The Linux /proc Filesystem CSE8343 – Fall 2001 Group A1 – Alex MacFarlane, Garrick Williamson, Brad Crabtree
Topics Introduction / History Problems Addressed Layout Process Information Kernel Information Kernel Configuration Implementation Programming for /proc Advantages & Disadvantages
Introduction What is /proc? –A pseudo-filesystem that acts as an interface to internal data structures in the kernel What is it used for? –Can be used to obtain information about the system –Can be used to change certain kernel parameters at runtime.
History The idea of a Process Filesystem –Used for reporting process information only –Seen in UNIXes such as Solaris /proc extends the concept A similar implementation available for various flavors of BSD, including FreeBSD /proc for Linux is the most actively developed
The Problem Modern kernel is highly complex Linux kernel has device drivers built-in An enormous amount of status information Many run-time configurable parameters How do we allow controlled access to kernel data and parameters and provide a familiar interface that programmers can easily adopt?
The Solution Create pseudo-filesystem to represent status information and configuration parameters as files Provides a unified ‘API’ for collecting status information and configuring drivers Control access through UNIX permissions No new libraries needed – simple filesystem calls are all that is necessary Quick, easy access via command line Not version- or configuration-specific
/proc Layout Two major subdivisions –Read-only files/directories –Configurable settings in /proc/sys/ Hierarchical Subdirectories for –Network –SCSI –IDE –Device Drivers –Etc…
# ls -la /proc
Process Information Each process has a /proc directory identified by its PID - /proc/PID/ Symlink /proc/self/ points to the process reading the file system Allows access to –Process status –Process memory information –Links to cwd, exe, root dir –CPU and Memory Map information (2.4 only)
Process Information (Example)
Kernel Information APMBusesCPUs Available Devices DMA Channels Filesystems Device Drivers Frame Buffer Devices IDE Subsystem Interrupts Memory Map I/O Ports ISA PnP Kernel Core Image Kernel Symbols Kernel Messages Load Averages Kernel Locks Memory Loaded Modules Mounted Filesystems NetworkingPartitionsRTCSCSI Statistics Swap Space SysV IPC TTY Drivers Uptime
Configuring the Kernel Read-write entries in /proc/sys/ Allow for tuning, monitoring and optimization of running kernel Modifiable only by root Parameters may be changed simply via ‘echo’ # cat /proc/sys/fs/file-max 4096 # echo 8192 > /proc/sys/fs/file-max # cat /proc/sys/fs/file-max 8192
Configuring the Kernel (Cont’d) Filesystem Data Miscellaneous Binary Formats General Kernel Parameters Virtual Memory Subsystem Device Specific Parameters Remote Procedure Calls Networking
Examples
Examples
Implementation Linux has virtual filesystem layer (VFS) VFS provides an abstraction layer between user processes and filesystems Allows for any filesystem to be used transparently in the system Filesystems don’t have to be physical /proc fileystem resides entirely in memory
Implementation – Linux VFS
Programming for /proc Simple filesystem representation allows for easy programming C calls uptimefp = myfopen (PROC_DIR "uptime"); fgets (line, sizeof (line), uptimefp); new.uptime = (unsigned long) (atof (strtok (line, " ")) * (unsigned long) HZ); Web interfaces <html><body> <? if ($fp = fopen('/proc/sys/kernel/hostname','r')) { $result = trim(fgets($fp, 4096)); echo gethostbyaddr(gethostbyname($result)); } ?> </body></html> Shell scripts – bash, PERL, etc.
Advantages & Disadvantages Advantages –Coherent, intuitive interface to the kernel –Great for tweaking and collecting status info –Easy to use and program for Disadvantages –Certain amount of overhead, must use fs calls Alleviated somewhat by sysctl() interface –User can possibly cause system instability