1、Chapter 21: The Linux System,Chapter 21: The Linux System,Linux History Design Principles Kernel Modules Process Management Scheduling Memory Management File Systems Input and Output Interprocess Communication Network Structure Security,Objectives,To explore the history of the UNIX operating system
2、from which Linux is derived and the principles which Linux is designed upon To examine the Linux process model and illustrate how Linux schedules processes and provides interprocess communication To look at memory management in Linux To explore how Linux implements file systems and manages I/O devic
3、es,History,Linux is a modern, free operating system based on UNIX standards First developed as a small but self-contained kernel in 1991 by Linus Torvalds, with the major design goal of UNIX compatibility Its history has been one of collaboration by many users from all around the world, correspondin
4、g almost exclusively over the Internet It has been designed to run efficiently and reliably on common PC hardware, but also runs on a variety of other platforms The core Linux operating system kernel is entirely original, but it can run much existing free UNIX software, resulting in an entire UNIX-c
5、ompatible operating system free from proprietary code Many, varying Linux Distributions including the kernel, applications, and management tools,The Linux Kernel,Version 0.01 (May 1991) had no networking, ran only on 80386-compatible Intel processors and on PC hardware, had extremely limited device-
6、drive support, and supported only the Minix file system Linux 1.0 (March 1994) included these new features: Support for UNIXs standard TCP/IP networking protocols BSD-compatible socket interface for networking programming Device-driver support for running IP over an Ethernet Enhanced file system Sup
7、port for a range of SCSI controllers for high-performance disk access Extra hardware support Version 1.2 (March 1995) was the final PC-only Linux kernel,Linux 2.0,Released in June 1996, 2.0 added two major new capabilities: Support for multiple architectures, including a fully 64-bit native Alpha po
8、rt Support for multiprocessor architectures Other new features included: Improved memory-management code Improved TCP/IP performance Support for internal kernel threads, for handling dependencies between loadable modules, and for automatic loading of modules on demand Standardized configuration inte
9、rface Available for Motorola 68000-series processors, Sun Sparc systems, and for PC and PowerMac systems 2.4 and 2.6 increased SMP support, added journaling file system, preemptive kernel, 64-bit memory support,The Linux System,Linux uses many tools developed as part of Berkeleys BSD operating syste
10、m, MITs X Window System, and the Free Software Foundations GNU project The min system libraries were started by the GNU project, with improvements provided by the Linux community Linux networking-administration tools were derived from 4.3BSD code; recent BSD derivatives such as Free BSD have borrowe
11、d code from Linux in return The Linux system is maintained by a loose network of developers collaborating over the Internet, with a small number of public ftp sites acting as de facto standard repositories,Linux Distributions,Standard, precompiled sets of packages, or distributions, include the basi
12、c Linux system, system installation and management utilities, and ready-to-install packages of common UNIX tools The first distributions managed these packages by simply providing a means of unpacking all the files into the appropriate places; modern distributions include advanced package management
13、 Early distributions included SLS and Slackware Red Hat and Debian are popular distributions from commercial and noncommercial sources, respectively The RPM Package file format permits compatibility among the various Linux distributions,Linux Licensing,The Linux kernel is distributed under the GNU G
14、eneral Public License (GPL), the terms of which are set out by the Free Software Foundation Anyone using Linux, or creating their own derivative of Linux, may not make the derived product proprietary; software released under the GPL may not be redistributed as a binary-only product,Design Principles
15、,Linux is a multiuser, multitasking system with a full set of UNIX-compatible tools Its file system adheres to traditional UNIX semantics, and it fully implements the standard UNIX networking model Main design goals are speed, efficiency, and standardization Linux is designed to be compliant with th
16、e relevant POSIX documents; at least two Linux distributions have achieved official POSIX certification The Linux programming interface adheres to the SVR4 UNIX semantics, rather than to BSD behavior,Components of a Linux System,Components of a Linux System (Cont.),Like most UNIX implementations, Li
17、nux is composed of three main bodies of code; the most important distinction between the kernel and all other components The kernel is responsible for maintaining the important abstractions of the operating system Kernel code executes in kernel mode with full access to all the physical resources of
18、the computer All kernel code and data structures are kept in the same single address space,Components of a Linux System (Cont.),The system libraries define a standard set of functions through which applications interact with the kernel, and which implement much of the operating-system functionality
19、that does not need the full privileges of kernel code The system utilities perform individual specialized management tasks,Kernel Modules,Sections of kernel code that can be compiled, loaded, and unloaded independent of the rest of the kernel A kernel module may typically implement a device driver,
20、a file system, or a networking protocol The module interface allows third parties to write and distribute, on their own terms, device drivers or file systems that could not be distributed under the GPL Kernel modules allow a Linux system to be set up with a standard, minimal kernel, without any extr
21、a device drivers built in Three components to Linux module support: module management driver registration conflict resolution,Module Management,Supports loading modules into memory and letting them talk to the rest of the kernel Module loading is split into two separate sections: Managing sections o
22、f module code in kernel memory Handling symbols that modules are allowed to reference The module requestor manages loading requested, but currently unloaded, modules; it also regularly queries the kernel to see whether a dynamically loaded module is still in use, and will unload it when it is no lon
23、ger actively needed,Driver Registration,Allows modules to tell the rest of the kernel that a new driver has become available The kernel maintains dynamic tables of all known drivers, and provides a set of routines to allow drivers to be added to or removed from these tables at any time Registration
24、tables include the following items: Device drivers File systems Network protocols Binary format,Conflict Resolution,A mechanism that allows different device drivers to reserve hardware resources and to protect those resources from accidental use by another driver The conflict resolution module aims
25、to: Prevent modules from clashing over access to hardware resources Prevent autoprobes from interfering with existing device drivers Resolve conflicts with multiple drivers trying to access the same hardware,Process Management,UNIX process management separates the creation of processes and the runni
