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Xen and the Art of Virtualization
 

Xen and the Art of Virtualization

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    Xen and the Art of Virtualization Xen and the Art of Virtualization Presentation Transcript

    • Xen and the Art of Virtualization Paul Barham et. al. University of Cambridge Computer Laboratory Presentor: Devdatta Kulkarni
    • Outline
      • Overview of Xen’s approach of virtualization
      • Detailed Design of Xen
      • Experimentation and Evaluation
      • Discussion
    • Problem Statement
      • Current approaches to virtualization have performance problems
      • Goal: Provide performance as close as possible to non-virtualized system setup
      • Solution: Use paravirtualization by selectively exposing real hardware
    • Xen’s Virtualization Approach
      • Full Virtualization is a performance evil
        • Advantages of exposing real hardware
          • Bettter support time-sensitive tasks with real and virtual times
          • Performance improvement by using superpages or page coloring
      • Provide an abstraction that is “similar” to underlying hardware
    • Implications of Providing “Similar” and not “Exact” Hardware abstraction
      • Changes required to guest OSes
      • No changes are required to guest applications
    • Terminology Used
      • Guest OS: OSes that Xen can host
      • Domain: Running virtual machine within which a guest OS executes
      • Xen: Hypervisor
    • Virtual Machine Interface
      • Memory Management
      • CPU
      • Device I/O
    • Memory Management Interface
      • Guest OSes responsible for managing h/w page tables
      • Xen restricted to 64 MB section of every address space preventing TLB flush while entering hypervisor
    • CPU Interface
      • Xen executes in ring 0 : Most privileged mode
      • Guest OSes execute in ring 1
      • Page Fault Handling
        • Write the faulting address into Extended Stack frame on the guest OS stack
      • System Calls: Fast Exception Handlers bypassing Xen
        • Double Faulting code is terminated by Xen
    • Device I/O
      • Data transferred from each Domain via Xen using shared-memory, asynchronous buffer descriptor rings
      • Light weight event delivery mechanism which involves “callbacks” in the guest OSes
    • Xen System Architecture
    • Control Transfer: Hypercalls and Events
      • Hypercalls: similar to system calls
      • Events: similar to Unix signals
        • Event-callback handler invoked by Xen
    • Data Transfer: I/O Rings
      • Guiding Principles:
        • Resource management/accountability
        • Event notification
      • I/O Descriptor Rings:
        • Both XEN and Guest OS act as producers and consumers for the data
    • I/O rings Structure
    • Subsystem Virtualization
      • CPU Scheduling
        • Borrowed Virtual Time (BVT) scheduling
          • Low latency wakeup
      • Time and Timers
        • Real time, virtual time and wall-clock time
    • Virtual Address Translation
      • Register Guest OS page tables directly with MMU
      • Restrict guest OS to read only access
      • Type and reference count associated with each machine page frame
      • A page frame can be reused only when unpinned and its reference count is zero
      • Use batch updates in a single hypercall
        • Commit updates before TLB flush
    • Physical Memory
      • Statically partitioned between domains
        • XenoLinux uses balloon driver to adjust the memory
      • Physical Memory
        • Virtual, contiguous
      • Hardware Memory
        • Actual hardware memory, sparse
      • Physical to Hardware : Done by guest OS
      • Hardware to Physical : Done by Xen
        • Using a shared translation array
    • Network
      • Xen provides abstrations for
        • Virtual firewall-router (VFR)
        • Virtual network interfaces
          • 2 I/O rings of buffer descriptors
          • Each direction has associated rule
            • (<pattern>, <action>)
        • Transmission: Guest OS enqueues buffer descriptor onto transmit ring
        • Receiption: Guest OS exchanges unused page frame for each packet it receives
    • Disk
      • Domains access storage through virtual block devices (VBDs)
      • VBD
        • Ownership and access control information
        • Accessed via I/O ring mechanism
        • Requests can be reordered by guest OS and also by Xen
      • Translation table maintained in Xen
      • Batching done by Xen to improve performance
    • Evaluation
      • Entities used in experiments
        • XenoLinux port (based on Linux 2.4.21)
        • Vmware Workstation 3.2 running on top of Linux host OS
        • User-mode Linux (UML) on Linux host
      • Experimental Setup
          • Dell 2650 dual processor 2.4GHz Xeon server with 2GB RAM
          • Broadcom Tigon 3 Gigabit Ethernet NIC
          • Single Hitachi DK 32EJ 146GB 10k RMP SCSI disk
    • Relative Performance
    • Operating System Benchmarks Process Related Performance
    • Operating System Benchmarks Context Switch Performance
    • Operating System Benchmarks File and VM system latencies
    • Network Performance
    • Concurrent Virtual Machines
    • Concurrent Virtual Machines
    • Scalability
    • Summary
      • Xen shows performance results close to native Linux system
      • Improved performance achieved at the cost of
        • Modifying OS code
    • Discussion Questions
      • Xen or VMware ?
        • Performance
        • Virtualization support in hardware
        • CPUs not having multiple privilege execution rings