From e1769f27643ea0a174e341ae92a41279728556c0 Mon Sep 17 00:00:00 2001 From: Sebastian Huber Date: Thu, 3 Sep 2015 14:45:44 +0200 Subject: smp: Documentation Close #2274. --- doc/user/smp.t | 65 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 65 insertions(+) diff --git a/doc/user/smp.t b/doc/user/smp.t index de07f3c2d9..5427403221 100644 --- a/doc/user/smp.t +++ b/doc/user/smp.t @@ -466,6 +466,71 @@ architecture please consult the @cite{RTEMS CPU Architecture Supplement}. The only remaining user of task variables in the RTEMS code base is the Ada support. So basically Ada is not available on RTEMS SMP. +@subsection OpenMP + +OpenMP support for RTEMS is available via the GCC provided libgomp. There is +libgomp support for RTEMS in the POSIX configuration of libgomp since GCC 4.9 +(requires a Newlib snapshot after 2015-03-12). In GCC 6.1 or later (requires a +Newlib snapshot after 2015-07-30 for provided self-contained +synchronization objects) there is a specialized libgomp configuration for RTEMS +which offers a significantly better performance compared to the POSIX +configuration of libgomp. In addition application configurable thread pools +for each scheduler instance are available in GCC 6.1 or later. + +The run-time configuration of libgomp is done via environment variables +documented in the @uref{https://gcc.gnu.org/onlinedocs/libgomp/, libgomp +manual}. The environment variables are evaluated in a constructor function +which executes in the context of the first initialization task before the +actual initialization task function is called (just like a global C++ +constructor). To set application specific values, a higher priority +constructor function must be used to set up the environment variables. + +@example +@group +#include + +void __attribute__((constructor(1000))) config_libgomp( void ) +@{ + setenv( "OMP_DISPLAY_ENV", "VERBOSE", 1 ); + setenv( "GOMP_SPINCOUNT", "30000", 1 ); + setenv( "GOMP_RTEMS_THREAD_POOLS", "1$2@@SCHD", 1 ); +@} +@end group +@end example + +The environment variable @env{GOMP_RTEMS_THREAD_POOLS} is RTEMS specific. It +determines the thread pools for each scheduler instance. The format for +@env{GOMP_RTEMS_THREAD_POOLS} is a list of optional +@code{[$]@@} configurations +separated by @code{:} where: + +@itemize @bullet +@item @code{} is the thread pool count for this scheduler +instance. +@item @code{$} is an optional priority for the worker threads of a +thread pool according to @code{pthread_setschedparam}. In case a priority +value is omitted, then a worker thread will inherit the priority of the OpenMP +master thread that created it. The priority of the worker thread is not +changed by libgomp after creation, even if a new OpenMP master thread using the +worker has a different priority. +@item @code{@@} is the scheduler instance name according to the +RTEMS application configuration. +@end itemize + +In case no thread pool configuration is specified for a scheduler instance, +then each OpenMP master thread of this scheduler instance will use its own +dynamically allocated thread pool. To limit the worker thread count of the +thread pools, each OpenMP master thread must call @code{omp_set_num_threads}. + +Lets suppose we have three scheduler instances @code{IO}, @code{WRK0}, and +@code{WRK1} with @env{GOMP_RTEMS_THREAD_POOLS} set to +@code{"1@@WRK0:3$4@@WRK1"}. Then there are no thread pool restrictions for +scheduler instance @code{IO}. In the scheduler instance @code{WRK0} there is +one thread pool available. Since no priority is specified for this scheduler +instance, the worker thread inherits the priority of the OpenMP master thread +that created it. In the scheduler instance @code{WRK1} there are three thread +pools available and their worker threads run at priority four. + @subsection Thread Dispatch Details This section gives background information to developers interested in the -- cgit v1.2.3