The Linux The Linux /proc/proc Filesystem Filesystem

CSE8343 – Fall 2001CSE8343 – Fall 2001

Group A1 – Alex MacFarlane, Group A1 – Alex MacFarlane, Garrick Williamson, Brad CrabtreeGarrick Williamson, Brad Crabtree

TopicsTopics

Introduction / HistoryIntroduction / History

Problems AddressedProblems Addressed

LayoutLayout

Process InformationProcess Information

Kernel InformationKernel Information

Kernel ConfigurationKernel Configuration

ImplementationImplementation

Programming for Programming for /proc/proc

Advantages & DisadvantagesAdvantages & Disadvantages

IntroductionIntroduction

What is What is /proc/proc??– A pseudo-filesystem that acts as an interface A pseudo-filesystem that acts as an interface

to internal data structures in the kernelto internal data structures in the kernel

What is it used for?What is it used for?– Can be used to obtain information about the Can be used to obtain information about the

systemsystem– Can be used to change certain kernel Can be used to change certain kernel

parameters at runtime.parameters at runtime.

HistoryHistory

The idea of a Process FilesystemThe idea of a Process Filesystem– Used for reporting process information onlyUsed for reporting process information only– Seen in UNIXes such as SolarisSeen in UNIXes such as Solaris

/proc/proc extends the concept extends the concept

A similar implementation available for A similar implementation available for various flavors of BSD, including FreeBSDvarious flavors of BSD, including FreeBSD

/proc/proc for Linux is the most actively for Linux is the most actively developeddeveloped

The ProblemThe Problem

Modern kernel is highly complexModern kernel is highly complex

Linux kernel has device drivers built-inLinux kernel has device drivers built-in

An enormous amount of status informationAn enormous amount of status information

Many run-time configurable parametersMany run-time configurable parameters

How do we allow controlled access to How do we allow controlled access to kernel data and parameters and provide a kernel data and parameters and provide a familiar interface that programmers can familiar interface that programmers can easily adopt?easily adopt?

The SolutionThe Solution

Create pseudo-filesystem to represent status Create pseudo-filesystem to represent status information and configuration parameters as filesinformation and configuration parameters as filesProvides a unified ‘API’ for collecting status Provides a unified ‘API’ for collecting status information and configuring driversinformation and configuring driversControl access through UNIX permissionsControl access through UNIX permissionsNo new libraries needed – simple filesystem No new libraries needed – simple filesystem calls are all that is necessarycalls are all that is necessaryQuick, easy access via command lineQuick, easy access via command lineNot version- or configuration-specificNot version- or configuration-specific

/proc/proc Layout Layout

Two major subdivisionsTwo major subdivisions– Read-only files/directoriesRead-only files/directories– Configurable settings in Configurable settings in /proc/sys//proc/sys/

Hierarchical Subdirectories forHierarchical Subdirectories for– NetworkNetwork– SCSISCSI– IDEIDE– Device DriversDevice Drivers– Etc…Etc…

# ls -la /proc# ls -la /proc

Process InformationProcess Information

Each process has a Each process has a /proc/proc directory directory identified by its PID - identified by its PID - /proc/PID//proc/PID/Symlink Symlink /proc/self//proc/self/ points to the process points to the process reading the file systemreading the file systemAllows access toAllows access to– Process statusProcess status– Process memory informationProcess memory information– Links to cwd, exe, root dirLinks to cwd, exe, root dir– CPU and Memory Map information (2.4 only)CPU and Memory Map information (2.4 only)

Process Information (Example)Process Information (Example)

Kernel InformationKernel InformationAPMAPM BusesBuses CPUsCPUs Available Available

DevicesDevicesDMA DMA ChannelsChannels

FilesystemsFilesystems Device Device DriversDrivers

Frame Buffer Frame Buffer DevicesDevices

IDE IDE SubsystemSubsystem

InterruptsInterrupts

Memory MapMemory Map I/O PortsI/O Ports ISA PnPISA PnP Kernel Core Kernel Core ImageImage

