This release of HP-UX 11.0 includes the ability to "gang schedule"
MPI (Message Passing Interface) applications and multithreaded processes.The
gang scheduler permits a set of MPI processes, or multiple threads
from a single process, to be scheduled concurrently as a group.
Only applications using the HP-UX 11.0 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.
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 applications.
An environment variable enables and disables the HP-UX gang
scheduler. The variable is defined as: MP_GANG [ON] | [OFF]
Setting MP_GANG
to ON enables
gang scheduling and setting it to OFF
disables it. No action is taken if MP_GANG
is not set, or if it is set to an undefined value. 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.
Thread and process priorities for gangs are managed identically
to timeshare policy. The timeshare priority 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).
Refer to the gang_sched(7) manpage for details about
HP-UX gang scheduling.
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