Advanced Operating Systems

Advanced Operating Systems
Lecture 3: OS design University of Tehran Dept. of EE and Computer Engineering By: Dr. Nasser Yazdani Univ. of Tehran Distributed Operating Systems

Distributed Operating Systems
How to design an OS Some general guides and experiences. References “The Computer for the 21st Century”, Mark Weiser “Exokernel: An Operating System Architecture for Application Level Resource Management”, Dawson R., Engler M, Frans Kaashoek, et al. “On Micro-Kernel Constructions“, Univ. of Tehran Distributed Operating Systems

Outline New applications/requirements Organizing operating systems Some microkernel examples Object-oriented organizations Spring Organization for multiprocessors Univ. of Tehran Distributed Operating Systems

New vision Two important problems: location and scale. Ubiquitous computing: tiny kernels of functionality Virtual Reality Mobility Intelligent devices distributed computing" make networks appear like disks, memory, or other nonnetworked devices. Univ. of Tehran Distributed Operating Systems

Ubiquitous computing Transparent computing is the ultimate goal Computers should disappear into the background Computation becomes part of the environment Computing everywhere Desktop, Laptop, Palmtop Cars, Cell phones Shoes, Clothing, Walls (paper / paint) Connectivity everywhere Broadband Wireless Mobile everywhere Users move around Disposable devices Mobile internet access Wireless communication Hand-held devices Bluetooth Univ. of Tehran Distributed Operating Systems

Ubiquitous Computing Structure Resource and service discovery critical User location an issue Interface discovery Disconnected operation Ad-hoc organization Security Small devices with limited power Intermittent connectivity Agents Sensor Networks Univ. of Tehran Distributed Operating Systems

Grid Computing Federated system No single controlling authority Scheduling Processors, bandwidth and other resources Policy is an important issue Reliability, security, of who can use, and what one is willing to use. Systems Globus toolkit Condor Related but not grid – CORBA, DCOM, DCE Applications Distributed supercomputing Univ. of Tehran Distributed Operating Systems

Peer-to-Peer Computing
Locating Cooperative elements Scalability OS support Security Policies

P2P File Sharing Issues Naming Data discovery Availability Security Encryption Fault tolerance Conflict resolution Replication Data discovery: Routing Univ. of Tehran Distributed Operating Systems

Other Peer to Peer Technologies
Ad-hoc networking Untrusted nodes used to relay messages Multiple routes (distributed and replicated) Extends range, reduces power, increases aggregate bandwidth. Increases latency, management more difficult. Sensor networks An application of ad-hoc networking Add processing/reduction in the network Univ. of Tehran Distributed Operating Systems

What is the big deal? Performance Border crossings are expensive Change in locality Copying between user and kernel buffers Application requirements differ in terms of resource management Univ. of Tehran Distributed Operating Systems

Operating System Organization
What is the best way to design an operating system? Put another way, what are the important software characteristics of an OS? What should be in OS kernel or application or partitioning. Is there a minimal set for kernel? Univ. of Tehran Distributed Operating Systems

Important OS Software Characteristics
Correctness and simplicity Power and completeness Performance Extensibility and portability Flexibility Scalability Suitability for distributed and parallel systems Compatibility with existing systems Security and fault tolerance Univ. of Tehran Distributed Operating Systems

Common OS Organizations
Monolithic Virtual machine Layered designs Kernel designs Microkernels Object-Oriented Note that individual OS components can be organized these ways Trade off between generality and specialization

What are we shooting for?
OS should be thin (like a microkernel) providing only mechanisms not embodying policies (i.e. management) Fine grain access to system resources while avoiding border crossings as much as possible (like DOS) Allow flexible extensions for management of resources (like a microkernel) without sacrificing safety (like a monolithic kernel) Univ. of Tehran Distributed Operating Systems

Monolithic OS Design Build OS as single combined module
Hopefully using data abstraction, compartmentalized function, etc. OS lives in its own, single address space Examples DOS early Unix systems most VFS file systems

Pros/Cons of Monolithic OS Organization
Highly adaptable (at first . . .) Little planning required Potentially good performance Hard to extend and change Eventually becomes extremely complex Eventually performance becomes poor Highly prone to bugs

Virtual Machine Organizations
A base operating system provides services in a very generic way One or more other operating systems live on top of the base system Using the services it provides To offer different views of system to users Examples - IBM’s VM/370, the Java interpreter

