Xen (Virtual Machine Monitor) Operating systems laboratory Esmail asyabi- April 2015

Cloud Computing environments

Virtualization technology Consolidation Isolation Migration

virtual machines running on a Xen Project Hypervisor are known as “domains” special domain known as dom0 is responsible for controlling the hypervisor and starting other guest operating systems. These other guest operating systems are called domUs, this is because these domains are “unprivileged” in the sense they cannot control the hypervisor or start/stop other domains.

Consolidation Virtual machine monitors enable server consolidation Consolidated domains have wildly different workloads Streaming Media Server Voice-over-IP Server Application Server Web Server Database Server... Virtualized Host Guest Domain 1 Guest Domain 2 Guest Domain 3 Guest Domain 4... consolidation

Hypervisor – thin virtualization layer Driver Domain – hardware control Guest domains given restricted access Xen Architecture Hardware Driver Domain Xen Control Software Guest Domain User Software Guest Domain User Software Guest Domain User Software Xen Hypervisor

Xen Architecture

9 Scheduling I/O in Virtual Machine Monitors© 2008 Diego Ongaro Receiving a Network Packet time Hardware Driver Domain Xen Control Software Guest Domain User Software Guest Domain User Software Guest Domain User Software Xen Hypervisor Hardware Interrupt Virtual Interrupt Driver Domain Target Domain Scheduler Virtual Interrupt Packet Arrives

VM life cycle

11 Scheduling I/O in Virtual Machine Monitors© 2008 Diego Ongaro Scheduler Fairness Virtual machine monitors must fairly support both computation and I/O domains  Xen supports computation domains well, but has mixed results with I/O domains The scheduler plays a significant role in I/O performance  Improvements for I/O are not obvious  Some of Xen’s current scheduler optimizations for I/O are beneficial, others are not

12 Scheduling I/O in Virtual Machine Monitors© 2008 Diego Ongaro Credits are assigned to each domain  Approximate the fraction of processor resources each domain will receive  Do not indicate when each domain will receive its fraction Scheduler increments/decrements credits  Periodically deducts credits from running domain  Adds credits when majority of credits in the system have been consumed Xen’s Credit Scheduler

13 Scheduling I/O in Virtual Machine Monitors© 2008 Diego Ongaro Scheduler Operation Domain states  Under – domain has credits remaining  Over – domain is over its credit allowance Domains are run in FIFO order by state  Over domains only run if no under domains  A domain may run for up to 30ms if it has enough credits After running, return to the run queue by state  Behind all other domains in the same state  Regardless of runtime or remaining credits This approach is biased against I/O O2O1 Run Queue U4U2U3 Next domain to run U1

14 Scheduling I/O in Virtual Machine Monitors© 2008 Diego Ongaro Scheduler “Optimizations” for I/O I/O Domain Requirements  Low latency  High bandwidth  Independent of other domains’ workloads “Optimizations”  Boosting idle domains Included in Xen  Ordering the run queue by credits Proposed for Xen  Tickling the scheduler Included in Xen

15 Scheduling I/O in Virtual Machine Monitors© 2008 Diego Ongaro Boosting Idle Domains Initially, scheduler had no special features for I/O domains Additional scheduling state: boost  Higher priority than under  Used when an idle domain is sent a virtual interrupt With boost O2O1U1U4U2U3 I1 U5 O2O1U1U4U2U3 I1 B1 Next domain to run Next domain to run

16 Scheduling I/O in Virtual Machine Monitors© 2008 Diego Ongaro Impact of Boost Boost improves latency for I/O domains

Ordering the Run Queue I/O domains tend to quickly block Within each state, sorts domains by credits remaining Short-running I/O domains are re-inserted near the head of the run queue

18 Scheduling I/O in Virtual Machine Monitors© 2008 Diego Ongaro Impact of Ordering the Run Queue Ordering the run queue can reduce latency for I/O domains under larger loads Surprisingly, often complements boost

Project1 Disable Boost mechanism Define a QoS strategy Modify the scheduler in such a way it can support your Predefined fairness strategy.