gang_sched.7 (2010 09)

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gang_sched(7) gang_sched(7)

NAME

gang_sched - Gang Scheduler

DESCRIPTION

The gang scheduler permits a set of MPI (Message Passing Interface) processes, or multiple threads from

a single process, to be scheduled concurrently as a group.

Gang scheduling is enabled and disabled by setting the

MP_GANG environment variable to ON or OFF.

The gang scheduling feature can signiﬁcantly improve parallel application performance in loaded

timeshare environments that are oversubscribed. Oversubscription occurs when the total number of

runnable parallel threads, runnable MPI processes, and other runnable processes exceeds the number of

processors in the system.

Gang scheduling also permits low-latency interactions among threads in shared-memory parallel applica-

tions.

Only applications using the HP-UX V11.0 MPI or pthread libraries can be gang scheduled. Because HP

compiler parallelism is primarily built on the pthread library, programs compiled with HP compilers can

beneﬁt from gang scheduling.

INTERFACE

The HP-UX gang scheduler is enabled and disabled using an environment variable. The variable is

deﬁned as:

MP_GANG [ON | OFF]

Setting MP_GANG to ON enables gang scheduling and setting it to OFF disables it. If MP_GANG

is not set,

or if it is set to an undeﬁned value, no action is taken.

Gang scheduling is a process attribute that is inherited by child processes created by

fork (see fork (2)).

The state of gang scheduling for a process can change only following a call to

exec

(see exec(2)).

BEHAVIOR

After the

MP_GANG environment variable is set to ON, any MPI or pthread application to execute and ﬁnd

this variable will enable gang scheduling for that process.

Only the pthread and MPI libraries query the

MP_GANG variable--the operating system does not.

Gang scheduling is an inherited process attribute. When a process with gang scheduling enabled creates

a child process, the following occurs:

• The child process inherits the gang scheduling attribute.

• A new gang is formed for the child process. The child does not become part of its parent’s gang.

The gang scheduler is engaged only when a gang consists of multiple threads. For a pthread application,

this is when a second thread is created. For an MPI application, it is when a second process is added.

As a process creates threads, the new threads are added to the process’s gang if gang scheduling is

enabled for the process. However, once the size of a gang equals the number of processors in the system,

the following occurs:

• New threads or processes are not added to the gang.

• The gang remains intact and continues to be gang scheduled.

• The spill-over threads are scheduled with the regular timeshare policies.

• If threads in the gang exit (thus making room available), the spill-over threads are not added

into the gang. However, newly created threads are added into the gang when room is available.

MPI processes are allocated statically at the beginning of execution. When

MP_GANG is set to ON, all

processes in an MPI application are made part of the same gang.

Thread and process priorities for gangs are managed identically to timeshare policy. The timeshare prior-

ity scheduler determines when to schedule a gang and adheres to the timeshare policies.

Although it is likely that scheduling a gang will preempt one or more higher priority timeshare threads,

over the long run the gang scheduler policy is generally fair. All threads in a gang will have been highest

priority by the time a gang is scheduled. Because all threads in a gang must execute concurrently, some

threads do not execute when they are highest priority (the threads must wait until all other threads have

also been selected, allowing other processes to run ﬁrst).

HP-UX 11i Version 3: September 2010 − 1 − Hewlett-Packard Company 1

Summary of content (4 pages)

PAGE 1
gang_sched(7) gang_sched(7) NAME gang_sched - Gang Scheduler DESCRIPTION The gang scheduler permits a set of MPI (Message Passing Interface) processes, or multiple threads from a single process, to be scheduled concurrently as a group. Gang scheduling is enabled and disabled by setting the MP_GANG environment variable to ON or OFF. The gang scheduling feature can significantly improve parallel application performance in loaded timeshare environments that are oversubscribed.
PAGE 2
gang_sched(7) gang_sched(7) Gangs are scheduled for a single time-slice. The time-slice is the same for all threads in the system, whether gang-scheduled or not. When a single gang executes on a system, the gang’s threads are assigned to processors in the system and are not migrated to different processors. In an oversubscribed system with multiple gangs, all gangs are periodically moved in order to give an equalized percentage of CPU time to each of the different threads.
PAGE 3
gang_sched(7) gang_sched(7) • If a gang-scheduled process is selected to be swapped out, the process will not be gang-scheduled when it is swapped back in. • Realtime processes are not gang-scheduled. • Gang scheduling is only supported for processes with timeshare scheduling policies. • When a gang-scheduled process contains the maximum number of threads (or the maximum number of processes, for MPI applications), threads or processes created after this point are not scheduled as part of the gang.
PAGE 4
(Notes) A (Notes) gA 4 Hewlett-Packard Company −1− HP-UX 11i Version 3: September 2010