Pros/Cons of Virtual Machine Organizations
Allows multiple OS personalities on a single machine Good OS development environment Can provide good portability of applications Significant performance problems Especially if more than 2 layers Lacking in flexibility

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 Univ. of Tehran Distributed Operating Systems

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 Univ. of Tehran Distributed Operating Systems

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… Univ. of Tehran Distributed Operating Systems

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 Univ. of Tehran Distributed Operating Systems

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? Univ. of Tehran Distributed Operating Systems

Layered OS Design Design tiny innermost layer of software
Next layer out provides more functionality Using services provided by inner layer Continue adding layers until all functionality required has been provided Examples Multics Fluke layered file systems and comm. protocols

Pros/Cons of Layered Organization
More structured and extensible Easy model and development Performance: Layer crossing can be expensive In some cases, unnecessary layers, duplicated functionality.

Kernel OS Designs Similar to layers, but only two OS layers
Kernel OS services Non-kernel OS services Move certain functionality outside kernel file systems, libraries Unlike virtual machines, kernel doesn’t stand alone Examples - Most modern Unix systems

Pros/Cons of Kernel OS Organization
Many advantages of layering, without disadvantage of too many layers Easier to demonstrate correctness Not as general as layering Offers no organizing principle for other parts of OS, user services Kernels tend to grow to monoliths

Object-Oriented OS Design
Design internals of OS as set of privileged objects, using OO methods Sometimes extended into application space Tends to lead to client/server style of computing Examples Mach (internally) Spring (totally)

Object-Oriented Organizations
Object-oriented organization is increasingly popular Well suited to OS development, in some ways OSes manage important data structures OSes are modularizable Strong interfaces are good in OSes Univ. of Tehran Distributed Operating Systems

Object-Orientation and Extensibility
One of the main advantages of object-oriented programming is extensibility Operating systems increasingly need extensibility So, again, object-oriented techniques are a good match for operating system design Univ. of Tehran Distributed Operating Systems

How object-oriented should an OS be?
Many OSes have been built with object-oriented techniques E.g., Mach and Windows NT But most of them leave object orientation at the microkernel boundary No attempt to force object orientation on out-of-kernel modules Univ. of Tehran Distributed Operating Systems

Pros/Cons of Object Oriented OS Organization
Offers organizational model for entire system Easily divides system into pieces Good hooks for security Can be a limiting model Must watch for performance problems Not widely used yet

Microkernel OS Design Like kernels, only less number of abstractions exported (threads, address space, communication channel) Try to include only small set of required services in the microkernel Moves even more out of innermost OS part Like parts of VM, IPC, paging, etc. System services (e.g. VM manager) implemented as servers on top High comm overhead between services implemented at user level and microkernel limits extensibility in practice Examples - Mach, Amoeba, Plan 9, Windows NT, Chorus, Spring, etc.

Pros/Cons of Microkernel Organization
Those of kernels, plus: Minimizes code for most important OS services Offers model for entire system Microkernels tend to grow into kernels Requires very careful initial design choices Serious danger of bad performance

Organizing the Total System
In microkernel organizations, much of the OS is outside the microkernel But that doesn’t answer the question of how the system as a whole gets organized How do you fit together the components to build an integrated system? While maintaining all the advantages of the microkernel Univ. of Tehran Distributed Operating Systems

Some Important Microkernel Designs
Micro-ness is in the eye of the beholder Mach Spring Amoeba Plan 9 Windows NT

Mach Mach didn’t start life as a microkernel
Became one in Mach 3.0 Object-oriented internally Doesn’t force OO at higher levels Microkernel focus is on communications facilities Much concern with parallel/distributed systems

Mach Model User processes User space Software emulation layer 4.3BSD
SysV emul. HP/UX emul. other emul. Kernel space Microkernel

What’s In the Mach Microkernel?
Tasks & Threads Ports and Port Sets Messages Memory Objects Device Support Multiprocessor/Distributed Support

Mach Tasks An execution environment providing basic unit of resource allocation Contains Virtual address space Port set One or more threads

Mach Task Model Address space Process User space Thread Process port
Bootstrap port Exception port Registered ports Kernel

Mach Threads Basic unit of Mach execution Runs in context of one task
All threads in one task share its resources Unix process similar to Mach task with single thread

Task and Thread Scheduling
Very flexible Controllable by kernel or user-level programs Threads of single task can execute in parallel On single processor Multiple processors User-level scheduling can extend to multiprocessor scheduling

