thr_setconcurrency(3thread)

thr_setconcurrency -- request a level of concurrency

Synopsis

   cc [options] -Kthread file
   
   #include <thread.h>
   

   int thr_setconcurrency(int new_level);

Description

thr_setconcurrency tells the implementation the number of lightweight processes (LWPs) that the user would like available for running multiplexed threads.

thr_setconcurrency sets to new_level the requested level, or degree, of concurrency, which is the number of LWPs that the user would like available to execute multiplexed threads in the process. The requested concurrency is a hint to the implementation as to the level of concurrency expected by the user; the implementation might use this value to affect the number of LWPs available for running multiplexed threads. thr_create(3thread) with the THR_INCR_CONC flag is another way to control concurrency.

Parameters

new_level: the requested level of concurrency

new_level parameter

new_level must be a non-negative integer. thr_setconcurrency interprets it as follows:

If new_level is zero, thr_setconcurrency sets the level of concurrency to the default level.
If new_level is greater than the current number of LWPs, in this implementation, thr_setconcurrency will create LWPs until the number of LWPs in the pool equals new_level, unless new_level exceeds a system limit. In that case, LWPs will be created only up to the system limit. However, if new_level exceeds the number of threads in the process, thr_setconcurrency will create new LWPs so that the number of LWPs equals the number of threads in the process.
If new_level is less than the current number of LWPs, thr_setconcurrency will allow LWPs to terminate themselves as the result of aging.
If new_level is equal to the current number of LWPs, thr_setconcurrency sets the new level of concurrency, but does not create or terminate any LWPs.

When the number of LWPs becomes greater than the number of threads in the process, the concurrency level might automatically decay over time to be equal to or less than the number of threads in the process. This process is called ``aging''. In this implementation, if an LWP is inactive for a certain length of time, it will terminate itself if:

the number of LWPs is greater than the requested concurrency level
or the number of LWPs is greater than the number of threads in the process

Usage

The Threads Library ensures that a sufficient number of threads are active so that the process can continue to make progress. While this conserves system resources, it might not produce the most effective level of concurrency. thr_setconcurrency permits the application to give the Threads Library a hint about the desired level of concurrency.

Return values

thr_setconcurrency returns zero for success and an error number for failure, as described below.

Errors

If the following condition is detected, thr_setconcurrency returns the corresponding value:

EINVAL: new_level is negative.

References

Intro(3thread), thr_create(3thread), thr_getconcurrency(3thread)

Notices

The creation or termination of LWPs is not synchronous with the call to thr_setconcurrency. If new_level causes a system limit to be exceeded no error is returned.

The Threads Library will always ensure that an LWP is available to run multiplexed threads.