V if it is less than a threshold more likely to

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Unformatted text preview: l a�er dom2, the I/O requests can be processed by a single scheduling. à Reduce context switch overhead. 37 Sor�ng Example 38 MRG Scheduler v  Possible problem of deferring dom0: v  A group of VMs are not reques�ng any I/O. v  Causes delay to the other I/O pending VMs. v  Solu�on: v  Predict the I/O request likelihood of a VM. v  It is based on running �me of its last scheduling. v  If it is less than a threshold -­‐> more likely to request I/O next scheduling. Figure 5 from [2] 39 MRG Scheduler 40 Hierarchical Scheduling v  To improve fairness: v  First level: v  The CPU �me allocated for a VM depends on its credit v  Allocate credits for groups: and its MR group credit. v  The weight of a group is the sum of each of its VMs credits. v  It provides fair CPU �me to each group, and then to each v  User allocated credits. VM belonging to a group à hierarchical scheduling. v  VM credits are normalized as v  The scheduler keeps track of the running �me of each group Cg × (Ci / ∑ C j ) v  Second Level: v  Within each group’s allo�ed �me, only VMs of this group can run. v  Allocate CPU to to VMs in a group using their credits. v  à tasks in the same group will progress around the same rate. 41 42 7 9/17/13 Example For Group 1: CPU alloca�on for the group: 2/6 of overall CPU �me. Within group sharing ra�o: 2:1. CPU alloca�on for VMs: 2/9 and 1/9 of overall CPU �me. (2/3).(2/6)=(4/18)=(2/9) Hierarchical Scheduling v  Possible problem: v  It is possible that all the VMs in a group are blocked in the wai�ng list a�er reques�ng too many I/O. v  à Wasted CPU �me. v  Solu�on: v  A �meout counter is associated with each group. v  Start the �meout when all VMs within the group are blocked. v  If the �meout expires and they are s�ll all blocked à the group yields the CPU to the next group. Figure 6 from [2] 43 Se�ng the Timeout Counter 44 Se�ng the Timeout Counter v  Credit-­‐remaining ra�o (CRR) for each group: v  Se�ng it too low: v  à The scheduler switches between groups quickly. CRR = v  à Pending I/O are not given a chance to complete. ∑VM _ remaining _ credit Total _ group _ credit v  The frac�on of CPU credits remaining for the group. v  Affects group-­‐level fairness. v  If CRR is close to 1 à the group is far from using all its credits. While if it is close to zero à the group has used most of its credits. v  Se�ng it too large à the system is underu�lized. v  tw = wakeup latency between the �me when all VMs were v  The scheduler must select it to balance these trade-­‐offs. blocked and the �me when any of them was boosted. (ini�alized with 1 ms). v  The �meout: t = tw . CRR 45 46 Hierarchical Scheduling v  Addi�onal extensions: Third Approach v  The current running group is selected as the group of the VM in the head of the running queue. v  The scheduler must only select VMs from the gr...
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