Mach Ports Basic Mach object reference mechanism
Kernel-protected communication channel Tasks communicate by sending messages to ports Threads in receiving tasks pull messages off a queue Ports are location independent Port queues protected by kernel; bounded

Port Rights mechanism by which tasks control who may talk to their ports Kernel prevents messages being set to a port unless the sender has its port rights Port rights also control which single task receives on a port Univ. of Tehran Distributed Operating Systems

Port Sets A group of ports sharing a common message queue A thread can receive messages from a port set Thus servicing multiple ports Messages are tagged with the actual port A port can be a member of at most one port set Univ. of Tehran Distributed Operating Systems

Mach Messages Typed collection of data objects Sent to particular port
Unlimited size Sent to particular port May contain actual data or pointer to data Port rights may be passed in a message Kernel inspects messages for particular data types (like port rights)

Mach Memory Objects A source of memory accessible by tasks
May be managed by user-mode external memory manager a file managed by a file server Accessed by messages through a port Kernel manages physical memory as cache of contents of memory objects

Mach Device Support Devices represented by ports
Messages control the device and its data transfer Actual device driver outside the kernel in an external object

Mach Multiprocessor and DS Support
Messages and ports can extend across processor/machine boundaries Location transparent entities Kernel manages distributed hardware Per-processor data structures, but also structures shared across the processors Intermachine messages handled by a server that knows about network details

Mach’s NetMsgServer User-level capability-based networking daemon Handles naming and transport for messages Provides world-wide name service for ports Messages sent to off-node ports go through this server Univ. of Tehran Distributed Operating Systems

NetMsgServer in Action
User space User space User process User process NetMsgServer NetMsgServer Kernel space Kernel space Receiver Sender Univ. of Tehran Distributed Operating Systems

Mach and User Interfaces
Mach was built for the UNIX community UNIX programs don’t know about ports, messages, threads, and tasks How do UNIX programs run under Mach? Mach typically runs a user-level server that offers UNIX emulation Either provides UNIX system call semantics internally or translates it to Mach primitives

Windows NT More layered than some microkernel designs
NT Microkernel provides base services Executive builds on base services via modules to provide user-level services User-level services used by privileged subsystems (parts of OS) true user programs

Windows NT Diagram Executive Microkernel Hardware User Mode Win32
Processes Protected Subsystems User Mode Win32 POSIX Kernel Mode Executive Microkernel Hardware

NT Microkernel Thread scheduling Process switching
Exception and interrupt handling Multiprocessor synchronization Only NT part not preemptible or pageable All other NT components runs in threads

NT Executive Higher level services than microkernel
Runs in kernel mode but separate from the microkernel itself ease of change and expansion Built of independent modules all preemptible and pageable

NT Executive Modules Object manager Security reference monitor
Process manager Local procedure call facility (a la RPC) Virtual memory manager I/O manager

Typical Activity in NT Win32 Protected Client Subsystem Process
Executive Kernel Hardware

Windows NT Threads Executable entity running in an address space
Scheduled by kernel Handled by kernel’s dispatcher Kernel works with stripped-down view of thread - kernel thread object Multiple process threads can execute on distinct processors--even Executive ones

Microkernel Process Objects
A microkernel proxy for the real process Microkernel’s interface to the real process Contains pointers to the various resources owned by the process e.g., threads and address spaces Alterable only by microkernel calls

Microkernel Thread Objects
As microkernel process objects are proxies for the real object, microkernel thread objects are proxies for the real thread One per thread Contains minimal information about thread Priorities, dispatching state Used by the microkernel for dispatching

More On Microkernels Microkernels were the research architecture of the 80s But few commercial systems of the 90s really use microkernels To some extent, “microkernel” is now a dirty word in OS design Why?

