Multithreaded Programming Guide

Set Thread Concurrency Level

By default, Solaris threads attempt to adjust the system execution resources (LWPs) used to run unbound threads to match the real number of active threads. While the Solaris threads package cannot make perfect decisions, it at least ensures that the process continues to make progress.

When you have some idea of the number of unbound threads that should be simultaneously active (executing code or system calls), tell the library through thr_setconcurrency(). To get the number of threads being used, use thr_getconcurrency().


thr_setconcurrency(3THR) provides a hint to the system about the required level of concurrency in the application. The system ensures that a sufficient number of threads are active so that the process continues to make progress.

#include <thread.h>

int new_level;
int ret;

ret = thr_setconcurrency(new_level);

Unbound threads in a process might or might not be required to be simultaneously active. To conserve system resources, the threads system ensures by default that enough threads are active for the process to make progress, and that the process will not deadlock through a lack of concurrency.

Because this might not produce the most effective level of concurrency, thr_setconcurrency() permits the application to give the threads system a hint, specified by new_level, for the desired level of concurrency.

The actual number of simultaneously active threads can be larger or smaller than new_level.

Note that an application with multiple compute-bound threads can fail to schedule all the runnable threads if thr_setconcurrency() has not been called to adjust the level of execution resources.

You can also affect the value for the desired concurrency level by setting the THR_NEW_LWP flag in thr_create(). This effectively increments the current level by one.

Return Values

Returns a zero when it completes successfully. Any other returned value indicates that an error occurred. When any of the following conditions is detected, thr_setconcurrency() fails and returns the corresponding value.


The specified concurrency level would cause a system resource to be exceeded.


The value for new_level is negative.