26、ng of a new program into two distinct operations. The fork system call creates a new process A new program is run after a call to execve Under UNIX, a process encompasses all the information that the operating system must maintain t track the context of a single execution of a single program Under L
27、inux, process properties fall into three groups: the processs identity, environment, and context,Process Identity,Process ID (PID). The unique identifier for the process; used to specify processes to the operating system when an application makes a system call to signal, modify, or wait for another
28、process Credentials. Each process must have an associated user ID and one or more group IDs that determine the processs rights to access system resources and files Personality. Not traditionally found on UNIX systems, but under Linux each process has an associated personality identifier that can sli
29、ghtly modify the semantics of certain system calls Used primarily by emulation libraries to request that system calls be compatible with certain specific flavors of UNIX,Process Environment,The processs environment is inherited from its parent, and is composed of two null-terminated vectors: The arg
30、ument vector lists the command-line arguments used to invoke the running program; conventionally starts with the name of the program itself The environment vector is a list of “NAME=VALUE” pairs that associates named environment variables with arbitrary textual values Passing environment variables a
31、mong processes and inheriting variables by a processs children are flexible means of passing information to components of the user-mode system software The environment-variable mechanism provides a customization of the operating system that can be set on a per-process basis, rather than being config
32、ured for the system as a whole,Process Context,The (constantly changing) state of a running program at any point in time The scheduling context is the most important part of the process context; it is the information that the scheduler needs to suspend and restart the process The kernel maintains ac
33、counting information about the resources currently being consumed by each process, and the total resources consumed by the process in its lifetime so far The file table is an array of pointers to kernel file structures When making file I/O system calls, processes refer to files by their index into t
34、his table,Process Context (Cont.),Whereas the file table lists the existing open files, the file-system context applies to requests to open new files The current root and default directories to be used for new file searches are stored here The signal-handler table defines the routine in the processs
35、 address space to be called when specific signals arrive The virtual-memory context of a process describes the full contents of the its private address space,Processes and Threads,Linux uses the same internal representation for processes and threads; a thread is simply a new process that happens to
36、share the same address space as its parent A distinction is only made when a new thread is created by the clone system call fork creates a new process with its own entirely new process context clone creates a new process with its own identity, but that is allowed to share the data structures of its
37、parent Using clone gives an application fine-grained control over exactly what is shared between two threads,Scheduling,The job of allocating CPU time to different tasks within an operating system While scheduling is normally thought of as the running and interrupting of processes, in Linux, schedul
38、ing also includes the running of the various kernel tasks Running kernel tasks encompasses both tasks that are requested by a running process and tasks that execute internally on behalf of a device driver As of 2.5, new scheduling algorithm preemptive, priority-based Real-time range nice value,Relat
39、ionship Between Priorities and Time-slice Length,List of Tasks Indexed by Priority,Kernel Synchronization,A request for kernel-mode execution can occur in two ways: A running program may request an operating system service, either explicitly via a system call, or implicitly, for example, when a page
40、 fault occurs A device driver may deliver a hardware interrupt that causes the CPU to start executing a kernel-defined handler for that interrupt Kernel synchronization requires a framework that will allow the kernels critical sections to run without interruption by another critical section,Kernel S
41、ynchronization (Cont.),Linux uses two techniques to protect critical sections: 1. Normal kernel code is nonpreemptible (until 2.4) when a time interrupt is received while a process is executing a kernel system service routine, the kernels need_resched flag is set so that the scheduler will run once
42、the system call has completed and control is about to be returned to user mode 2. The second technique applies to critical sections that occur in an interrupt service routines By using the processors interrupt control hardware to disable interrupts during a critical section, the kernel guarantees th
43、at it can proceed without the risk of concurrent access of shared data structures,Kernel Synchronization (Cont.),To avoid performance penalties, Linuxs kernel uses a synchronization architecture that allows long critical sections to run without having interrupts disabled for the critical sections en
44、tire duration Interrupt service routines are separated into a top half and a bottom half. The top half is a normal interrupt service routine, and runs with recursive interrupts disabled The bottom half is run, with all interrupts enabled, by a miniature scheduler that ensures that bottom halves neve
45、r interrupt themselves This architecture is completed by a mechanism for disabling selected bottom halves while executing normal, foreground kernel code,Interrupt Protection Levels,Each level may be interrupted by code running at a higher level, but will never be interrupted by code running at the s
46、ame or a lower level User processes can always be preempted by another process when a time-sharing scheduling interrupt occurs,Process Scheduling,Linux uses two process-scheduling algorithms: A time-sharing algorithm for fair preemptive scheduling between multiple processes A real-time algorithm for
47、 tasks where absolute priorities are more important than fairness A processs scheduling class defines which algorithm to apply For time-sharing processes, Linux uses a prioritized, credit based algorithm The crediting rule factors in both the processs history and its priority This crediting system a
48、utomatically prioritizes interactive or I/O-bound processes,Process Scheduling (Cont.),Linux implements the FIFO and round-robin real-time scheduling classes; in both cases, each process has a priority in addition to its scheduling class The scheduler runs the process with the highest priority; for
49、equal-priority processes, it runs the process waiting the longest FIFO processes continue to run until they either exit or block A round-robin process will be preempted after a while and moved to the end of the scheduling queue, so that round-robing processes of equal priority automatically time-sha
50、re between themselves,Symmetric Multiprocessing,Linux 2.0 was the first Linux kernel to support SMP hardware; separate processes or threads can execute in parallel on separate processorsTo preserve the kernels nonpreemptible synchronization requirements, SMP imposes the restriction, via a single kernel spinlock, that only one processor at a time may execute kernel-mode code,