Microkernel Construction
Most Microkernels do not perform well Is it inherent in the approach or Implementation? IPC, microkernel bottleneck, can implemented an order of magnitude faster. Not supervise memory Minimal address space management, grant, map, flush. Fast kernel-User Switch, usually us but 3 in L3 implementation

Exokernel Traditional operating systems fix the interface and implementation of OS abstractions. Abstractions must be overly general to work with diverse application needs. FIXED Hardware Applications Interface Abstractions Univ. of Tehran Distributed Operating Systems

Example Traditional OS FIXED Hardware Apache Interface Abstractions SQL Server Univ. of Tehran Distributed Operating Systems

The Issues Performance Denies applications the advantages of domain-specific optimizations Flexibility Restricts the flexibility of application builders Functionality Discourages changes to the implementations of existing abstractions Univ. of Tehran Distributed Operating Systems

Performance Example: A DB can have predictable data access patterns, that doesn't fit with OS LRU page replacement, causing bad performance. Cao et al. Found that application-controlled file caching can reduce running time by as much as 45%. There is no single way to abstract physical resources or to implement an abstraction that is best for all applications. OS is forced to make trade-offs Performance improvements of application-specific policies could be substantial – Relational databases and garbage collectors sometimes have very predictable data access patterns – LRU? – Application-controlled file caching can reduce application running time by as much as 45%. Univ. of Tehran Distributed Operating Systems

Flexibility Fixed high-level abstractions hide information from applications. Makes it difficult or impossible for applications to implement their own resource management abstractions. Hidden information - Low-level exceptions, timer interrupts, page faults, raw device I/O, etc. Univ. of Tehran Distributed Operating Systems

Functionality Only one available interface between applications and hardware resources. Because all applications must share one set of abstractions, changes to these abstractions occur rarely, if ever Univ. of Tehran Distributed Operating Systems

The Solution Separate protection from management Allow user level to manage resources Application libraries implement OS abstractions Exokernel exports resources Low level interface Protects, does not manage Expose hardware Univ. of Tehran Distributed Operating Systems

Exokernel Philosophy Applications know better than Operating Systems what the goal of their resource management decisions should be Applications should be given as much control as possible over those decisions Implementation view Exokernel HW Frame Buffer | TLB | Network | Memory | Disk Univ. of Tehran Distributed Operating Systems

Example Exokernel – Application level resource management Library OS Chosen from available Apache Interface Abstractions SQL Server Library OS Customized for SQLServer Interface Abstractions Exokernel Hardware Univ. of Tehran Distributed Operating Systems

Implementation Overview
Library O.S., which uses the low-level exokernel interface to implement higher-level abstractions. Library O.S. Exokernel HW Frame Buffer | TLB | Network | Memory | Disk Univ. of Tehran Distributed Operating Systems

Implementation Overview
Applications link to library kernel, leveraging their higher-level abstractions. Library O.S. Library O.S. Application Application Exokernel HW Frame Buffer | TLB | Network | Memory | Disk Univ. of Tehran Distributed Operating Systems

End-to-End Argument “if something has to be done by the user program itself, it is wasteful to do it in a lower level as well.” Why should the OS do anything that the user program can do itself? In other words - all an OS should do is securely allocate resources. Univ. of Tehran Distributed Operating Systems

Exokernel design Univ. of Tehran Distributed Operating Systems

Exokernel tasks Track ownership Guard all resources through bind points Revoke access to resources Univ. of Tehran Distributed Operating Systems

Design principle Expose hardware (securely) Expose allocation Expose names Expose revocation Univ. of Tehran Distributed Operating Systems

Secure binding Decouples authorization from use Allows kernel to protect resource without understanding their semantics Example: TLB entry Virtual to physical mapping performed in the library (above exokernel) Binding loaded into the kernel; used multiple times Example: packet filter Predicates loaded into the kernel Checked on each packet arrival Univ. of Tehran Distributed Operating Systems

Implementing secure bindings
Hardware mechanisms Capability for physical pages of a file Frame buffer regions (SGI) Software caching Exokernel large software TLB overlaying the hardware TLB Downloading code into kernel Avoid expensive boundary crossings Similar to the SPIN idea Univ. of Tehran Distributed Operating Systems

Examples of secure binding
Physical memory allocation (hardware supported binding) Library allocates physical page Exokernel records the allocator and the permissions and returns a “capability” – an encrypted cypher Every access to this page by the library requires this capability Page fault: Kernel fields it Kicks it up to the library Library allocated a page – gets an encrypted capability Library calls the kernel to enter a particular translation into the TLB by presenting the capability Univ. of Tehran Distributed Operating Systems

Download code into kernel to establish secure binding Packet filter for demultiplexing network packets Exactly similar to SPIN How to ensure authenticity? Only trusted servers (library OS) can download code into the kernel Other use of downloaded code Execute code on behalf of an app that is not currently scheduled E.g. application handler for garbage collection could be installed in the kernel Univ. of Tehran Distributed Operating Systems

