Chorus and other Microkernels Presented by: Jonathan Tanner and Brian Doyle Articles By: Jon Udell Peter D. Varhol Dick Pountain

What is a Microkernel? Provides IPC, Scheduling, Real-Time Events and Memory Management All else is a service which can be plugged in as an external module at run-time.

Monolithic vs. Micro Scheduler Memory Manager IPC Device Drivers File System Monolithic Scheduler Memory Manager IPC Real-Time Events Microkernel Device Drivers File System

Windows NT Micro or no? Has Core Kernel Size All Modules ARE NOT simply user level Security, I/O and others run in Executive Mode.

Win NT Kernel Scheduler Memory Manager IPC Security File System Kernel Space Executive / Privileged Space User Level Space I/O Netscape

The Chorus Microkernel Aspirations of a New Operating System –Multitasking –Networking –Fault Tolerance –Symmetric Multiprocessing –Massive Parallelism –Binary Compatibility with Industry Standard Software –Object Oriented Design

The Chorus Microkernel Chorus Systems are built on a tiny nucleus (typically only Kbytes in size vs. Monolithic which can be 400 Kbytes or more) which includes. –Scheduling –Memory Management –Real-Time Events –Communications

Chorus Lexicon Actor - The Equivalent of a Unix process; it provides an execution context for one or more threads. Ports - Queues attached to actors by which threads of one actor send messages to threads of another. Site - The basic unit of computing hardware, consisting of one or more processors, memory, and I/O devices. Thread - The unit of execution in Chorus. It has the same meaning as it does in Windows NT. A Thread does not need a private address space but does need its own stack. Under Chorus the Actor owns the address space.

Chorus Microkernel Communication Medium threads ports Actors Site

The Chorus Microkernel Multitasking Real-Time Executive –Allocates Local Processors –Schedules Threads Using Priority-Based Preemptive Scheme –Optional Time-Slicing –API for Thread Creation/Destruction –Synchronization via Semaphores, Spin Locks, and Mutexes. Chorus Philosophy - Provide a variety of efficient low-level mechanisms, leaving the choice of performance trade-offs to the sub-system builder.

The Chorus Microkernel The Memory Manager –Offers Segments and Regions –The virtual address space of an actor is divided into contiguous regions that map a portion of a segment into physical memory. –System actors called “Mappers” manage segments, allocating regions as needed.

The Chorus Microkernel The Supervisor –Dispatches Interrupts, Exceptions and Traps. –These events are dispatched to dynamically defined device drivers and other real-time event handlers at run time. –Response time is fast enough for Chorus to be applied to real-time control systems.

The Chorus Microkernel The Interprocess Communications (IPC) –Delivers messages between ports –Two Communication Modes Simple, non-blocking, asynchronous send/receive protocol in which messages are not acknowledged. RPC (Remote Procedure Call) with full client/server semantics.

The Chorus Microkernel Above the Kernel - Everything else in the OS is a server. (Operating in User Mode or Kernel Mode) –File Managers –Stream and Socket Managers –Device Drivers –Unix System V

The Chorus Microkernel The ability to support conventional Operating Systems as Sub-Systems means you could develop “Multiple Personalities” –OS/2 –Unix –Windows IBM is basing its future OS strategy on a similar idea, implementing it on the Mach-3.0 microkernel.

Chorus Systems Unix A Trend in OS Development - Restructuring of traditional “monolithic” operating systems into independent servers. –Primary Concerns Efficiency: Can a microkernel-based modular operating system provide performance comparable to that of the monolithic kernel. Compatibility: Portability, Standardization and Compatible Interfaces are needed for applications as well as device drivers and streams modules.

Chorus Systems Unix Supervisor Actors: –Share the supervisor address space. –Can execute in kernel mode. –Are truly separate entities from the nucleus; compiled, linked and loaded independently. –Utilize privileged instructions and connected handlers. Connected Handlers: –Created dynamically by Supervisor Actors to catch hardware interrupts, system call traps, and program exceptions. –Allows for a common interface for the nucleus. –Interrupt processing time is greatly reduced, allowing real-time applications to be implemented outside the nucleus.

Chorus Systems Unix Chorus Unix - Split into four servers: (Supervisor Actors) –Process Manager: A Unix process was implemented by the Chorus Actor. –File Manager –Device Manager –Socket Manager Unix System Calls - Implemented by a process- level library.

Chorus Systems Unix Some Early Problems –Unix Signals: Mono-threaded actors used priority messages. –Device Drivers: Needed to reside within the kernel.

Chorus Systems Unix Experience Gained: Implementing a rich operating system environment such as Unix provides insight into the basic operating system services that a microkernel must provide. –Supervisor Actors –Threads –Connected Handlers