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What is a Microkernel?
A Microkernel is a small Operating System core that provides the foundation for modular extensions.
The philosophy underlying the microkernel is that only absolutely essential core operating system functions should be in the kernel. Less essential services and application are built on the microkernel and execute in user mode. Although the dividing line between what is in and what is outside the microkernel varies from one design to next. The common characteristic is that many services that traditionally have been part of the operating system are now external subsystems that interact with the kernel and with each other; these include device drivers, file systems, virtual memory manager, windowing system, and security services.
Operating System components external to the microkernel are implemented as server processes; these interact with each other on peer basis, typically by means of messages passed through the microkernel. Thus the microkernel functions as a message exchange. It validates messages, passes them between components, and grants access to hardware. The microkernel also performs a protection function; it prevents message passing unless exchange is allowed.
Advantages and Benefits of microkernel organization:
- Uniform Interfaces.
- Extensibility.
- Flexibility.
- Portability.
- Reliability.
- Distributed System Support.
- Support for Object Oriented Operating System.
Microkernel Performance:
One potential and often cited disadvantage of microkernel is that it takes longer to build and send a message via the microkernel, and accept and decode the reply, then to make the single service call. But owing to the significant role of other factors it is difficult to generalize about the performance penalty, if any.
Actually much depends on the size and functionality of microkernel. These penalties persisted despite efforts to optimize the microkernel code. One response to this problem was to enlarge the microkernel by reintegrating the critical servers and drivers back into the operating system. However this workaround reduces the performance penalty at the expense of the strengths of microkernel design: minimal interfaces, flexibility, and so on.
Microkernel Design:
The microkernel must include those functions that depend directly on the hardware and those functions needed to support the servers and applications operating in user mode. These functions fall into the general categories of low-level memory management. InterProcess Communication (IPC), and i/o and interrupt management.
Low Level Memory Management:
The microkernel has to control the network concept of address space to make it possible to implement protection at the process level. Eg, a virtual memory module outside the microkernel decides when to bring a page into memory and which page already in memory is to be replaced; the microkernel maps these page references into a physical address in main memory.
The concept that paging and virtual memory management can be performed external to the kernel was introduced. When a thread in the application references a page not in main memory, a page fault occurs and execution traps to the kernel. The kernel then sends a message to the pager process indicating which page had been referenced. The pager can decide to load that page and allocate a page frame for that purpose. The pager and kernel must interact to map the pager’s local operations onto physical memory. Once the page is available the pager sends a resume message to the application.
This technique enables a non-kernel process to map files and databases into user address spaces without invoking the kernel. Application specific memory sharing policies can be implemented outside the kernel.
The three microkernel operations that can support external paging and virtual memory management are:
- Grant
The owner of an address space can grant a number of its pages to another process. The kernel removes these pages from the grantor’s address space and assigns them to the designated process.
- Map
A process can map any of its pages into the address space of another process, so that both processes have access to the pages. This creates shared memory between the two processes. The kernel maintains the assignment of these pages to the original owner but provides a mapping to permit access by other process.
- Flush
A process can reclaim any pages that were granted or mapped to other processes.
A microkernel has hence become an important part of almost all modern Operating Systems Architecture.
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