Visible resource revocation
Most resources are visibly revoked E.g. processor; physical page Library can then perform necessary action before relinquishing the resource E.g. needed state saving for a processor E.g. update of page table Univ. of Tehran Distributed Operating Systems

Abort protocol Repossession exception passed to the library OS Repossession vector Gives info to the library OS as to what was repossessed so that corrective action can be taken Library OS can seed the vector to enable exokernel to autosave (e.g. disk blocks to which a physical page being repossessed should be written to) Univ. of Tehran Distributed Operating Systems

Aegis – an exokernel Univ. of Tehran Distributed Operating Systems

Aegis – processor time slice
Linear vector of time slots Round robin An application can mark its “position” in the vector for scheduling Timer interrupt Beginning and end of time slices Control transferred to library specified handler for actual saving/restoring Time to save/restore is bounded Penalty? loss of a time slice next time! Univ. of Tehran Distributed Operating Systems

Aegis – processor environments
Exception context Program generated Interrupt context External: e,g. timer Protected entry context Cross domain calls Addressing context Guaranteed mappings implemented by software TLB mimicking the library OS page table Univ. of Tehran Distributed Operating Systems

Aegis performance Univ. of Tehran Distributed Operating Systems

Aegis - Address translation
On TLB miss Kernel installs hardware from software TLB for guaranteed mappings Otherwise application handler called Application establishes mapping TLB entry with associated capability presented to the kernel Kernel installs and resumes execution of the application Univ. of Tehran Distributed Operating Systems

ExOS – library OS IPC abstraction VM Remote communication using ASH (application specific safe handlers) Takeaway: significant performance improvement possible compared to a monolithic implementation Univ. of Tehran Distributed Operating Systems

The Exokernel A thin veneer that multiplexes and exports physical resources securely. Simplicity allows efficiency The lower the level of a primitive, the more efficiently it can be implemented, and the more latitude it grants to implementers of higher level abstractions. Univ. of Tehran Distributed Operating Systems

The Exokernel Resource management is restricted to allocation, revocation, sharing ownership tracking Univ. of Tehran Distributed Operating Systems

Library operating systems
Use the low level exokernel interface Higher level abstractions Special purpose implementations An application can choose the library which best suits its needs, or even build its own. Univ. of Tehran Distributed Operating Systems

Another Example an unmodified UNIX application linked against the ExOS libOS and a specialized exokernel application using its own TCP and file system libraries. Applications communicate with the kernel using low-level physical names (e.g., block numbers); the kernel interface is as close to the hardware as possible. Univ. of Tehran Distributed Operating Systems

Design Challenge How can an Exokernel allow libOSes to freely manage physical resources while protecting them from each other? Track ownership of resources Secure bindings – libOS can securely bind to machine resources Guard all resource usage Revoke access to resources Univ. of Tehran Distributed Operating Systems

Secure Bindings Exokernel allows libOSes to bind resources using secure bindings Multiplex resources securely Protection for mutually distrusted apps Efficient Univ. of Tehran Distributed Operating Systems

Secure Bindings Secure Binding – a protection mechanism that decouples authorization from actual use of a resource Allows the kernel to protect resources without having to understand them Univ. of Tehran Distributed Operating Systems

Guard all resource usage
Invisible resource revocation -Efficient – application layer not involved -Traditional OS Visible resource revocation -Allows libOS to guide deallocation and track availability of resources. -Exokernel Univ. of Tehran Distributed Operating Systems

Revoke access to resources
Abort protocol – Allows exokernel to break secure bindings of an uncooperative libOS by force Univ. of Tehran Distributed Operating Systems

Conclusion An Exokernel securely multiplexes available hardware raw hardware among applications Application level library operating systems implement higher-level traditional OS abstractions LibOSes can specialize an implementation to suit a particular application Univ. of Tehran Distributed Operating Systems

Conclusion The lower the level of a primitive… …the more efficiently it can be implemented … the more latitude it gives to higher level abstractions So, separate management from protection and… …implement protection at a low level (exokernel) … implement management at a higher level (libOS) Univ. of Tehran Distributed Operating Systems

Some Features It is possible to have different libOSes, for example, one could export a Unix API and another a Windows API Univ. of Tehran Distributed Operating Systems

Exokernel vs. Microkernel
A micro-kernel provides abstractions to the hardware such as files, sockets, graphics etc. An exokernel provides almost raw access to the hardware. Univ. of Tehran Distributed Operating Systems

