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Microkernels, virtualization, exokernels

Microkernels, virtualization, exokernels. Tutorial 1 – CSC469. Monolithic kernel vs Microkernel. Monolithic OS kernel. Application. System call. What was the main idea? What were the problems?. User mode. VFS. Microkernel. IPC, file system. Application IPC. Unix server. Device

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Microkernels, virtualization, exokernels

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  1. Microkernels, virtualization, exokernels Tutorial 1 – CSC469

  2. Monolithic kernel vs Microkernel Monolithic OS kernel Application System call • What was the main idea? • What were the problems? User mode VFS Microkernel IPC, file system Application IPC Unix server Device driver File server Scheduler, virtual memory Kernel mode Device drivers, dispatcher … IPC, virtual memory Hardware Hardware

  3. Exokernels • Motivation • OSes hide machine info behind abstractions (processes, files, address spaces, IPC) • These abstractions are hardcoded => restrictive • Idea • Separate protection from management • Application-level (untrusted) resource management • VM, IPC implemented at application level • Library OSes implement all the abstractions • Basically, it’s a minimal kernel that multiplexes securely the hardware resources

  4. Virtual Machine Monitors (VMM) • Definitions: • A VMM is a hardware virtualization technique that allows multiple operating systems, termed guests, to run concurrently on a host computer • A VMM is a software layer that runs on a host platform and provides an abstraction of a complete computer system to higher-level software. • Also called Hypervisor

  5. VMM types • Type 1: run directly on the host's hardware • Type 2: run within a conventional operating system environment VMWare workstation, VirtualPC, User-Mode Linux, UMLinux Disco, VMWare ESX Server, Xen

  6. Disco • Goals • Extend modern OS to run efficiently on shared memory multiprocessors without large changes to the OS • VMM can run multiple copies of Silicon Graphics IRIX operating system on a Stanford Flash shared memory multiprocessor

  7. Problem • Commodity OS's not well-suited for ccNUMA (1997) • Do not scale: Lock contention, memory architecture • Do not isolate/contain faults: more processors => more failures • Customized operating systems • Take time to build, lag hardware • Cost a lot of money

  8. Solution • Add a virtual machine monitor (VMM) • Commodity OSesrun in their own virtual machines (VMs) • Communicate through distributed protocols • VMM uses global policies to manage resources • Moves memory between VMs to avoid paging • Schedules virtual processors to balance load

  9. Advantages • Scalability • Flexibility • Hide NUMA effect • Fault Containment • Compatibility with legacy applications

  10. Disco

  11. VM challenges • Overheads • Instruction execution, exception processing, I/O • Memory • Code and data of hosted operating systems • Replicated buffer caches • Resource management • Lack of information • Idle loop, lock busy-waiting • Page usage • Communication and sharing • Not really a problem anymore because of distributed protocols

  12. Disco interface • VCPUs provide abstraction of a MIPS R10000 processor • Emulates all instructions, the MMU, trap architecture • Enabling/disabling interrupts, accessing privileged registers -> Memory-based interface to VMM • Physical memory • Contiguous address space, starting at address 0 • Physical-to-machine address translation, second (software) TLB

  13. Disco interface (cont’d) • I/O devices • Each VM assumes exclusive access to I/O devices • Virtual devices exclusive to VM • Physical devices multiplexed between virtual ones • Special interface to SCSI disks and network devices • Interpose on DMA calls • Disk: • Set of virtualized disks to be mounted by VMs • Copy-on-write disks; for persistent disks, uses NFS • Network: • Virtual subnet across all virtual machines • Uses copy-on-write mappings => reduces copying, allows sharing

  14. Xen virtualization • Technically, two kinds • Paravirtualization • Guests run a modified OS • High performance on x86 • Hardware-assisted virtualization • CPUs that support virtualization • Unmodified guest OSes

  15. Xen infrastructure

  16. How does it compare to Disco? • Three main differences • Less complete virtualization • Domain0 to initialize/manage VMs, incl. to set policies • Strong performance isolation • Other • Interface is pretty close to hardware and enables low-overhead high-performance virtualization • Need to change more OS code than in Disco • All the cool details • “Xen and the art of virtualization” – SOSP’03

  17. Questions • What’s the difference between a hypervisor and an exokernel?

  18. Questions • What about an exokernel and a microkernel? • Performance? • What about fault isolation?

  19. Questions • What is the difference between a hypervisor (VMM) and a microkernel?

  20. Questions • Can a microkernel be used to implement a hypervisor?

  21. Questions • Can a hypervisor be used to implement a microkernel?

  22. Hypervisors for servers • Type 1 or Type 2? • Hyper-V: “MicroKernelized” Hypervisor Design • VMWare ESX Server: “Monolithic” hypervisor architecture

  23. Hypervisor design Monolithic Hypervisor Microkernel Hypervisor VM1 (Admin) VM2 VMn VM1 (Admin) VM2 VMn Virt Stack Hypervisor Virtualization Stack Drivers Drivers Drivers Drivers Hypervisor Hardware Hardware • Both true Type 1 hypervisors – no host OS • The hardware is the physical machine; OSs are all virtual

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