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An fast realtime interactive fractal zoomer

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3.2 XaoS thread library

This description should be usefull for those, who want to port XaoS into multiprocessor platforms and those, who want to implement some filter or other relativly computational expensive code. Note that thread library should be mapped into nothread calls, in case host does not allows multithreading or it is not SMP architecture (since this library is used only to distribute calcualtion into other CPUs)

XaoS thread library is simple map of few functions required by XaoS to system library for threads.

It has following variables:

Variable: ethreads
This is set to 1 in case that threads are enabled

Variable: nthreads
Number of threads

It and following calls:

Function: void xth_init (int threads)
This function initializes threading library (starts threads, sets ethread to 1 and nthreads to n. Threads parameter should be set to 0 -- autodetection or number of threads users wants. In case threads is set to 1, threading library is disabled and following functions are mapped into ther nothread_ equivalents defined in xthread.h.

Note that threads are not identical -- there is main thread (one that called xth_init) that comunicates with drivers, controls calcualtion etc. and other tasks that are waiting to orders from main task. They also can't use functions from xthread library.

Function: void xth_uninit (void)
This function uninitializes thread library -- kills child threads, sets ethread to 0 and nthreads to 1.

Function: void xth_function (xfunction *function, void *data, int range)
This function is used in case, engine wants to perform some operation at image in parael. It is expected to wait until all threads are ready and start function at all threads including control one with following paramters: data -- this parameter is same as data passed to xth_function, taskinfo -- pointer to structure taskinfo, that is platform depended (defined in xthread.h) but must have at least field n, that holds number of thread (control thread has 0 and other numbers in range 1 -- nthreads). Next two parameters is range of image, function is expected to do action. Xth_function is expected to divided range into nthreads equal pieces and pass always start of piece and start of next piece (range in case of last one). Function does not wait for other threads at the end and returns imediately to main thread after function returns.

This function is called approx 5--10 times per frame

Function: void xth_sync (void)
This functions waits until all threads are ready for next order from main task.

This function is called approx 5--10 times per frame

Function: void xth_bgjob (xfunction *function, void *data)
This function is expected to behave as follows: look if there is any thread waiting for orders, if so, ask him to call function with similiar conventions as in xth_function except that range parameters are set to 0. Otherwise it starts function in normally (at foreground).

This function is called once per frame.

Function: void xth_nthread (struct taskinfo *s)
This function should be used to determine number of current thread. Do not use taskinfo->n instead since in case threads are disabled, it should be defined to 0 and that allows optimizer to perform better optimizations. This function should be called by all threads.

Function: void xth_lock (int n)
Function: void xth_unlock (int n)
Lock/unlock lock number n. At least MAXSEMAPHORS locks must be available.

Note that locks are used always for very short fragments of code so they needs to be fast. So spinlocks are maybe better than Dijskra semaphors. Untested. They are called once per calculated line/row during zoom and once per approx 10 pixels during calculation of new image.

Function: void xth_sleep (int n, int l)
It is expected to atomically unlock lock l and sleep in queue n. At least MAXCONDS queues must be available. After it is waked up, lock l again. This mechanizm is used by calcualtion of new image algorithm, but it is designed to minimize its calls, so I expect they should be called once or twice.

Function: void xth_wakeup (int n)
Wake up some thread from queue n. Lock used by sleep calls is locked in this cases. Function should also wake up all threads if such operation is not supported by host API. With luck, this function should not be called at all. It should be called by new image caluclation routines in case queue is empty. This happends in case of 50 threads but happeds rarely at two or eight threads according to my tests.

Function: void xth_wakeall (int n)
Similiar to wakeup but wake up all threads.

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