V also not true in pay as you go sengs 19 assumpons

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Unformatted text preview: from 1/3 to 1 quickly à increases the average progress score. v  Ex: network, and disk I/O. v  Needless specula�on can reduce throughput. v  Also not true in “pay-­‐as-­‐you-­‐go” se�ngs. 19 Assump�ons Invalidated 20 Assump�ons Invalidated v  Example: 30% of reducers finish, while the others are v  The assump�on that a straggler’s likelihood depend on s�ll in the copy phase. its progress score. v  Average progress score = (0.3).(1) + (0.7).(1/3) ~= 53% v  New and fast tasks can be chosen over old and slow tasks. v  The threshold jumped from (33 – 20 = 13%) to (53 – 20 = v  It should also consider progress rate. 33%) à all the remaining 70% of the reducers are considered for specula�on. à task slots will be filled from an arbitrary selec�on. v  Average progress score limit is 100% à Threshold = 80%. à Tasks with progress score > 80% can never be considered for specula�on. v  True stragglers may have not been chosen. v  Network overloaded with unnecessary copying. 21 LATE Scheduler 22 LATE Scheduler v  Goal: reduce the MR job response �me. v  Determining which nodes are slow: v  A node’s total performed work = sum of progress scores v  Approach: for both completed and in-­‐progress tasks. 1.  Priori�ze the specula�ve tasks based on their es�mated v  If total < SlowNodeThreshold à it is a straggler. finish �me. v  To reduce resources cost: 2.  Schedule the tasks on fast nodes (avoid stragglers). v  specula�veCap: a cap on the number of running v  Es�ma�ng a task’s finish �me: specula�ve tasks at a �me. v  Progress rate = progressScore / T v  Use the SlowTaskThreshold as an addi�onal condi�on for running specula�ve tasks. v  Remaining �me = (1 – progressScore) / progressRate v  If there are only fast tasks that are running à no need to v  Assump�on: the task’s progress rate is constant (may not speculate them. hold). 23 24 4 9/17/13 LATE Scheduler LATE Scheduler v  Algorithm: v  Se�ng parameters: (by prac�ce) v  When a node asks for a task v  Specula�veCap = 10% of available task slots. v  slowNodeThreshold = 25th percen�le of node progress. v  If the number of running specula�ve tasks < specula�veCap v  If the node’s total progress > slowNodeThreshold v  slowTaskThreshold = 25th percen�le of task progress rates. v  Rank running tasks (that are not currently speculated, and have run for more than 1 minute) by es�mated remaining �me v  Choose the highest-­‐ranked task with progress rate < slowTaskThreshold, then send it the asking node. v  Note: it does not consider data locality such as in Hadoop. 25 LATE Scheduler 26 Incorrect Es�mated Times -­‐ Example v  Some incorrect es�mated �mes: v  The es�mated remaining �me for a task was based on the progress rate: (1 – ProgressScore) / ProgressRate. v  This assumed constant progress rate per task, whic...
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