A server hog is a user, program or system that places excessive load on a server

Server (computing)

In the context of client-server architecture, a server is a computer program running to serve the requests of other programs, the "clients". Thus, the "server" performs some computational task on behalf of "clients"...

 such that the server performance as experienced by other clients

Client (computing)

A client is an application or system that accesses a service made available by a server. The server is often on another computer system, in which case the client accesses the service by way of a network....

 is degraded, or such that the server itself is so heavily loaded that it fails to perform routine housekeeping for its own maintenance.

History

The term server hog is colloquial rather than precisely delineated. The sensibility, if not the term itself, almost certainly dates back to the early years of time-sharing

Time-sharing

Time-sharing is the sharing of a computing resource among many users by means of multiprogramming and multi-tasking. Its introduction in the 1960s, and emergence as the prominent model of computing in the 1970s, represents a major technological shift in the history of computing.By allowing a large...

 computer systems in the 1960s when it was common for a single institutional mainframe

Mainframe computer

Mainframes are powerful computers used primarily by corporate and governmental organizations for critical applications, bulk data processing such as census, industry and consumer statistics, enterprise resource planning, and financial transaction processing.The term originally referred to the...

 to control many interactive terminals. In such an environment server lag

Lag

Lag is a common word meaning to fail to keep up or to fall behind. In real-time applications, the term is used when the application fails to respond in a timely fashion to inputs...

 is acutely perceived. Furthermore, in many operating environments, scarce server resources such as CPU-seconds were often metered and charged against the account of the user running the program. An unintentional server hog could prove extremely costly in financial terms. These programs were often called run-away programs or endless loops.

Resource contention

Server performance has many dimensions. Any subsystem that becomes excessively loaded can compromise the performance of other clients contending for that subsystem. Common forms of hardware contention include CPU cycles, interrupt latency, I/O bandwidth, available system memory, or aggregate system memory bandwidth. At the software level, contention can arise for buffers

Buffer (computer science)

In computer science, a buffer is a region of a physical memory storage used to temporarily hold data while it is being moved from one place to another. Typically, the data is stored in a buffer as it is retrieved from an input device or just before it is sent to an output device...

, queues, spool

Spooling

In computer science, spool refers to the process of placing data in a temporary working area for another program to process. The most common use is in writing files on a magnetic tape or disk and entering them in the work queue for another process. Spooling is useful because devices access data at...

s, or page table

Page table

A page table is the data structure used by a virtual memory system in a computer operating system to store the mapping between virtual addresses and physical addresses. Virtual addresses are those unique to the accessing process...

s.

Known hogs

It is accepted practice that servers are appropriately sized by system administrators for the workload (or mixture of workloads) expected, and server performance is closely monitored to establish performance baselines. The server load might include well known server hogs, such as system backup. These tasks are generally scheduled for time periods of light demand, such as in the very early hours on a Sunday morning, with an accepted administrative policy to discourage or prohibit other demands on the server during those time periods.

Unexpected hogs

More often, the term server hog is used to designate an unusual load condition where the server performance falls short of the culturally accepted baseline. A common scenario in the early years of computing was an overload condition known as thrashing where the aggregate server performance becomes severely degraded, such as when two departments of a large company attempt to run a heavy report concurrently on the same mainframe. In such a situation, the designation of the server hog becomes a political matter of pointing fingers, as the termination of either long-running report would restore the server to normal performance.

Internet era

In the internet

Internet

The Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...

 era, the nature of server loads greatly changed, as the clients became increasingly dispersed geographically, and often increasingly anonymous, as for example, any member of the public with internet access can request a web server in any part of the world to deliver a web page. In this context, a server hog most commonly designates a malicious server hog--a program written expressly for the purpose of overloading a remote server with excessive requests or excessively difficult requests (such as complex search). Use of a deliberate server hog is known as a denial-of-service attack

Denial-of-service attack

A denial-of-service attack or distributed denial-of-service attack is an attempt to make a computer resource unavailable to its intended users...

, a behaviour exhibited by many viruses

Computer virus

A computer virus is a computer program that can replicate itself and spread from one computer to another. The term "virus" is also commonly but erroneously used to refer to other types of malware, including but not limited to adware and spyware programs that do not have the reproductive ability...

, worms

Computer worm

A computer worm is a self-replicating malware computer program, which uses a computer network to send copies of itself to other nodes and it may do so without any user intervention. This is due to security shortcomings on the target computer. Unlike a computer virus, it does not need to attach...

 and trojan horses

Trojan horse (computing)

A Trojan horse, or Trojan, is software that appears to perform a desirable function for the user prior to run or install, but steals information or harms the system. The term is derived from the Trojan Horse story in Greek mythology.-Malware:A destructive program that masquerades as a benign...

. It is also possible for a petulant or vindictive computer user to manually overload a remote server by unleashing a crap flood.

Bots

A special case is that of a run-away bot

Bot

Bot or BOT may refer to:-Computing:* Bot, another also name for a Web crawler* Bots , an open-source EDI software* BOTS, a computer game* Internet bot, a computer program that does automated tasks...

, a program that was designed to be helpful by automating a drudgerous task, but due to poor programming or poorly understood circumstances, goes out of control and hammers a server unceasingly at high rate. A common case is a web spider which accesses too many pages on a web server too quickly at the expense of the server's intended audience.