Getting Started with QNX Neutrino - QNX Software Systems

Threads and processes
© 2009, QNX Software Systems GmbH & Co. KG.
This is not a problem for Neutrino — Neutrino “limits” you to 32767 threads per
process! However, each thread must have a unique stack. If your stack is a reasonable
size (say 8 KB), you’ll have used 1280 × 8 KB (10 megabytes!) of stack. And for
what? There are only 4 processors in your SMP system. This means that only 4 of the
1280 threads will run at a time — the other 1276 threads are waiting for a CPU. (In
reality, the stack will “fault in,” meaning that the space for it will be allocated only as
required. Nonetheless, it’s a waste — there are still other overheads.)
A much better solution to this would be to break the problem up into 4 pieces (one for
each CPU), and start a thread for each piece:
int num_lines_per_cpu;
int num_cpus;
int
main (int argc, char **argv)
{
int cpu;
... // perform initializations
// get the number of CPUs
num_cpus = _syspage_ptr -> num_cpu;
num_lines_per_cpu = num_x_lines / num_cpus;
for (cpu = 0; cpu < num_cpus; cpu++) {
pthread_create (NULL, NULL,
do_one_batch, (void *) cpu);
}
}
... // display results
void *
do_one_batch (void *c)
{
int cpu = (int) c;
int x1;
}
for (x1 = 0; x1 < num_lines_per_cpu; x1++) {
do_line_line (x1 + cpu * num_lines_per_cpu);
}
Here we’re starting only num_cpus threads. Each thread will run on one CPU. And
since we have only a small number of threads, we’re not wasting memory with
unnecessary stacks. Notice how we got the number of CPUs by dereferencing the
“System Page” global variable _syspage_ptr. (For more information about what’s in
the system page, please consult QSS’s Building Embedded Systems book or the
include file).
Coding for SMP or single processor
The best part about this code is that it will function just fine on a single-processor
system — you’ll create only one thread, and have it do all the work. The additional
overhead (one stack) is well worth the flexibility of having the software “just work
faster” on an SMP box.
44 Chapter 1 • Processes and Threads April 30, 2009