Proposal: rename for in C and C++ to serial_for
No more incumbent “for.” (it was voted off the island)
(let’s assume parallel_for == cilk_for in this discussion)
Consider:
serial_for (i=0; i < n; i++) { body }
vs.
parallel_for (int i=0; i < n; i++) { body }
- serial_for allows the values of n and i to be modified inside body, parallel_for does not
- serial_for allows body of one iteration to affect body of a future iteration, parallel_for does not
- serial_for guarantees the exit value of i will be n, parallel_for does not
That’s the highlights.
Which should we use? serial_for or parallel_for?
What if I told you that every new computer today can reasonably be expected to work better with parallel_for than serial_for… and the difference will keep getting bigger in the future?
“Serial Traps” has been a topic between myself and two co-authors for an upcoming book on parallel programming.
I think we are coining phrase… and maybe this is the first coin off the presses. Maybe someone else already coined it and we didn’t know it? It’s hardly a new idea… but we constantly get reminder that it is not widely understood.
I am developing a list of “Serial Traps” which are use of a programming method, that has unintended consequences on parallel program performance. for as a serialization method – is one such trap.
I say “unintended” because I do believe large numbers, if not most, for loops are not written to depend on the features of serial_for that are not supported by parallel_for.
My favorite short example:
for (int i=0;i
This is full of unintended consequences when for means serial_for.
written as:
parallel_for (int i=0;i
or (better yet)
b[:] = a[:];
does what the programmer probably wanted – without implying unintended consequences that kill performance that super-optimizing compiler try to get back but succeed only some of the time.
Perhaps I should have proposed “Make for == parallel_for, and create serial_for to be used only for legacy needs.” Would that win more friends?
Original Source: http://software.intel.com/en-us/blogs/2011/08/16/lets-rename-for-to-serial_for/
