Linux Kernel continued...

 

Interprocess communications aka IPC occurs with the help of signals and pipes. Linux also supports System V IPC mechanisms. Signals notify events to one or more processes and can be used as a primitive way of communication and synchronization between user processes. Signals can also be used for job control.  Processes can choose to ignore most of the signals except for the well-known SIGSTOP and SIGKILL. The first causes a process to halt its execution. The second causes a process to exit. Defaults actions are associated with signals that the kernel completes. Signals are not delivered to the process until it enters running state from ready state. When a process exits a system call, the signals are then delivered. Linux is POSIX compatible so the process can specify which signals are blocked when a particular signal handling routine is called.

A pipe is a unidirectional, ordered and unstructured stream of data. Writers add data at one end and readers get it from the other end. An example is the command “ls | less” which paginates the results of the directory listing.

UNIX System V introduced IPC mechanisms in 1983 which included message queues, semaphores, and shared memory. The mechanisms all share common authentication methods and Linux supports all three. Processes access these resources by passing a unique resource identifier to the kernel via system calls.

Message queues allow one or more processes to write messages, which will be read by one or more processes. They are more versatile than pipes because the unit is a message rather than an unformatted stream of bytes and messages can be prioritized based on a type association.

Semaphores are objects that support atomic operations such as set and test. They are counters for controlled access to shared resources by multiple processes. Semaphores are most often used as locking mechanisms but must be used carefully to avoid deadlocking such as when a thread holds on to a lock and never releases it.

Shared memory is a way to communicate when that memory appears in the virtual address spaces of the participating processes. Each process that wishes to share the memory must attach to virtual memory via a system call and similarly must detach from the memory when it no longer needs the memory.

Linux has a symmetrical multiprocessing model. A multiprocessing system consists of a number of processors communicating via a bus or a network. There are two types of multiprocessing systems: loosely coupled or tightly coupled. Loosely coupled systems consists of processors that operate standalone. Each processor has its own bus, memory, and I/O subsystem, and communicates with other processes through the network medium. Tightly coupled systems consists of processors that share memory, bus, devices and sometimes cache. These can be symmetric and asymmetric. Asymmetric systems have a single master processor that controls the others. Symmetric systems are subdivided into further classes consisting of dedicated and shared cache systems.

Ideally, an SMP System with n processors would perform n times better than a uniprocessor system but in reality, no SMP is 100% scalable.

SMP systems use locks where multiple processors execute multiple threads at the same time. Locking must be limited to the smallest time possible. Another common technique is to use finer grain locking so that instead of locking a table, only a few rows are locked at a time. Linux 2.6 removes most of the global locks and locking primitives are optimized for low overheads.

Multiprocessors demonstrate cache coherency problem because each processor has an individual cache, and multiple copies of certain data exist in the system which can get out of sync.

Processor affinity improves system performance because the data and the resources accessed by the code will stay local to the processor’s cache due to warmth. Affinity helps to use these rather than fetch repeatedly. Use of processor affinity is accentuated in Non-uniform Memory Access architectures where some resources can be closer to a processor than others.