Microkernel - Performance

Performance

On most mainstream processors, obtaining a service is inherently more expensive in a microkernel-based system than a monolithic system. In the monolithic system, the service is obtained by a single system call, which requires two mode switches (changes of the processor's ring or CPU mode). In the microkernel-based system, the service is obtained by sending an IPC message to a server, and obtaining the result in another IPC message from the server. This requires a context switch if the drivers are implemented as processes, or a function call if they are implemented as procedures. In addition, passing actual data to the server and back may incur extra copying overhead, while in a monolithic system the kernel can directly access the data in the client's buffers.

Performance is therefore a potential issue in microkernel systems. Indeed, the experience of first-generation microkernels such as Mach and ChorusOS showed that systems based on them performed very poorly. However, Jochen Liedtke showed that Mach's performance problems were the result of poor design and implementation, and specifically Mach's excessive page cache footprint. Liedtke demonstrated with his own L4 microkernel that through careful design and implementation, and especially by following the minimality principle, IPC costs could be reduced by more than an order of magnitude compared to Mach. L4's IPC performance is still unbeaten across a range of architectures.

While these results demonstrate that the poor performance of systems based on first-generation microkernels is not representative for second-generation kernels such as L4, this constitutes no proof that microkernel-based systems can be built with good performance. It has been shown that a monolithic Linux server ported to L4 exhibits only a few percent overhead over native Linux. However, such a single-server system exhibits few, if any, of the advantages microkernels are supposed to provide by structuring operating system functionality into separate servers.

A number of commercial multi-server systems exist, in particular the real-time systems QNX and Integrity. No comprehensive comparison of performance relative to monolithic systems has been published for those multiserver systems. Furthermore, performance does not seem to be the overriding concern for those commercial systems, which instead emphasize reliably quick interrupt handling response times (QNX) and simplicity for the sake of robustness. An attempt to build a high-performance multiserver operating system was the IBM Sawmill Linux project. However, this project was never completed.

It has been shown in the meantime that user-level device drivers can come close to the performance of in-kernel drivers even for such high-throughput, high-interrupt devices as Gigabit Ethernet. This seems to imply that high-performance multi-server systems are possible.