Linux

Linux

Linux

Introduction

Linux is the second most sold and the fastest growing server operating system today (Rapoza). Among those switching to Linux are Amazon.com; Boeing's Research and Development division; Government offices of Jefferson County, CO; the city of Steamboat Springs, CO; and the New York Stock Exchange (Chen). Additionally, Governments of several countries are making the change from Microsoft to the less expensive Linux. Germany, for example, in order to cut costs and improve security, has contracted with IBM to provide computer systems based on Linux

Processes and threads

Threads are "light weight processes" (LWPs). The idea is a process has five fundamental parts: code ("text"), data (VM), stack, file I/O, and signal tables. "Heavy-weight processes" (HWPs) have a significant amount of overhead when switching: all the tables have to be flushed from the processor for each task switch. Also, the only way to achieve shared information between HWPs is through pipes and "shared memory". If a HWP spawns a child HWP using fork(), the only part that is shared is the text.

Threads reduce overhead by sharing fundamental parts. By sharing these parts, switching happens much more frequently and efficiently. Also, sharing information is not so "difficult" anymore: everything can be shared. There are two types of threads: user-level and kernel-level(lkml.org).

User-Level Threads

User-level avoids the kernel and manages the tables itself. Often this is called "cooperative multitasking" where the task defines a set of routines that get "switched to" by manipulating the stack pointer. Typically each thread "gives-up" the CPU by calling an explicit switch, sending a signal or doing an operation that involves the switcher. Also, a timer signal can force switches. User threads typically can switch faster than kernel threads [however, Linux kernel threads' switching is actually pretty close in performance] (www.linux.org).

Disadvantages. User-level threads have a problem that a single thread can monopolize the timeslice thus starving the other threads within the task. Also, it has no way of taking advantage of SMPs (Symmetric MultiProcessor systems, e.g. dual-/quad-Pentiums). Lastly, when a thread becomes I/O blocked, all other threads within the task lose the timeslice as well.

Memory management

The Linux memory manager implements demand paging with a copy-on-write strategy relying on the 386's paging support. A process acquires its page tables from its parent (during a fork) with the entries marked as read-only or swapped. Then, if the process tries to write to that memory space, and the page is a copy-on-write page, it is copied, and the page is marked read-write. An exec() results in the reading in of a page or so from the executable. The process then faults in any other pages it needs(lkml.org).

Each process has a page directory which means it can access 1 KB of page tables pointing to 1 MB of 4 KB pages which is 4 GB of memory. A process' page directory is initialized during a fork by copy_page_tables(). The idle process has its page directory initialized during the initialization sequence.

Each user process has a local descriptor table that contains a code segment ...