Exokernel Implementation Overview Allows the extension, specialization, and even replacement of abstractions. Example: Page Table implementations can vary from libOS to libOS, and applications can choose whichever is most suitable for their needs. Univ. of Tehran Distributed Operating Systems

Exokernel Implementation Principles Provide libOS'es maximum freedom while protecting them from each other. It is achieved through separation of protection and resource management. Resources should only be managed to the extent required for protection. LibOS'es handle how best to use resources, with exokernel arbitrating between competing libraries. LibOS's should be able to request specific physical resources (like specific physical pages). Resources should not be implicitly allocated; the LibOS should participate in every allocation. Univ. of Tehran Distributed Operating Systems

Exokernel Design Secure Bindings Downloading Code Visible Revocation Abort Protocol Univ. of Tehran Distributed Operating Systems

Exokernel Secure Bindings Protection mechanism that decouples authorization (bind time) from actual use of the resource (access time). Authorization performed at bind time. Expressed in simple operations that the exokernel can implement quickly and efficiently. Can protect resources without understanding them. Example: When a page fault occurs, virtual to physical address mapping is performed, the page is loaded by the exokernel (bind time), and then used multiple times (access time). Univ. of Tehran Distributed Operating Systems

Exokernel Downloading Code Code can be downloaded into the exokernel, for execution at defined events (like packet arrival). Reduces kernel crossings. Can execute even when the application isn't scheduled. Can initiate events (e.g. - initiate response message to packet) Example: A packet filter is downloaded into the exokernel (bind time), and then run on every incoming packet to determine the intended target application (access time), and can even initiate a response. Univ. of Tehran Distributed Operating Systems

Exokernel Visible Resource Revocation Traditionally, OS's revoke (deallocate) resources invisibly, without application involvement (e.g. - physical memory). Advantage: lower latency Disadvantage: applications cannot guide deallocation Exokernel uses visible revocation for most resources. The libraryOS is notified of the intention to deallocate, and has the capability of guiding the process. Example: libOS is told that exokernel will deallocate physical page “5”, it can use this information to update it's page table, or even to suggest a less important page for deallocation. Univ. of Tehran Distributed Operating Systems

Exokernel Abort Protocol Mechanism to take away resources when libOS's fail to respond satisfactorily to visible revocation requests. A Repossession Vector is used to keep track of forcibly deallocated resources. Library OS's can pre-load the vector with information that can be used to write state or data about the resource when it is deallocated (e.g. - define disk blocks for memory paging). OS's normally require certain allocations to be permanent, so exokernel can guarantee a small number of resources that cannot be forcibly deallocated. Example: page tables, exception areas Univ. of Tehran Distributed Operating Systems

Exokernel Implementation Aegis: Exokernel Exports: processor, physical memory, TLB, exceptions, interrupts, and network interface. ExOS: Library OS Implements: processes, virtual memory, user-level exceptions, interprocess abstractions, and network protocols (ARP,IP,UDP,NFS) Compared to Ultrix Univ. of Tehran Distributed Operating Systems

Exokernel Aegis Processor Time Slices Time Slices partitioned and allocated at the clock granularity. Scheduled using round robin. Advanced Scheduling can be implemented by libOS through requesting specific positions in the time slices. Long running apps can allocate contiguous time slices, while interactive apps can allocate several equidistant slices Univ. of Tehran Distributed Operating Systems

Exokernel Aegis Exceptions Interrupts Address Translations Guarantees address mappings for small number of pages, to simplify boot strapping. Protected Control Transfers For IPC abstractions Changes program counter to agreed location, sets appropriate data for context for callee, and donates current time slice. Dynamic Packet Filter Univ. of Tehran Distributed Operating Systems

Exokernel ExOS IPC Abstractions pipe: ExOS uses shared memory buffer, order of magnitude faster than Ultrix, which uses standard unix pipes. Application Level Virtual Memory 150x150 integer matrix mult – doesn't use any special ExOS or Aegis abilities – shows application level VM doesn't incur noticeable overhead (.1 second difference) All other tests performs comparably with Ultrix (reading pages, flipping protection bits, etc...) Downloaded code for networking handler Round Trip latency for RPC faster than FRPC Univ. of Tehran Distributed Operating Systems

