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Pthread attr setting doesn't work before thread create?
Post 302528300 by Loic Domaigne on Tuesday 7th of June 2011 12:43:30 AM
06-07-2011
This is my understanding how things are supposed to work . Warning: the last time I dealt intensively with such questions was 6 years ago, so I can't guarantee to get everything right.
POSIX differentiates between threads subject to process contention (PCS) and system contention scope (SCS). PCS threads corresponds to the user level threads that are scheduled by the thread library, whereas SCS threads to kernel level threads that are scheduled by the OS (I think, Solaris calls SCS threads LWP).
We may set different scheduling policy and priority to these PCS; the scheduling shall be always relative to the other PCS threads within the process. We may for instance have 3 PCS threads; thread 1 with SCHED_FIFO policy and priority 1, thread 2 with priority 2, and thread 3 with the default time sharing policy (SCHED_OTHER). Assume further that these 3 PCS threads are mapped onto 1 SCS thread. When this SCS thread gets scheduled by the OS, the thread library shall look which PCS thread should run: thread 2 shall be scheduled if runnable, otherwise thread 1, otherwise thread 3.
Now, how does nice() affects the whole thing? First nice() only operates at the process level, and doesn't really make sense for real-time policy. We expect therefore nice() to affect only SCS threads that are not subject to real-time policy. Indeed, POSIX states:
Back to our previous example. If I renice my process, and if my process is not subject to real-time policy, the corresponding SCS shall be scheduled less frequently (assuming I increased the nice level) compared to other SCS running on the system. So will the 3 PCS threads. From a system perspective, we can say that all 3 PCS have been impacted simultaneously by this renice operation. From the perspective of our PCS threads however, nothing has changed since the scheduling is always relative to other PCS threads (except perhaps that it feels like running on a slower CPU).
HTH, Loïc
POSIX differentiates between threads subject to process contention (PCS) and system contention scope (SCS). PCS threads corresponds to the user level threads that are scheduled by the thread library, whereas SCS threads to kernel level threads that are scheduled by the OS (I think, Solaris calls SCS threads LWP).
We may set different scheduling policy and priority to these PCS; the scheduling shall be always relative to the other PCS threads within the process. We may for instance have 3 PCS threads; thread 1 with SCHED_FIFO policy and priority 1, thread 2 with priority 2, and thread 3 with the default time sharing policy (SCHED_OTHER). Assume further that these 3 PCS threads are mapped onto 1 SCS thread. When this SCS thread gets scheduled by the OS, the thread library shall look which PCS thread should run: thread 2 shall be scheduled if runnable, otherwise thread 1, otherwise thread 3.
Now, how does nice() affects the whole thing? First nice() only operates at the process level, and doesn't really make sense for real-time policy. We expect therefore nice() to affect only SCS threads that are not subject to real-time policy. Indeed, POSIX states:
Quote:
[PS|TPS] 
Calling the nice() function has no effect on the priority of processes or threads with policy SCHED_FIFO or SCHED_RR. The effect on processes or threads with other scheduling policies is implementation-defined. 
The nice value set with nice() shall be applied to the process. If the process is multi-threaded, the nice value shall affect all system scope threads in the process.
Calling the nice() function has no effect on the priority of processes or threads with policy SCHED_FIFO or SCHED_RR. The effect on processes or threads with other scheduling policies is implementation-defined. The nice value set with nice() shall be applied to the process. If the process is multi-threaded, the nice value shall affect all system scope threads in the process.
HTH, Loïc
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LEARN ABOUT OSF1
pthread_attr_setscope
pthread_attr_setscope(3) Library Functions Manual pthread_attr_setscope(3) NAME
pthread_attr_setscope - Sets the contention scope attribute of the specified thread attributes object. LIBRARY
DECthreads POSIX 1003.1c Library (libpthread.so) SYNOPSIS
#include <pthread.h> int pthread_attr_setscope( pthread_attr_t *attr, int scope); STANDARDS
Interfaces documented on this reference page conform to industry standards as follows: IEEE Std 1003.1c-1995, POSIX System Application Program Interface PARAMETERS
Address of the thread attributes object whose contention scope attribute is to be modified. New value for the contention scope attribute of the thread attributes object specified by attr. DESCRIPTION
This routine uses the value specified in the scope argument to set the contention scope attribute of the thread attributes object specified in the attr argument. When creating a thread, use a thread attributes object to specify nondefault values for thread attributes. The contention scope attribute specifies the set of threads with which a thread must compete for processing resources. The contention scope attribute specifies whether the new thread competes for processing resources only with other threads in its own process, called process contention scope, or with all threads on the system, called system contention scope. On Tru64 UNIX, DECthreads supports both process contention scope and system contention scope threads. DECthreads selects at most one thread to execute on each processor at any point in time. DECthreads resolves the contention based on each thread's scheduling attributes (for example, priority) and scheduling policy (for example, round-robin). A thread created using a thread attributes object whose contention scope attribute is set to PTHREAD_SCOPE_PROCESS contends for processing resources with other threads within its own process that also were created with PTHREAD_SCOPE_PROCESS. It is unspecified how such threads are scheduled relative to threads in other processes or threads in the same process that were created with PTHREAD_SCOPE_SYSTEM contention scope. A thread created using a thread attributes object whose contention scope attribute is set to PTHREAD_SCOPE_SYSTEM contends for processing resources with other threads in any process that also were created with PTHREAD_SCOPE_SYSTEM. NOTES
The value of the contention scope attribute of a particular thread attributes object does not necessarily correspond to the actual schedul- ing contention scope of any existing thread in your multithreaded program. RETURN VALUES
If an error condition occurs, this routine returns an integer value indicating the type of error. Possible return values are as follows: Successful completion. The value specified by attr is not a valid thread attributes value, or the value specified by scope is not valid. An attempt was made to set the attribute to an unsupported value. ERRORS
None RELATED INFORMATION
Functions: pthread_attr_destroy(3), pthread_attr_init(3), pthread_attr_getscope(3), pthread_create(3) Manuals: Guide to DECthreads and Programmer's Guide delim off pthread_attr_setscope(3)