OS/Kernel Structure

1. OS/Kernel Structure Module 01

2. MS-DOS Layer Structure

3. Unix is a monolithic system • UNIX – the original UNIX operating system had limited structuring. The UNIX OS consists of two separable parts. • Systems programs • The kernel • Consists of everything below the system-call interface and above the physical hardware • Provides the file system, CPU scheduling, memory management, and other operating-system functions; a large number of functions for one level.

4. UNIX System Structure

5. System Structure – Layered Approach • The operating system is divided into a number of layers (levels), each built on top of lower layers. The bottom layer (layer 0), is the hardware; the highest (layer N) is the user interface. • With modularity, layers are selected such that each uses functions (operations) and services of only lower-level layers. • Why layering? • explicit structure allows identification, relationship of complex system’s pieces • modularization eases maintenance, develop, updating of system • change of implementation of layer’s service transparent to rest of system • e.g., change in gate procedure doesn’t affect rest of system • layering considered harmful?

6. Layered Structure of the THE OS • A layered design was first used in THE operating system. Its six layers are as follows:

7. OS/2 Layer Structure

8. Microkernels • Small operating system core • Contains only essential operating systems functions • Many services traditionally included in the operating system are now external subsystems • device drivers • file systems • virtual memory manager • windowing system • security services

10. Benefits of a Microkernel Organization • Uniform interface on request made by a process • All services are provided by means of message passing, not system calls. • Examples of opening a file • Creating threads • Extensibility • Allows the addition of new services • Flexibility • New features added • Existing features can be subtracted

11. Benefits of a Microkernel Organization • Portability • Changes needed to port the system to a new processor is changed in the microkernel - not in the other services • Reliability • Modular design • Small microkernel can be rigorously tested

12. Benefits of Microkernel Organization • Distributed system support • Message are sent without knowing what the target machine is • Object-oriented operating system • Components are objects with clearly defined interfaces that can be interconnected to form software

13. Microkernel Design • Microkernel must include functions that depend on the HW and functions needed to support the servers and applications operating in user mode. • Low-level memory management • mapping each virtual page to a physical page frame. Other memory functions like swapping, protection, paging, etc is done by VM service. • Inter-process communication • Using ports. A port is a mailbox associated with one receiver and multiple senders. • I/O and interrupt management • Interrupts are transformed into messages.

14. KLM • Examples of ukernels are Mach and Chorus • Are the basis for KLM (Kernel Loadable Modules) • Linux modules are located in /lib/modules and they have had the extension ".ko" since version 2.6. • Sound • Device Drivers • Network support (Apple Talk, Novel, etc) • Language support • VM (the virtualization layer) • etc

15. Virtual Machines • A virtual machine provides multiprogramming only by providing exact virtual copies of the bare hardware . • A virtual machine provides an interface identical to the underlying bare hardware. • The operating system creates the illusion of multiple processes, each executing on its own processor with its own (virtual) memory. • Virtual Machine: A machine implemented in software, not actual hardware. Also known as a machine emulator, not as OS simulator. • Each virtual machine can run any OS on top of it • You can run different OSs, each best suited for some task, on the same physical machine • Similar to Java approach. JVM is an application VM. • Virtual PC for Windows allows you to create separate “virtual machines” on top of your Windows desktop, where you can install virtually any PC-based operating system including OS/2, Linux, Solaris, NetWare or other versions of Windows. Each virtual machine emulates a complete hardware system – from processor to network card – in a self-contained, isolated software environment, enabling the simultaneous operation of otherwise incompatible systems.

16. Virtual Machines (Cont.) • The resources of the physical computer are shared to create the virtual machines. • CPU scheduling can create the appearance that users have their own processor. • Spooling and a file system can provide virtual card readers and virtual line printers. • A normal user time-sharing terminal serves as the virtual machine operator’s console. • Instruction set emulation • Benefits of VM OS • A big plus for SW development – can run and test SW simultaneously on multiple OS on a single machine, without rebooting. Testing can take place with different system configuration, like memory, disk space, etc. • IT professionals can safely migrate and deploy new operating systems – while continuing to run and support older or custom legacy applications. • Internet VPS • Cloud Computing – Amazon Elastic CC (or EC2) • Testing networked machines with multiple NEs (routers, FWs, etc) • Testing and parallel and distributed programming for cluster nodes • Sandboxing – play and test in a protected environment w/o risking damage to other kernels running. • Server consolidation: virtualization allowed a single server to replace multiple underutilized dedicated servers. • Any drawbacks?

