Timeslice Donation in Component-Based Systems

Transcription

1 Department of omputer Science Institute of Systems rchitecture Operating Systems Group Timeslice Donation in omponent-based Systems Udo Steinberg, lexander Böttcher, Bernhard Kauer

2 omponent-based System: NOV VM VM VM guest mode VMM VMM VMM pplications Partition Manager Device Drivers User Microhypervisor Kernel host mode Udo Steinberg Timeslice Donation in omponent-based Systems 2

3 omponent-based System: NOV VM VM VM guest mode VMM VMM VMM Multiple components operating on shared resources and interacting with each other through IP. pplications Partition Manager Device Drivers User Microhypervisor Kernel host mode Udo Steinberg Timeslice Donation in omponent-based Systems 3

4 Synchronous IP (Example: L4) c request S s During IP, client donates its timeslice to the server. Kernel switches from client to server S without changing the current timeslice. c response S s Effect is priority inheritance, but only until S is preempted If the kernel fails to recognize the dependency between and S after preemption, S will consume its own timeslice s instead of c. Udo Steinberg Timeslice Donation in omponent-based Systems 4

5 Problem #1: lassic Priority Inversion S Shared Resource Udo Steinberg Timeslice Donation in omponent-based Systems 5

6 Problem #1: lassic Priority Inversion L S Shared Resource Udo Steinberg Timeslice Donation in omponent-based Systems 6

7 Problem #1: lassic Priority Inversion H High-priority thread H blocked by low-priority thread L holding S. L Unbounded priority inversion if M prevents L from running and thus from releasing S. S Shared Resource Our Solution: Priority Inheritance Server inherits priority of its clients for the duration of their requests. Kernel tracks dependencies. Udo Steinberg Timeslice Donation in omponent-based Systems 7

8 Problem #1: lassic Priority Inversion M H High-priority thread H blocked by low-priority thread L holding S. L Unbounded priority inversion if M prevents L from running and thus from releasing S. S Shared Resource Our Solution: Priority Inheritance Server inherits priority of its clients for the duration of their requests. Kernel tracks dependencies. Udo Steinberg Timeslice Donation in omponent-based Systems 8

9 Problem #2: Time Quantum Depleted L H Thread L exhausts its time quantum while holding S. Blocks other threads, e.g. H, from acquiring the resource until time quantum has been replenished. Priority boosting will not help. S Shared Resource Our Solution: Bandwidth Inheritance lients donate not only their priority, but the entire timeslice (priority, time quantum,...) Udo Steinberg Timeslice Donation in omponent-based Systems 9

10 Old lgorithm for Dependency Tracking* U B V D E W Update required when: New node added or new link established Node removed or existing link broken Priority of a node changes X Inconsistent state until dependency tree has been fully updated. Z * Presented at ERTS 05 Udo Steinberg Timeslice Donation in omponent-based Systems 10

11 Old lgorithm for Dependency Tracking* U V D E W Update required when: New node added or new link established Node removed or existing link broken Priority of a node changes X Inconsistent state until dependency tree has been fully updated. Z * Presented at ERTS 05 Udo Steinberg Timeslice Donation in omponent-based Systems 11

12 Old lgorithm for Dependency Tracking* U V D E W Update required when: New node added or new link established Node removed or existing link broken Priority of a node changes X Inconsistent state until dependency tree has been fully updated. Z * Presented at ERTS 05 Udo Steinberg Timeslice Donation in omponent-based Systems 12

13 Old lgorithm for Dependency Tracking* U V D E W Update required when: New node added or new link established Node removed or existing link broken Priority of a node changes X Inconsistent state until dependency tree has been fully updated. Z * Presented at ERTS 05 Udo Steinberg Timeslice Donation in omponent-based Systems 13

14 Old lgorithm for Dependency Tracking* U V D E W Update required when: New node added or new link established Node removed or existing link broken Priority of a node changes X Inconsistent state until dependency tree has been fully updated. Z * Presented at ERTS 05 Udo Steinberg Timeslice Donation in omponent-based Systems 14

