schedulabiliy es can be performed in linear ime (under cerain simplifying assumpions) EDF is opimal among all scheduling algorihms ha uses dynamic prioriies under he assumpion ha D i = T i for all asks (shown by C. L. Liu & J. W. Layland in 1973) RM EDF Example: RM versus EDF Missed deadline τ 1:( C1 = 2, T1 = 5) τ :( C = 4, T = 7)

6 EDA421/DIT171 - Parallel and Disribued Real-Time Sysems, Chalmers/GU, 2011/2012 Lecure #4 Updaed March 16, 2012 When asks are no longer independen (i.e., hey access shared sofware/hardware objecs for which muual exclusion is enforced) he scheduler mus be exended wih special mechanisms. μ 1 R 1 τ1 τ 2 τ 3 R 1 Prioriy inversion phenomenon: normal execuion criical region H M L H blocked 1 prioriy (H) > prioriy (M) > prioriy (L) H and L share muex resource R 2 Blocking ime for H no bounded by execuion of criical region Resolving resource conflics: (while also avoiding prioriy/deadline inversion) Off-line resource scheduling: Inelligen algorihms ha are configured o generae schedules wih no need for conflic resoluion a run-ime. Examples: branch-and-bound (B&B) algorihms On-line resource access proocols: Blocking proocols using dynamic adjusmens of ask prioriies. Examples: Prioriy Inheriance Proocol, Deadline Inheriance Proocol, Prioriy Ceiling Proocol, Immediae Ceiling Prioriy Proocol, Sack Resource Policy Non-blocking proocols using rery loops. Examples: lock-free and wai-free objec sharing Prioriy Inheriance Proocol: (Sha, Rajkumar & Lehoczky, 1990) Basic idea: When a ask τ i blocks one or more higherprioriy asks, i emporarily assumes (inheris) he highes prioriy of he blocked asks. Advanage: Prevens medium-prioriy asks from preemping τ i and prolonging he blocking duraion experienced by higher-prioriy asks. Disadvanage: May deadlock: prioriy inheriance can cause deadlock Chained blocking: he highes-prioriy ask may be blocked once by every oher ask execuing on he same processor. 6

7 EDA421/DIT171 - Parallel and Disribued Real-Time Sysems, Chalmers/GU, 2011/2012 Lecure #4 Updaed March 16, 2012 Prioriy Ceiling Proocol: (Sha, Rajkumar & Lehoczky, 1990) Basic idea: Each resource is assigned a prioriy ceiling equal o he prioriy of he highes-prioriy ask ha can lock i. Then, a ask τ i is allowed o ener a criical region only if is prioriy is higher han all prioriy ceilings of he resources currenly locked by asks oher han τ i. When he ask τ i blocks one or more higher-prioriy asks, i emporarily inheris he highes prioriy of he blocked asks. Advanage: No deadlock: prioriy ceilings preven deadlocks No chained blocking: a ask can be blocked a mos he duraion of one criical region. H M L Prioriy Ceiling Proocol: normal execuion criical region R 3 M blocks on R 3 L inheris he prioriy of M prioriy (H) > prioriy (M) > prioriy (L) H sequenially accesses resources R 1 and R 2 M accesses resource R 3 L accesses resource R 3 and ness R 2 R 1 R 2 ceiling blocking R 3 R 2 R 2 R 3 H blocked because is prioriy is no higher han ceiling for R 2 L inheris he prioriy of H R 3 Disribued PCP: (Rajkumar, Sha & Lehoczky, 1988) All criical regions associaed wih he same global resource are bound o a specified synchronizaion processor. A ask migraes o he synchronizaion processor o execue he criical region (using remoe-procedure calls) Advanage: deadlock-free algorihm Disadvanage: large overhead for message-passing proocol All criical regions associaed wih he same global resource are execued a a prioriy equal o he semaphore s prioriy ceiling shor blocking imes Lock-Free and Wai-Free Objec Sharing: If several asks aemp o access a lock-free objec concurrenly, and if a subse of hese asks sop aking seps, hen one of he remaining asks complees is access in a finie number of seps. If several asks aemp o access a wai-free objec concurrenly, and if a subse of hese asks sop aking seps, hen each of he remaining asks complee heir access in a finie number of seps. 7

8 EDA421/DIT171 - Parallel and Disribued Real-Time Sysems, Chalmers/GU, 2011/2012 Lecure #4 Updaed March 16, 2012 Lock-Free Objec Sharing: (Anderson e al., 1996) Basic idea: The lock-free objec sharing scheme is implemened using rery loops. Objec accesses are implemened using esand-se or compare-and-swap insrucions ypically found in modern RISC processors. Advanage: Resource accesses are non-blocking Deadlock-free Avoids prioriy inversion Requires no kernel-level suppor Disadvanage: Poenially unbounded rery loops Wai-Free Objec Sharing: (Anderson e al., 1997) Basic idea: The wai-free objec sharing scheme is implemened using a helping sraegy where one ask helps one or more oher asks o complee an operaion. Before beginning an operaion, a ask mus announce is inenions in an announce variable. While aemping o perform is own operaions, a ask mus also help any previously-announced operaion (on is processor) o complee execuion. Advanage: Non-blocking, deadlock-free, and prioriy-inversion-free Requires no kernel-level suppor Precludes waiing dependencies among asks Non-exisence of opimal on-line shared-resource scheduler: (Mok, 1983) When here are muual exclusion consrains in a sysem, i is impossible o find an opimal on-line scheduling algorihm (unless i is clairvoyan). Complexiy of shared-resource feasibiliy es: (Mok, 1983) The problem of deciding feasibiliy for a se of periodic asks which use semaphores o enforce muual exclusion is NP-hard. 8