I have written a program using C which has two threads. Initially it was,
for(int i=0;i<n;i++){
long_operation(arr[i]);
}
then I divided the loop into two threads, two execute concurrently.
One thread will carry out the operation for arr[0] to arr[n/2],
another thread will work for arr[n/2] to arr[n-1].
long_operation function is thread safe.
Initially I was using join but it was taking higher sys time for futex
system call, which I observed using strace command.
So i removed the join and use two volatile variable in the two threads
to keep track whether thread is completed or not and a busy loop in
the thread spawning function to halt the execution of critical
section. And I made the thread detachable.
It improved performance a little bit. but when i used time command,
the sys part was taking,
real 0m31.368s
user 0m53.738s
sys 0m15.203s
but when i checked using the strace command the output was,
% time seconds usecs/call calls errors syscall
55.79 0.000602 9 66 clone
44.21 0.000477 3 177 write
------ ----------- ----------- --------- --------- ---------------
100.00 0.001079 243 total
So the time command was showing that around 15 seconds CPU spend in
kernel within the process. But the strace command showing almost 0
seconds was utilized for system calls.
Then why 15 seconds was wasted in kernel? I have an dual-core
hyper-threaded Intel CPU. and kernel version 4.7.9