Exokernel ExOS Extensibility Extensible Page-Table structures Implemented inverted page tables Extensible Schedulers Stride Scheduling (proportional share scheduling) The processes are succesfully scheduled at a ration of 3:2:1 Univ. of Tehran Distributed Operating Systems

Exokernel Conclusion Experiments with Aegis and ExOS show Simple exokernel primitives can be implemented efficiently Fast low-level hardware multiplexing can be implemented efficiently Traditional OS abstractions can be implemented as User Level Applications can create special-purpose implementations by modifying libraries Univ. of Tehran Distributed Operating Systems

Exokernel Other Exokernel Work Porting Multithreading Libraries to an Exokernel System Ernest Artiaga, Albert Serra, Marisa Gil Dept. of Computer Architecture Universitat Politecnica de Catalunya ACM SIGOPS European Workshop, ACM 2000, pp Ported Cthreads to Exokernel Slightly faster execution than without threading Univ. of Tehran Distributed Operating Systems

Exokernel Other Exokernel Work Fast and Flexible Application-Level Networking on Exokernel System Gergory Ganger, Dawson Engled, et al. CMU, Stanford, MIT and Vividon, Inc. ACM Transactions on Computer Systems, vol. 20, no. 1, pp , 2002 Implemented TCP, HTTP server, and web benchmarking tool TCP: % higher throughput HTTP: 3-8 higher throughput Benchmarking: Can produce loads 2-8 times heavier Univ. of Tehran Distributed Operating Systems

Key points of the paper Microkernel should provide minimal abstractions Address space, threads, IPC Abstractions machine independent but implementation hardware dependent for performance Myths about inefficiency of micro-kernel stem from inefficient implementation and NOT from microkernel approach Univ. of Tehran Distributed Operating Systems

What abstractions? Determining criterion: Functionality not performance Hardware and microkernel should be trusted but applications are not Hardware provides page-based virtual memory Kernel builds on this to provide protection for services above and outside the microkernel Principles of independence and integrity Subsystems independent of one another Integrity of channels between subsystems protected from other subsystems Univ. of Tehran Distributed Operating Systems

Microkernel Concepts Hardware provides address space mapping from virtual page to a physical page implemented by page tables and TLB Microkernel concept of address spaces Hides the hardware address spaces and provides an abstraction that supports Grant? Map? Flush? These primitives allows building a hierarchy of protected address spaces Univ. of Tehran Distributed Operating Systems

Address spaces R R A2, P2 V2, NIL A1, P1 V1, R (P1, v1) (P1, v1) map A3, P3 V3, R R (P2, v2) A2, P2 V2, R (P3, v3) (P1, v1) flush R A3, P3 V3, NIL (P2, v2) (P1, v1) grant Univ. of Tehran Distributed Operating Systems

Power and flexibility of address spaces Initial memory manager for address space A0 appears by magic (similar to SPIN core service BUT outside the kernel) and encompasses the physical memory Allow creation of stackable memory managers (all outside the kernel) Pagers can be part of a memory manager or outside the memory manager All address space changes (map, grant, flush) orchestrated via kernel for protection Device driver can be implemented as a special memory manager outside the kernel as well Univ. of Tehran Distributed Operating Systems

PT M2, A2, P2 M1, A1, P1 Map/grant PT PT M0, A0, P0 processor Microkernel Univ. of Tehran Distributed Operating Systems

Threads and IPC Executes in an address space PC, SP, processor registers, and state info (such as address space) IPC is cross address space communication Supported by the microkernel Classic method is message passing between threads via the kernel Sender sends info; receiver decides if it wants to receive it, and if so where Address space operations such as map, grant, flush need IPC Higher level communication (e.g. RPC) built on top of basic IPC Univ. of Tehran Distributed Operating Systems

Interrupts? Each hardware device is a thread from kernel’s perspective Interrupt is a null message from a hardware thread to the software thread Kernel transforms hardware interrupt into a message Does not know or care about the semantics of the interrupt Device specific interrupt handling outside the kernel Clearing hardware state (if privileged) then carried out by the kernel upon driver thread’s next IPC TLB handler? In theory software TLB handler can be outside the microkernel In practice first level TLB handler inside the microkernel or in hardware Univ. of Tehran Distributed Operating Systems

Unique IDs Kernel provides uid over space and time for Threads IPC channels Univ. of Tehran Distributed Operating Systems

Breaking some performance myths
Kernel user switches Address space switches Thread switches and IPC Memory effects Base system: 486 (50 MHz) – 20 ns cycle time Univ. of Tehran Distributed Operating Systems

