Implementing an Operating System
Turning away from the user and system call interfaces, let us now take a look at how to implement an operating system. In the next eight sections we will examine some general conceptual issues relating to implementation strategies. After that we will look at some low-level techniques that are often helpful.
12.3.1 System Structure
Probably the first decision the implementers have to make is what the system structure should be. We examined the main possibilities in Sec. 1.7, but will review them here. An unstructured monolithic design is really not a good idea, except maybe for a tiny operating system in, say, a refrigerator, but even there it is arguable.
A reasonable approach that has been well established over the years is a layered system. Dijkstra's THE system (Fig. 1-25) was the first layered operating system. UNIX (Fig. 10-3) and Windows 2000 (Fig. 11-7) also have a layered structure, but the layering in both of them is more a way of trying to describe the system than a real guiding principle that was used in building the system.
For a new system, designers choosing to go this route should first very carefully choose the layers and define the functionality of each one. The bottom layer should always try to hide the worst idiosyncracies of the hardware, as the HAL does in Fig. 11-7. Probably the next layer should handle interrupts, context switching, and the MMU, so above this level, the code is mostly machine independent. Above this, different designers will have different tastes (and biases). One possibility is to have layer 3 manage threads, including scheduling and interthread synchronization, as shown in Fig. 12-1. The idea here is that starting at layer 4 we have proper threads that are scheduled normally and synchronize using a standard mechanism (e.g., mutexes).
In layer 4 we might find the device drivers, each one running as a separate thread, with its own state, program counter, registers, etc., possibly (but not necessarily) within the kernel address space. Such a design can greatly simplify the I/O structure because when an interrupt occurs, it can be converted into an unlock on a mutex and a call to the scheduler to (potentially) schedule the newly readied thread that was blocked on the mutex. MINIX uses this approach, but in UNIX, Linux, and Windows 2000, the interrupt handlers run in a kind of no-man's land, rather than as proper threads that can be scheduled, suspended, etc. Since a huge amount of the complexity of any operating system is in the I/O, any technique for making it more tractable and encapsulated is worth considering.
Above layer 4, we would expect to find virtual memory, one or more file systems, and the system call handlers. If the virtual memory is at a lower level than the file systems, then the block cache can be paged out, allowing the virtual memory manager to dynamically determine how the real memory should be divided among user pages and kernel pages, including the cache. Windows 2000 works this way.
Figure 12-1. One possible design for a modern layered operating system.
While layering has its supporters among system designers, there is also another camp that has precisely the opposite view (Engler et al., 1995). Their view is based on the end-to-end argument (Saltzer et al., 1984). This concept says that if something has to be done by the user program itself, it is wasteful to do it in a lower layer as well.
Consider an application of that principle to remote file access. If a system is worried about data being corrupted in transit, it should arrange for each file to be checksummed at the time it is written and the checksum stored along with the file. When a file is transferred over a network from the source disk to the destination process, the checksum is transferred, too, and also recomputed at the receiving end. If the two disagree, the file is discarded and transferred again.
This check is more accurate than using a reliable network protocol since it also catches disk errors, memory errors, software errors in the routers, and other errors besides bit transmission errors. The end-to-end argument says that using a reliable network protocol is then not necessary, since the end point (the receiving process) has enough information to verify the correctness of the file itself. The only reason for using a reliable network protocol in this view is for efficiency, that is, catching and repairing transmission errors earlier.
The end-to-end argument can be extended to almost all of the operating system. It argues for not having the operating system do anything that the user program can do itself. For example, why have a file system? Just let the user read and write a portion of the raw disk in a protected way. Of course, most users like having files, but the end-to-end argument says that the file system should be a library procedure linked with any program that needs to use files. This approach allows different programs to have different file systems. This line of reasoning says that all the operating system should do is securely allocate resources (e.g., the CPU and the disks) among the competing users. The Exokernel is an operating system built according to the end-to-end argument (Engler et al., 1995).
A compromise between having the operating system do everything and the operating system do nothing is to have the operating system do a little bit. This design leads to a microkernel with much of the operating system running as user-level server processes as illustrated in Fig. 1-27. This is the most modular and flexible of all the designs. The ultimate in flexibility is to have each device driver also run as a user process, fully protected against the kernel and other drivers. Getting the drivers out of the kernel would eliminate the largest source of instability in any operating systembuggy third-party driversand would be a tremendous win in terms of reliability.
Of course, device drivers need to access the hardware device registers, so some mechanism is needed to provide this. If the hardware permits, each driver process could be given access to only those I/O devices it needs. For example, with memory-mapped I/O, each driver process could have the page for its device mapped in, but no other device pages. If the I/O port space can be partially protected, the correct portion of it could be made available to each driver.
Even if no hardware assistance is available, the idea can still be made to work. What is then needed is a new system call, available only to device driver processes, supplying a list of (port, value) pairs. What the kernel does is first check to see if the process owns all the ports in the list. If so, it then copies the corresponding values to the ports to initiate device I/O. A similar call can be used to read I/O ports in a protected way.
This approach keeps device drivers from examining (and damaging) kernel data structures, which is (for the most part) a good thing. An analogous set of calls could be made available to allow driver processes to read and write kernel tables, but only in a controlled way and with the approval of the kernel.
The main problem with this approach, and microkernels in general, is the performance hit all the extra context switches cause. However, virtually all work on microkernels was done many years ago when CPUs were much slower. Nowadays, applications that use every drop of CPU power and cannot tolerate a small loss of performance, are few and far between. After all, when running a word processor or Web browser, the CPU is probably idle 90% of the time. If a microkernel-based operating system turned an unreliable 900-MHz system into a reliable 800-MHz system, probably few users would complain. After all, most of them were quite happy only a few years ago when they got their previous computer, at the then-stupendous speed of 100 MHz.
With the client-server systems discussed above, the idea was to get as much out of the kernel as possible. The opposite approach is to put more modules into the kernel, but in a protected way. The key word here is protected, of course. We studied some protection mechanisms in Sec. 9.5.6 that were initially intended for importing applets over the Internet, but are equally applicable to inserting foreign code into kernel. The most important ones are sandboxing and code signing as interpretation is not really practical for kernel code.
Of course, an extensible system by itself is not a way to structure an operating system. However, by starting with a minimal system consisting of little more than a protection mechanism and then adding protected modules to the kernel one at a time until reaching the functionality desired, a minimal system can be built for the application at hand. In this view, a new operating system can be tailored to each application by including only the parts it requires. Paramecium is an example of such a system (Van Doorn, 2001).
Another issue relevant here is that of system threads, no matter which structuring model is chosen. It is sometimes convenient to allow kernel threads to exist, separate from any user process. These threads can run in the background, writing dirty pages to disk, swapping processes between main memory and disk, and so on. In fact, the kernel itself can be structured entirely of such threads, so that when a user does a system call, instead of the user's thread executing in kernel mode, the user's thread blocks and passes control to a kernel thread that takes over to do the work.
In addition to kernel threads running in the background, most operating systems start up many daemon processes in the background, too. While these are not part of the operating system, they often perform ''system'' type activities. These might including getting and sending email and serving various kinds of requests for remote users, such as FTP and Web pages.