OPS Class Lecture Notes

ops-class.org : Learn Operating Systems Online

• An operating system is a computer program that multiplexes hardware resources and implements useful abstractions.
• As a computer scientist or engineer you should know how computers really work.
• Programming operating systems will make you a better programmer and improve all of your subsequent work.
• While many operating systems concepts are elegantly simple, implementing them is not.
• Find a way to iterate quickly and limit the number of untested lines.
• Break your code into small, easily tested functions.
• Abstractions simplify application design by hiding undesirable properties, adding new capabilities, and organizing information.
• Threads abstract the CPU.
• Address space abstract memory.
• Files abstract the disk.
• Processes are the most fundamental operating system abstraction.
• The OS is responsible for isolating processes from each other.
• Sharing data requires synchronization mechanisms to ensure consistency.
• Pipes create a producer-consumer buffer between two processes.
• Signals are a limited form of asynchronous communication between processes.
• Processes contain threads; threads belong to processes.
• Process life cycle:
• Birth: fork()
• Change: exec()
• Death: exit()
• The Afterlife: wait()
• File handles store the current file offset, or the position in the file that the next read will come from or the next write will go to.
• fork() is the Unix system call that creates a new process.
• The OS creates the illusion of concurrency by quickly switching the processor(s) between multiple threads.
• A race condition is "when the output of a process is unexpectedly dependent on timing or other events".
• Concurrency: the illusion that multiple things are happening all at once.
• Atomicity: the illusion that a set of separate actions occurred all at once.
• A critical section contains a series of instructions that only one thread can be executing at any given time.
• Busy waiting prevents the thread in the critical section from making progress.
• Locks protect access to shared resources.
• Deadlock occurs when a thread or set of threads are waiting for each other to finish and thus nobody ever does.
• Processes that have exited but not had their exit code collected are called zombies.
• When the CPU is in kernel mode there are special instructions that can be executed.
• Hardware interrupts are used to signal that a particular device needs attention.
• The instructions that the processor executes when an interrupt fires are called the interrupt service routine.
• Interrupts are voluntary. Exceptions are non-voluntary.
• Multiple cores have emerged as a solution to thermal- and energy- management issues caused by transistor density.
• Timer interrupts generated by a timer device ensure that the operating system regains control of the system at regular intervals.
• Timer interrupts are the basis of preemptive scheduling: the operating system doesn't wait for the thread to stop running, instead it preempts it.
• A transition between two threads is called a context switch.
• Nobody wants to work with a jerk--no matter how talented you are.
• Scheduling is the process of choosing the next thread to run on the CPU.
• How the CPU is scheduled impacts every other part of the system.
• Humans are sensitive to responsiveness and continuity.
• Normally we cannot predict the future. Instead, use the past to predict the future.
• Time multiplexing: sharing a resource by dividing up access to it over time.
• Space multiplexing: sharing a resource by dividing it into smaller pieces.
• Process layout is specified by the Executable and Linker Format (ELF) file.
• Address spaces are meant to provide a private view of memory to each process.
• Introducing another level of indirection is a classic systems technique.
• A physical address points to memory. A virtual address points to something that acts like memory.
• Dynamic memory allocation is performed by the sbrk() system call.
• Common systems trick: when something is too slow, throw a cache at it.
• Virtual address translation gives the kernel the ability to remove memory from a process behind its back.
• We call the process of moving data back and forth from memory to disk to improve memory usage swapping.
• Flash is the future. But HDDs are still around.
• Hierarchical file systems are dead. Long live search!
• File systems are really maintaining a large and complex data structure using disk blocks as storage.
• Most files are small, but some can be very large.
• Almost every file system operation involves modifying multiple disk blocks.
• File system operations that modify multiple blocks may leave the file system in an inconsistent state if partially completed.
• Most systems papers have one or two big ideas and a lot of implementation.
• Several cheap things can be better than one expensive thing.
• Operating systems leak a lot of information between processes through the file system and other channels.