gang_sched(7) gang_sched(7)
NAME [Toc] [Back]
gang_sched - Gang Scheduler
DESCRIPTION [Toc] [Back]
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 applications.
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 [Toc] [Back]
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 [Toc] [Back]
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:
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gang_sched(7) gang_sched(7)
+ 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 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).
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.
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gang_sched(7) gang_sched(7)
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. This rebalancing occurs every
few seconds.
EXTERNAL INFLUENCES [Toc] [Back]
Environment Variables
The following environment variables affect gang scheduling of
processes:
+ MP_GANG enables (when set to ON) and disables (when set to
OFF) gang scheduling of processes. For details see the
INTERFACE section of this man page.
+ MP_NUMBER_OF_THREADS specifies the number of processors
available to execute programs compiled for parallel execution.
If not set, the default is the number of processors in the
system.
PERFORMANCE [Toc] [Back]
Gang scheduling ensures that all runnable threads and processes in a
gang are scheduled simultaneously. This improves the synchronization
latency in parallel applications. For instance, threads waiting at a
barrier do not have to wait for currently unscheduled threads.
However, applications with lengthy parallel regions and infrequent
synchronization may perform best when not gang scheduled. For those
applications, some threads can be scheduled even if all threads are
not scheduled at once.
A gang-scheduled application's performance can be affected by the
number of gang-scheduled applications on a system, and by the number
of threads in each. The gang scheduler assigns parallel applications
to CPUs using a "best fit" algorithm that attempts to minimize CPU
overlap among applications.
On systems with complex workloads including gangs of varying sizes, or
odd combinations of sizes, the workload may not optimally match the
number of CPUs available. In this situation an application may perform
better when not gang scheduled, thus enabling some threads to be
scheduled rather than waiting for all threads to be scheduled as a
gang.
Scheduling Overhead [Toc] [Back]
Gang scheduling incurs overhead when the scheduler collects a set of
threads, assigns a set of processors to the threads, and rendezvous
the set of threads and processors to achieve concurrent execution.
On an idle system, the gang scheduling overhead can be seen in the
execution time of a single parallel application.
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gang_sched(7) gang_sched(7)
Kernel Blocking of Threads [Toc] [Back]
If a thread from a gang blocks in the kernel, the thread's processor
is available to run other non-gang-scheduled threads. When the blocked
thread resumes and its gang is currently running, the thread can join
the other ganged threads without having to rendezvous again.
In a multi-gang environment, thread blocking can result in lower
throughput. This occurs if an application's threads block often in
the kernel for long periods of time.
Preempting by Realtime Threads [Toc] [Back]
Gang-scheduled threads can be preempted from execution by realtime
threads. This affects only the gang-scheduled thread running on the
processor being preempted by a realtime thread. The remaining threads
of the gang continue to run through the end of their time-slice.
RESTRICTIONS [Toc] [Back]
For this implementation of gang scheduling, the following restrictions
exist. Some of these may be removed in future releases.
+ Gang scheduling of processes being debugged is not supported.
When a debugger attaches to a process, gang scheduling for the
process is disabled. This avoids gang scheduling processes
with one or more threads stopped by a debugger.
+ Gang scheduling is completely shut down when Process Resource
Manager (PRM) is enabled.
+ 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. For details see the
BEHAVIOR section of this man page.
+ Multiprocess applications that do not use MPI are not
supported by the gang scheduler.
+ Gang scheduling is not supported for PTHREAD_SCOPE_PROCESS
threads. From release 11i Version 1.6 of HP-UX, the default
scheduling contention scope for threads is
PTHREAD_SCOPE_PROCESS. If any PTHREAD_SCOPE_PROCESS threads
are created by an application, the initial thread will be
treated as a PTHREAD_SCOPE_PROCESS.
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gang_sched(7) gang_sched(7)
FILES [Toc] [Back]
The following are libraries used in providing gang scheduling:
/usr/lib/libpthread.1 The pthread library.
/opt/mpi The directory containing MPI libraries and MPI
software. HP MPI is an optional product.
SEE ALSO [Toc] [Back]
fork(2), exec(2).
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