large amount of sys time when using posix threads

[Debajyoti Chatterjee <dchatterjee172 at gmail dot com>]

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