17. System Models Similar to standalone Hyber-V whereby a thin layer is installed first Virtual Machine (Classical Type originated by IBM VM370 in 1972) Non-virtual Machine Virtualization layer or Hypervisor or VMM Possible that it can span over multiple CPUs (even in a distributed fashion). Referred as “Platform Virtualization” • VmWare (from Vmware Inc.), • Virtual PC (Connectix, now Mircorsoft, Integrated within Windows 7 to run all Windows XP apps) • KVM (preferred choice for Red Hat and Ubuntu) • Xen (freeware) • Virtualbox from Sun Inc. • A complete list is available at http://en.wikipedia.org/wiki/Comparison_of_platform_virtual_machines

18. VMware architecture – Common Type (e.g. vmware)

19. Modern Virtualization • Intel VT and AMD-V processors have hardware assisted virtualization to yield better performance • Full vs. Partial Virtualization • Much of the underlying hardware is emulated, but not all • Paravirtualization • Guest OS has special calls to VM, called hybervisor calls to speed access • i.e. it provides an interface to VM • Faster as the virtualization layer becomes smaller

20. Windows Hyper-V (2008 Server)

21. Hyper-V Architecture (1/2) •  A partition is a logical unit of isolation in which an OS executes. • The virtualization stack runs in the parent partition and has direct access to the hardware devices. • The parent partition then creates the child partitions which host the guest OSs. A parent partition creates child partitions using the hypercall API. • A virtualized partition does not have access to the physical processor, nor does it handle its real interrupts. Instead, it has a virtual view of the processor and runs in Guest Virtual Address.

22. Hyper-V Architecture (2/2) • The hypervisor handles the interrupts to the processor, and redirects them to the respective partition using a logical Synthetic Interrupt Controller (SynIC). Hyper-V can hardware accelerate the address translation between various Guest Virtual Address-spaces by using an IOMMU (I/O Memory Management Unit) which operates independent of the memory management hardware used by the CPU. • Parent partitions run a Virtualization Service Provider (VSP), which connects to the VMBus and handles device access requests from child partitions. Child partition virtual devices internally run a Virtualization Service Client (VSC), which redirect the request to VSPs in the parent partition via the VMBus.

23. Microsoft Windows • Single-user multitasking • From Windows 2000 on, Windows development developed to exploit modern 32-bit and 64-bit microprocessors • Designed for single users who run multiple programs • Main drivers are: • Increased memory and speed of microprocessors • Support for virtual memory

24. Windows Architecture

25. Client/Server Model • Windows OS, protected subsystem, and applications all use a client/server model • Common in distributed systems, but can be used internal to a single system • Processes communicate via RPC

26. Windows Objects • Windows draws heavily on the concepts of object-oriented design. • Key Object Oriented concepts used by Windows are: • Encapsulation • Object class and instance

27. Traditional UNIX Kernel

28. System V Release 4 (SVR4)

29. Linux • Modular Monolithic Kernel • Although monolithic, the kernel is structures as a collection of modules • Loadable modules • An object file which can be linked and unlinked at run time • Characteristics: • Dynamic Linking • Stackable modules • For checking dependencies when unloading

30. Example of loading two modules: FAT & VFAT VFAT is a client of FAT

31. *next: Pointer to the following module. All modules are organized into a single linked list. • *name: Pointer to module name. • size:Module size in memory pages. • usecount:Module usage counter. • The counter is incremented when an operation involving the module’s functions is started and decremented when the operation terminates. • flags:Module flags. • nsyms: Number of exported symbols. • ndeps: Number of referenced modules • *syms: Pointer to this module’s symbol table. • *deps: Pointer to list of modules the are referenced by this module. • *refs: Pointer to list of modules that use this module.

34. Linux Kernel Components