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OS Structures - Xen

OS Structures - Xen. Xen. Key points. Goal: extensibility akin to SPIN and Exokernel goals Main difference: support running several commodity operating systems on the same hardware simultaneously without sacrificing performance or functionality Why? Application mobility

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OS Structures - Xen

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  1. OS Structures - Xen

  2. Xen

  3. Key points • Goal: extensibility akin to SPIN and Exokernel goals • Main difference: support running several commodity operating systems on the same hardware simultaneously without sacrificing performance or functionality • Why? • Application mobility • Server consolidation • Co-located hosting facilities • Distributed web services • ….

  4. Old idea • VM 370 • Virtualization for binary support for legacy apps • Why resurgence today? • Companies want a share of everybody’s pie • IBM zSeries “mainframes” support virtualization for server consolidation • Enables billing and performance isolation while hosting several customers • Microsoft has announced virtualization plans to allow easy upgrades and hosting Linux! • You can see the dots connecting up • From extensibility (a la SPIN) to virtualization

  5. Possible virtualization approaches • Standard OS (such as Linux, Windows) • Meta services (such as grid) for users to install files and run processes • Administration, accountability, and performance isolation become hard • Retrofit performance isolation into OSs • Linux/RK, QLinux, SILK • Accounting resource usage correctly can be an issue unless done at the lowest level (e.g. Exokernel) • Xen approach • Multiplex physical resource at OS granularity

  6. Full virtualization • Virtual hardware identical to real one • Relies on hosted OS trapping to the VMM for privileged instructions • Pros: run unmodified OS binary on top • Cons: • supervisor instructions can fail silently in some hardware platforms (e.g. x86) • Solution in VMware: Dynamically rewrite portions of the hosted OS to insert traps • need for hosted OS to see real resources: real time, page coloring tricks for optimizing performance, etc…

  7. Xen principles • Support for unmodified application binaries • Support for multi-application OS • Complex server configuration within a single OS instance • Paravirtualization for strong resource isolation on uncooperative hardware (x86) • Paravirtualization to enable optimizing guest OS performance and correctness

  8. Xen: VM management • What would make VM virtualization easy • Software TLB • Tagged TLB =>no TLB flush on context switch X86 does not have either • Xen approach • Guest OS responsible for allocating and managing hardware PT • Xen top 64MB of every address space. Why?

  9. x86 Memory Management • Segments • CS (code); SS (stack); DS, ES, FS, GS (all data) • Base address and limit checking • Segment base added to CPU address is the linear 32-bit virtual address • 4KB pages • Top 10 bits of address: page table • Next 10 bits of address: page table entry

  10. Creating a new PT by Guest OS • allocate and initialize a page and registers it with Xen to serve as the new PT • all subsequent updates to this page (i.e. PT) via Xen • can batch updates to reduce the cost of entering and exiting Xen • Segmentation by guest OS virtualized similarly

  11. Xen: CPU virtualization • Four privilege levels in x86 • Ring 0 (Xen) • Ring 1 (guest OS) • Ring 3 (apps of the guest OS) • Privileged instructions • Validated and executed in Xen (e.g. installing a new PT) • Exceptions • Registered with Xen once • Called directly without Xen intervention • All syscalls from apps to guest OS handled this way • Page fault handlers are special • Faulting address can be read only in ring 0 • Xen reads the faulting address and passes it via stack to the OS handler in ring 1

  12. Xen: Device I/O virtualization • Set of clean and simple device abstractions • Allows buffers to be passed directly to/from guest OS to I/O devices • Event delivery mechanism for sending asynchronous notifications to the guest OS

  13. Details of subsystem virtualization • Control transfer • Data transfer These are used in the virtualization of all the subsystems

  14. Control transfer • Hypercalls from guest OS to Xen • E.g. set of page table updates • Events for notification from Xen to guest OS • E.g. data arrival on network; virtual disk transfer complete • Events may be deferred by a guest OS (similar to disabling interrupts)

  15. Data transfer – I/O rings • Resource management and accountability • CPU time • Demultiplex data to the domains quickly upon interrupt • Account computation time for managing buffers to the appropriate domain • Memory buffers • relevant domains provide memory for I/O to Xen • Xen pins page frames during I/O transfer

  16. I/O descriptors indirectly reference the actual memory buffers

  17. Each request: unique ID from guest OS • Response: use this unique ID • Network packets • Represented by a set of requests • Responses to these signal packet reception • Disk requests • May be reordered for flexibility • Zero copy semantics via the descriptors • Domain my enqueue multiple entries before invoking a hypercall requests • Ditto for responses from Xen

  18. CPU scheduling • Base Xen scheduler uses BVT algorithm [Duda & Cheriton, SOSP 99] • Work conserving for different domains (ensuring accountability) with a low-latency wake up on event arrival using virtual time warping

  19. Time and timers • Guest OSs have access to Xen’s • Real time (cycle counter accuracy for real time tasks in guest OSs) • Virtual time (enables time slicing within the guest OS) • Wall clock time (real time + domain changeable offset) • Guest OS maintain their own internal timer queues and use a pair of Xen timers (real and virtual)

  20. Virtual address translation • VMware solution • Each domain with shadow page table • Hypervisor go between the VMM and MMU • Xen solution (similar to Exokernel) • Register guest OS PT directly with MMU • Guest OS has read only access • All mods to the PT via Xen • Type associated with a page frame • PD, PT, LDT, GDT, RW • All except RW: read-only access to the page for guest OS • Guest OS can retask a page only when ref count is 0

  21. Physical memory • At domain creation, hardware pages “reserved” • Domain can increase/decrease its quota • Xen does not guarantee that the hardware pages are contiguous • Guest OS can maintain “physical memory” that is contiguous mapped to the “hardware pages”

  22. Network • Each guest OS has two I/O rings • Receive and transmit • Transmit • Enqueue descriptor on the transmit ring • Points to the buffer in guest OS space • No copying into Xen • Page pinned till transmission complete • Round robin packet scheduler across domains • Receive • Exchange received page for a page of the guest OS in the receive ring • No copying

  23. Disk • Batches of requests from competing domain taken and scheduled • Since Xen has knowledge of disk layout, requests may be reordered • No copying into Xen • “Reoder barrier” to prevent reordering (may be necessary for higher level semantics such as write ahead log)

  24. Performance

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