12-26-2002
The size of the LWP pool has a critical impact on the performance of the many-to-many model: if the number of LWPs in the pool is nearly equal to the number of threads, the implementation will act much like the one-to-one model. Conversely, if there are very few LWPs in the pool, the implementation will act like the many-to-one model.
Of particular concern is the risk of deadlock with an excessively small pool: one thread may block on a resource in the kernel and go to sleep, and by so doing block the LWP needed to run the resource-holder. To solve this problem, the threads package makes a minimal guarantee to the threads programmer: progress will always be made. This is implemented through the use of the SIGWAITING signal. When the kernel realizes that all of a process's LWPs are blocked at the kernel level, it drops a SIGWAITING on the process. Upon receipt of the signal, the user-level threads package decides whether or not to create a new LWP, on the basis of the number of runnable threads. The SIGWAITING mechanism makes no guarantees about optimal use of LWPs on a multiprocessor. Specifically, a process may have many more runnable user-level threads than it has LWPs, but it does not receive a SIGWAITING until all LWPs are blocked. Thus, even if there are processors available and work to be done, the SIGWAITING mechanism does not guarantee that there is a sufficient number of LWPs to run the user threads on the available processors. If the programmer wishes to use unbound threads and take advantage of all available processors, he or she is required to advise the library on the number of LWPs required.
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LEARN ABOUT HPUX
pthread_default_stacksize_np
pthread_default_stacksize_np(3T) pthread_default_stacksize_np(3T)
NAME
pthread_default_stacksize_np() - change the default stacksize
SYNOPSIS
PARAMETERS
new_size The new default stack size.
old_size Pointer to where the old default stack size is returned.
DESCRIPTION
The function allows an application to change the default value for the stacksize attribute. This function must be called before any
threads have been created. The new default stack size is passed in the new_size parameter. If not NULL, the previous default stack size is
returned in old_size. If new_size is zero, this function can be used (at any time) to query the current default stack size.
On HP-UX, threads with default stack sizes are cached after they terminate. The next time a thread is created with a default stack size, a
cached thread (and its stack) are reused. This can result in significant performance improvements for
However, if the default stack size is not appropriate for an application, it cannot take advantage of this performance enhancement. By
using the function, the threads library will change the default stack size so that it matches the applications needs. This allows the
application to utilize the performance benefit of cached threads.
RETURN VALUE
Upon successful completion, returns zero. Otherwise, an error number is returned to indicate the error (the variable is not set).
ERRORS
If any of the following occur, the function returns the corresponding error number:
[EINVAL] The value specified by new_size is less than
[EPERM] The calling process has already created threads (this must be called before any threads are created).
WARNINGS
If the environment variable is set with a valid value for default stack size, then it takes precedence over the invocation of interface.
However, it returns the current default stack size in old_size.
The maximum default stack size value that may be set through the environment variable is restricted to 10 MB; any value greater than 10 MB
will be rounded down to 10 MB.
Care should be taken while setting the default stack size limit since this could impact the maximum number of threads that can be created
in a process.
AUTHOR
was developed by HP.
SEE ALSO
pthread_attr_getstacksize(3T), pthread_attr_setstacksize(3T).
STANDARDS CONFORMANCE
Pthread Library pthread_default_stacksize_np(3T)