Gang scheduling
Encyclopedia
In Computer science
, Gang scheduling is a scheduling algorithm for parallel systems that schedules related thread
s or process
es to run simultaneously on different processor
s. Usually these will be threads all belonging to the same process, but they may also be from different processes, for example when the processes have a producer-consumer relationship, or when they all come from the same MPI
program.
Gang scheduling is used so that if two or more threads or processes communicate with each other, they will all be ready to communicate at the same time. If they were not gang-scheduled, then one could wait to send or receive a message to another while it is sleeping, and vice-versa. When processors are over-subscribed and gang scheduling is not used within a group of processes or threads which communicate with each other, it can lead to situations where each communication event suffers the overhead of a context switch.
Technically, gang scheduling is based on a data structure called the Ousterhout matrix. In this matrix each row represents a time slice, and each column a processor. The threads or processes of each job are packed into a single row of the matrix. During execution, coordinated context switching is performed across all nodes to switch from the processes in one row to those in the next row.
Gang scheduling is stricter than Coscheduling
. It requires all threads of the same process to run concurrently, while Coscheduling allows for fragments, which are sets of threads that do not run concurrently with the rest of the gang.
Gang scheduling was implemented and used in production mode on several parallel machines, most notably the Connection Machine
CM-5.
Computer science
Computer science or computing science is the study of the theoretical foundations of information and computation and of practical techniques for their implementation and application in computer systems...
, Gang scheduling is a scheduling algorithm for parallel systems that schedules related thread
Thread (computer science)
In computer science, a thread of execution is the smallest unit of processing that can be scheduled by an operating system. The implementation of threads and processes differs from one operating system to another, but in most cases, a thread is contained inside a process...
s or process
Process (computing)
In computing, a process is an instance of a computer program that is being executed. It contains the program code and its current activity. Depending on the operating system , a process may be made up of multiple threads of execution that execute instructions concurrently.A computer program is a...
es to run simultaneously on different processor
Central processing unit
The central processing unit is the portion of a computer system that carries out the instructions of a computer program, to perform the basic arithmetical, logical, and input/output operations of the system. The CPU plays a role somewhat analogous to the brain in the computer. The term has been in...
s. Usually these will be threads all belonging to the same process, but they may also be from different processes, for example when the processes have a producer-consumer relationship, or when they all come from the same MPI
Message Passing Interface
Message Passing Interface is a standardized and portable message-passing system designed by a group of researchers from academia and industry to function on a wide variety of parallel computers...
program.
Gang scheduling is used so that if two or more threads or processes communicate with each other, they will all be ready to communicate at the same time. If they were not gang-scheduled, then one could wait to send or receive a message to another while it is sleeping, and vice-versa. When processors are over-subscribed and gang scheduling is not used within a group of processes or threads which communicate with each other, it can lead to situations where each communication event suffers the overhead of a context switch.
Technically, gang scheduling is based on a data structure called the Ousterhout matrix. In this matrix each row represents a time slice, and each column a processor. The threads or processes of each job are packed into a single row of the matrix. During execution, coordinated context switching is performed across all nodes to switch from the processes in one row to those in the next row.
Gang scheduling is stricter than Coscheduling
Coscheduling
Coscheduling is a mechanism proposed for concurrent systems that schedules related processes to run on different processors at the same time. If an application consists of a collection of processes working closely together, and if some but not all of the processes are scheduled for execution, the...
. It requires all threads of the same process to run concurrently, while Coscheduling allows for fragments, which are sets of threads that do not run concurrently with the rest of the gang.
Gang scheduling was implemented and used in production mode on several parallel machines, most notably the Connection Machine
Connection Machine
The Connection Machine was a series of supercomputers that grew out of Danny Hillis' research in the early 1980s at MIT on alternatives to the traditional von Neumann architecture of computation...
CM-5.