Task Scheduling of Real- Time Media Processing with Hardware-Assisted Virtualization Heikki Holopainen

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1 Task Scheduling of Real- Time Media Processing with Hardware-Assisted Virtualization Heikki Holopainen Aalto University School of Electrical Engineering Degree Programme in Communications Engineering Supervisor: Prof. Raimo Kantola Advisor: M.Sc. Joakim Haldin Master s thesis presentation

2 Table of contents Motivation and objectives Background Virtualization Real-time processing Task scheduling Research methods Prototype system Results Conclusions Future work Questions? 2

3 Motivation and objectives Telecommunication service providers are shifting towards Virtualized Network Functions (VNF) that are run in virtual machines on standard general purpose hardware. Objectives: To find out how virtualization affects the performance and characteristics of a network function offering speech transcoding. Evaluate performance of different task scheduling algorithm combinations with the Virtualized Network Function (VNF) offering speech transcoding. Furthermore, different vcpu related configurations are measured. 3

4 Background In digital telecommunication networks speech is transported as packets, typically with 5-30 ms frame time lengths. Transcoding is required when speech communication happens between two networks that are adopting different speech coding standards. Quality of service degrades with packet loss and delays. Multitasking and task scheduling are required to process multiple calls simultaneously. 4

5 Real-time tasks and processing Real-time processing adds timing constraints to tasks in the form of deadlines Hard (deadlines cannot be missed and consequences of missing a deadline can be fatal e.g. avionics, cars, airbags etc.) Soft (deadline can be missed, but the utility of the computed result decreases e.g. media processing, phone calls etc.) Tasks are split into jobs Attributes Release time, start time, finishing time, deadline, execution time, response time, lateness Release patterns (periodic, sporadic and aperiodic) HARD real-time SOFT real-time 5

6 Linux scheduling algorithms Real-time class schedulers First In First Out (FIFO) SCHED_FIFO Default Linux real-time priority scheduler In Linux tasks are executed from highest priority to lowest priority (99 priority levels) Task in each priority queue executes until completion, after which next task is selected Separate run queue for each priority Round-Robin (RR) SCHED_RR SCHED_FIFO with time slices and a circular run queue 6

7 Linux scheduling algorithms Fair-class schedulers CFS (Completely Fair Scheduler) Each task is allocated a share of the processor according to its priority (nice value) CFS Implements two scheduling policies (+ SCHED_IDLE) SCHED_OTHER (The default scheduling policy for fair scheduling class tasks) SCHED_BATCH (Scheduling policy for batch style processes) Uses red-black tree instead of priority queues and introduces virtual runtime (vruntime) 7

8 CFS and vruntime Vruntime determines how long the process has got to execute on a CPU Task with the smallest vruntime is always scheduled next Always the left most task in rbtree After execution task is placed back into the run queue according to its vruntime Vruntime of an executing task increases monotonically after each scheduler tick 8

9 Virtualization Definition: Create a virtual version of (a computing resource or facility). [1] Virtualization provides the means to separate the hardware from the Operating System (OS), and allows multiple virtual operating system instances to run on the same physical hardware. Virtual Machines (VM) are hosted on a software called a Virtual Machine Monitor (VMM), which imitates the dedicated physical hardware and manages the physical system resources between the VMs. [1] 4 9

Scheduling presents problems in virtualization Semantic gap & transparency issues Independent scheduling on both layers

10 Virtualization challenges Full virtualization presents an identical Interface of a physical machine to the virtual machines Virtual machine doesn t know it s virtualized. Unmodified (guest) OS can be used Black box to the VMM. Scheduling presents problems in virtualization Semantic gap & transparency issues Independent scheduling on both layers (host and the guest) Scarce or no information about the workload characteristics Virtualization types Hardware-assisted full virtualization 10

Virtualization overheads and hardware assistance Hardware-assisted virtualization related operating mode changes VMM intervention Emulation of privileged & sensitive

I/O VT-D, Intel Virtualization Technology for Directed I/O (IOMMU) CPU Enables device pass-through, SR-IOV, etc.