Kernel-user switches Machine instruction for entering and exiting 107 cycles Mach measures 900 cycles for kernel-user switch Why? Empirical proof L3 kernel ~ 123 cycles (accounting for some TLB, cache misses) Where did the remaining 800 cycles go in MACH? Kernel overhead (construction of the kernel, and inherent in the approach) Univ. of Tehran Distributed Operating Systems

Address space switches
Primer on TLBs AS tagged TLB (MIPS R4000) vs untagged TLB (486) Untagged TLB requires flush on AS switch Instruction and data caches Usually physically tagged in most modern processors so TLB flush has no effect Address space switch Complete reload of Pentium TLB ~ 864 cycles Univ. of Tehran Distributed Operating Systems

Do we need a TLB flush always? Implementation issue of “protection domains” SPIN implements protection domains as Modula names within a single hardware address space Liedtke suggests similar approach in the microkernel in an architecture-specific manner PowerPC: use segment registers => no flush Pentium or 486: share the linear hardware address space among several user address spaces => no flush There are some caveats in terms of size of user space and how many can be “packed” in a 2**32 global space Univ. of Tehran Distributed Operating Systems

Upshot? Address space switching among medium or small protection domains can ALWAYS be made efficient by careful construction of the microkernel Large address spaces switches are going to be expensive ALWAYS due to cache effects and TLB effects, so switching cost is not the most critical issue Univ. of Tehran Distributed Operating Systems

Thread switches and IPC
Univ. of Tehran Distributed Operating Systems

Segment switch (instead of AS switch) makes cross domain calls cheap Univ. of Tehran Distributed Operating Systems

Memory Effects – System

Capacity induced MCPI Univ. of Tehran Distributed Operating Systems

Portability Vs. Performance
Microkernel on top of abstract hardware while portable Cannot exploit hardware features Cannot take precautions to avoid performance problems specific to an arch Incurs performance penalty due to abstract layer Univ. of Tehran Distributed Operating Systems

Examples of non-portability
Same processor family Use address space switch implementation TLB flush method preferable for 486 Segment register switch preferable for Pentium => 50% change of microkernel! IPC implementation Details of the cache layout (associativity) requires different handling of IPC buffers in 486 and Pentium Incompatible processors Exokernel on R4000 (tagged TLB) Vs. 486 (untagged TLB) => Microkernels are inherently non-portable Univ. of Tehran Distributed Operating Systems

Summary Minimal set of abstractions in microkernel Microkernels are processor specific (at least in implementation) and non-portable Right abstractions and processor-specific implementation leads to efficient processor-independent abstractions at higher layers Univ. of Tehran Distributed Operating Systems

Performance Univ. of Tehran Distributed Operating Systems

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 …. Univ. of Tehran Distributed Operating Systems

Multiprocessor OS Synchronization Communication Scheduling We have seen these issues already in the other readings in this section of the course Univ. of Tehran Distributed Operating Systems

Key Issues Modern parallel machines Large system sizes stressing bottlenecks in system software (e.g. global data structures) Higher memory latencies NUMA effects (i.e. symmetric assumption does not hold Cache hierarchy Write sharing expensive due coherence traffic False sharing due to large cache lines Univ. of Tehran Distributed Operating Systems

Thesis of Tornado paper
In designing multiprocessor OS Pay attention to locality Reduce shared system data structures Reduce distance between accessing processor and target memory module Univ. of Tehran Distributed Operating Systems

Effect of global data structure – shared counter

Tornado design approach
Object-oriented design for scalability Clustered objects Protected procedure call with a view to preserving locality while ensuring concurrency Semi automatic garbage collection for localizing locking OS objects have multiple implementations Low overhead version when scalability is not required Resort to scalable implementation when performance critical Optimize common case Object invocation should be fast; object creation/destruction can be slower Page fault handling should be fast; memory region creation/deletion can be slower Univ. of Tehran Distributed Operating Systems

Next Lecture Process and Thread “Cooperative Task Management Without Manual Stack Management”, by Atul Adya, et.al. “Capriccio: Scalable Threads for Internet Services”, by Ron Von Behrn, et. al. “The Performance Implication of Thread Management Alternative for Shared-Memory Multiprocessors”, Thomas E. Anderson, et.al. Univ. of Tehran Distributed Operating Systems

Advanced Operating Systems

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Advanced Operating Systems

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