Kernel Kernel SymbolsSymbols

Kernel Kernel MessagesMessages

Load Load AveragesAverages

Kernel LocksKernel Locks MemoryMemory Loaded Loaded ModulesModules

Mounted Mounted FilesystemsFilesystems

NetworkingNetworking PartitionsPartitions RTCRTC SCSISCSI

StatisticsStatistics Swap SpaceSwap Space SysV IPCSysV IPC TTY DriversTTY Drivers UptimeUptime

Configuring the KernelConfiguring the Kernel

Read-write entries in Read-write entries in /proc/sys//proc/sys/

Allow for tuning, monitoring and optimization of Allow for tuning, monitoring and optimization of running kernelrunning kernel

Modifiable only by rootModifiable only by root

Parameters may be changed simply via ‘echo’Parameters may be changed simply via ‘echo’# cat /proc/sys/fs/file-max # cat /proc/sys/fs/file-max

4096 4096

# echo 8192 > /proc/sys/fs/file-max # echo 8192 > /proc/sys/fs/file-max

# cat /proc/sys/fs/file-max # cat /proc/sys/fs/file-max

8192 8192

Configuring the Kernel (Cont’d)Configuring the Kernel (Cont’d)

Filesystem DataFilesystem Data

Miscellaneous Binary FormatsMiscellaneous Binary Formats

General Kernel ParametersGeneral Kernel Parameters

Virtual Memory SubsystemVirtual Memory Subsystem

Device Specific ParametersDevice Specific Parameters

Remote Procedure CallsRemote Procedure Calls

NetworkingNetworking

ExamplesExamples

ExamplesExamples

ImplementationImplementation

Linux has virtual filesystem layer (VFS)Linux has virtual filesystem layer (VFS)

VFS provides an abstraction layer VFS provides an abstraction layer between user processes and filesystemsbetween user processes and filesystems

Allows for any filesystem to be used Allows for any filesystem to be used transparently in the systemtransparently in the system

Filesystems don’t have to be physicalFilesystems don’t have to be physical

/proc/proc fileystem resides entirely in memory fileystem resides entirely in memory

Implementation – Linux VFSImplementation – Linux VFS

Programming for Programming for /proc/proc

Simple filesystem representation allows for Simple filesystem representation allows for easy programmingeasy programmingC callsC callsuptimefp = myfopen (PROC_DIR "uptime");uptimefp = myfopen (PROC_DIR "uptime");fgets (line, sizeof (line), uptimefp);fgets (line, sizeof (line), uptimefp);new.uptime = new.uptime =

(unsigned long) (atof (strtok (line, " ")) * (unsigned long) (unsigned long) (atof (strtok (line, " ")) * (unsigned long) HZ);HZ);

Web interfacesWeb interfaces<html><body><html><body><? if ($fp = fopen('/proc/sys/kernel/hostname','r')) {<? if ($fp = fopen('/proc/sys/kernel/hostname','r')) {

$result = trim(fgets($fp, 4096));$result = trim(fgets($fp, 4096));echo gethostbyaddr(gethostbyname($result)); } ?>echo gethostbyaddr(gethostbyname($result)); } ?>

</body></html></body></html>

Shell scripts – bash, PERL, etc.Shell scripts – bash, PERL, etc.

Advantages & DisadvantagesAdvantages & Disadvantages

AdvantagesAdvantages– Coherent, intuitive interface to the kernelCoherent, intuitive interface to the kernel– Great for tweaking and collecting status infoGreat for tweaking and collecting status info– Easy to use and program forEasy to use and program for

DisadvantagesDisadvantages– Certain amount of overhead, must use fs callsCertain amount of overhead, must use fs calls

Alleviated somewhat by sysctl() interfaceAlleviated somewhat by sysctl() interface

– User can possibly cause system instabilityUser can possibly cause system instability