11 Virtualization overheads and hardware assistance Hardware-assisted virtualization related operating mode changes VMM intervention Emulation of privileged & sensitive instructions VM Exits & Entries Interrupts cause a lot of VM exits and entries I/O intensive workloads suffer the most I/O, interrupt, memory and CPU related overheads Solutions I/O VT-D, Intel Virtualization Technology for Directed I/O (IOMMU) CPU Enables device pass-through, SR-IOV, etc. Intel Virtualization Technology (Intel VT) VT-x VMX, Virtual Machine extensions Interrupts APICv Memory Second Level Address Translation (Extended Page Tables) IOMMU, VT-D Intel VMX (mode changes) Instruction types Interrupt deliveryin virtualization 11

Dual scheduling problem Lock-holder pre-emption Lock holding vcpu gets pre-empted

prolonged synchronization latencies vcpu stacking Lock holding vcpu gets scheduled

latencies Multiple solutions proposed, but many of the solutions present new

12 Dual scheduling problem Lock-holder pre-emption Lock holding vcpu gets pre-empted Lock waiting vcpu cannot progress before lock holder releases the lock Leads to prolonged synchronization latencies vcpu stacking Lock holding vcpu gets scheduled on same run queue after the lock waiting vcpu Leads to increased synchronization latencies Multiple solutions proposed, but many of the solutions present new problems Co-scheduling problems Priority inversion CPU fragmentation Relaxed co-scheduling 12

100-350 encodings per core One thread per call Packets every 20 ms Timestamps from three points of processing for each

13 Research methods Prototype system Two computers (Intel x86) Physical call starter element Virtualized encoder element Gigabit network Virtualization VMM/Hypervisor: KVM + QEMU VT-d, VT-x Multiple simultaneous encodings AMR & AMR-WB codecs encodings per core One thread per call Packets every 20 ms Timestamps from three points of processing for each packet NIC (Hardware timestamp) Before encoding had started After encoding had finished Virtual Machine configurations Measurement setup 13

14 Call encoder and timestamping 14

15 Measurements Different combinations of default Linux scheduling algorithms vcpus scheduled with FIFO, RR, CFS (OTHER & BATCH) Encoded call threads scheduled with CFS and FIFO Measured cases vcpu scheduler effects on guest scheduling Performance of different scheduling algorithm combinations Pinned vs unpinned vcpus Single vs multiple vcpus 15

16 Results - vcpu scheduler effects on guest scheduling Bare metal 4 pcpu FIFO scheduled 4 vcpus CFS scheduled 4 vcpu 525 AMR-WB call encodings 16

17 Results - performance of different scheduling algorithm combinations Results for CFS scheduled call threads Fair schedulers performed better 173

18 Results - performance of different scheduling algorithm combinations Measurements for FIFO scheduled call threads Fair schedulers performed better as vcpu schedulers 18

19 Results - pinned vs unpinned vcpus - Mitigated task (vcpu) migrations - Preserved cache locality - Better response times - Lower packet loss 1 vcpu 3 vcpus 19

20 Results - single vs multiple vcpus Increased synchronization latencies 20

21 Conclusions vcpu scheduler effects on guest scheduling Scheduling algorithm used to schedule the vcpu threads is visible in the guest scheduling characteristics Performance of different combinations CFS (SCHED_OTHER) scheduled vcpu threads gave the best overall performance with FIFO scheduled call threads. Pinned vs unpinned vcpus Pinning benefitted the system as a whole Single vs multiple vcpus With single vcpu used vcpu scheduling algorithm was not as important as with multiple vcpus. No linear performance increase, more overhead due to increased synchronization latencies inside the guest OS 21

22 Future work More scheduler combinations vcpu scheduling specific schedulers More complex real-time schedulers Testing with different parameters Scheduler related parameters Hypervisors related parameters Effects of interrupt delivery Better quality of service analysis methods PESQ 22

23 Thank You Questions?

Lecture Topics. Announcements. Today: Advanced Scheduling (Stallings, chapter ) Next: Deadlock (Stallings, chapter

Lecture Topics. Announcements. Today: Advanced Scheduling (Stallings, chapter ) Next: Deadlock (Stallings, chapter Lecture Topics Today: Advanced Scheduling (Stallings, chapter 10.1-10.4) Next: Deadlock (Stallings, chapter 6.1-6.6) 1 Announcements Exam #2 returned today Self-Study Exercise #10 Project #8 (due 11/16)