A glossary of various terms and acronyms related to the Linux kernel. If you know something, please create yourself an account (UserPreferences) and add a term in alphabetical order. Don't be afraid to improve other people's explanations, the goal is to have a high quality document for the readers of this site. If everybody who reads this page adds a term each week, this glossary should be complete within a few months...

• Classifier: also called filter, classifies a network packet by inspecting it, used by QDiscs.
• Copy-on-Write: (also: COW) reuse and share existing objects and copy them not until a modification is required.
• Current: a kernel variable which points to the task_struct structure of the process currently running on this CPU.
• Device Mapper: A technology for presenting arbitrary groupings of underlying sectors on physical devices in a consistent logical fashion usable by higher level algorithms. Heavily used by kernel technologies such as LVM.
• ISR: interrupt service routine, the function in each device driver that gets called when an interrupt happens.
• Jiffies: An incrementing counter representing system "uptime" in ticks - or the number of timer interrupts since boot. Ultimately the entire original concept of a jiffy will likely vanish as systems use timer events only when necessary and become "jiffyless".
• kswapd: a kernel thread that frees up memory by evicting data from caches and paging out userspace memory, part of the virtual memory subsystem.

• Linux Device Drivers, 2nd Edition: [http://www.xml.com/ldd/chapter/book/].

• LKM: Linux Kernel Module. A (often dynamically loadable at system runtime) kernel extension ("driver") to support, for example, some kind of new hardware device or generic software abstraction.

• LKML: Linux Kernel Mailing List. The primary virtual watering hole (meeting ground) for kernel developers to share ideas and bounce opinions off oneanother during the course of the kernel development process. FAQ at [http://www.tux.org/lkml/].

• LVM: Logical volume management. A technology for providing an arbitrary logical view of underlying data storage in a fashion supporting resizing and restructuring of storage on the fly. Currently in version 2, originally written by Sistina (now Redhat).

• LXR: a cross-reference tool that can be used to navigate the Linux kernel source code, available at [http://lxr.linux.no/ lxr.linux.no].

• mem_map: A contiguous virtual array of struct pages representing the entirity of physical memory pages available within a system.
• MMU: memory management unit, part of the CPU that is needed for virtual memory.
• Page table: data structure used by the MMU to translate virtual memory addresses to physical memory addresses.
• Process descriptor: kernel data structure that describes/accounts process data related to a single process.

• QDisc: [http://lartc.org/howto/lartc.qdisc.html Queueing Discipline], queues packets before they are sent out to the network device, enforces QoS requirements, provides traffic shaping and prioritizing capabilities.

• QoS: Quality of Service, method to define the importance/priority of network services

• Scheduler: the part of the kernel that chooses a suitable process to run on the cpu, see the [http://lxr.linux.no/ident?i=schedule schedule()] function.

• Shared/Paged Socket Buffer: (also: pskb) Socket Buffer with uncontinuous data buffer, used for zero copy, TSO and Scatter/Gather capable network cards.
• Socket Buffer: (also: skb) data structure used to hold the data and attributes of a network packet.
• System call: (also: syscall) the way a program transitions from userspace into kernel space, to call a kernel space function.
• Virtual memory: every process in the system gets its own memory address space, independant of the other processes.
• Zero-Copy: A special networking code path where data is sent to the network directly from userspace memory; this avoids unnecessary copying of data and improves performance.