15 New Requirements Dependency updates must be preemptible operations Prevents DoS attacks via updates to long call chains ccounting Dependency tracking must be accounted to the client that initiated the request Move expensive tracking operations from the IP path into slower paths (scheduler) Udo Steinberg Timeslice Donation in omponent-based Systems 15

16 Improved Dependency Tracking H T T Helping Donation Donation S S Donation: Kernel follows explicit link from to S Both mechanisms are transitive. Helping: H retries its operation; switches to S if rendezvous fails Udo Steinberg Timeslice Donation in omponent-based Systems 16

17 Blocked Resource Holder B D Z Udo Steinberg Timeslice Donation in omponent-based Systems 17

18 Blocked Resource Holder B D Initially Z is running consumes highest-priority timeslice B When Z blocks B added to list of timeslices blocked on Z Z b When scheduler selects D,, These timeslices become blocked on Z as well Udo Steinberg Timeslice Donation in omponent-based Systems 18

19 Blocked Resource Holder B D Initially Z is running consumes highest-priority timeslice B When Z blocks B added to list of timeslices blocked on Z Z b d When scheduler selects D,, These timeslices become blocked on Z as well Udo Steinberg Timeslice Donation in omponent-based Systems 19

20 Blocked Resource Holder B D Initially Z is running consumes highest-priority timeslice B When Z blocks B added to list of timeslices blocked on Z Z b d c When scheduler selects D,, These timeslices become blocked on Z as well Udo Steinberg Timeslice Donation in omponent-based Systems 20

21 Blocked Resource Holder B D Initially Z is running consumes highest-priority timeslice B When Z blocks B added to list of timeslices blocked on Z Z b d c a When scheduler selects D,, These timeslices become blocked on Z as well Udo Steinberg Timeslice Donation in omponent-based Systems 21

22 Staggered Wakeup B D Z b d c a Udo Steinberg Timeslice Donation in omponent-based Systems 22

23 Staggered Wakeup d c a B D Z unblocks Highest-priority timeslice B added back to runqueue. Other timeslices are not released yet and remain linked to B. Z When B is removed from the runqueue, next timeslice D is released. Udo Steinberg Timeslice Donation in omponent-based Systems 23

24 Staggered Wakeup D c a Z unblocks Highest-priority timeslice B added back to runqueue. Other timeslices are not released yet and remain linked to B. Z When B is removed from the runqueue, next timeslice D is released. Udo Steinberg Timeslice Donation in omponent-based Systems 24

25 Direct Switching B D an the kernel switch directly from Z back to without scheduler invocation? Kernel must know if incoming link of current timeslice and reply link are the same. Z Donation ounter: =2, B=1, =0, D=0 ount number of consecutive donation links during traversal Per-PU donation counter indicates how often kernel can switch back directly. ounter update is the only overhead added to IP Udo Steinberg Timeslice Donation in omponent-based Systems 25

26 Livelock Detection Z Udo Steinberg Timeslice Donation in omponent-based Systems 26

27 Livelock Detection calls ; calls Z Deadlock occurs if Z calls back On most systems the deadlock will go unnoticed, freezing,, and Z. Z Helping mechanism will turn the deadlock into a livelock Per-PU helping counter tracks number of helping links for the current timeslice. Livelock detected if helping counter exceeds # of threads. Udo Steinberg Timeslice Donation in omponent-based Systems 27

28 onclusion Timeslice Donation mechanism in NOV facilitates: Priority Inheritance Bandwidth Inheritance Minimal Overhead: IP: Donation counter update (1 dd/sub Operation) Donation Link: Pointer Dereference (1 ache Miss) Helping Link: Thread Switch Dependency tracking is a preemptible operation that is accounted to the thread that initiated the IP request. Udo Steinberg Timeslice Donation in omponent-based Systems 28