how can i keep variables private in subroutines which are called in a parallel section of openmp?

Question

i am calling a nested for loop as follows:

do ir = 1,Nr
    do iom = iom1, iom2
       xyz(1) = xo(1) + xom(iom)*r
       xyz(2) = xo(2) + yom(iom)*r
       xyz(3) = xo(3) + zom(iom)*r
       call FUNSUB(xyz,Nprop,PropXYZ)
    enddo
enddo

where FUNSUB evaluates a property in the following manner:

id = 1
do l=0,lmax
    do m=-l,l
        id = id + 1
        Prop(id) = RHO * Ylm(l,m,xl(1),xl(2),xl(3))
    enddo
enddo

now i am trying to parallelize this with something of the form

!$OMP parallel do reduction(+:Prop) private(ir, l, m, j, iom, r, wrad, xyz, PropOm, PropOm1, PropXYZ)

where some of the other private variables seen here are left out of my example code for brevity. i am intentionally trying to make l and m private so that inside the FUNSUB loops, the properties are evaluated independently. i find, however, that they are shared between threads. i have put in some debugging options and found that id continually goes above the bounds of Prop and have found that l also goes about lmax as well as m being outside the bounds of (-l,l).

my question is: how do i go about making sure that l and m are kept private and NOT shared from the main routine? it seems that the problem is that it is an entirely different subroutine which holds these values and that, somehow these private declarations of variables do not carry over

Please post a full code of the FUNSUB. Correct usage of an external function in OpenMP region should not be a problem. — Vladimir F Героям слава
– Vladimir F Героям слава, Commented Jun 23, 2012 at 19:17

Community · Accepted Answer · 2017-05-23 12:22:35Z

3

I think your problem is that in Fortran, local subroutine variables can be static and that the !$OMP does not propagate to called functions.

Your code should work as expected if you copy-paste FUNSUB into the calling loops, i.e. if you manually inline it.

Update

After reading-up a bit on the looniness of how local variables may or may not be implemented by the compiler, I guess your best option is to declare FUNSUB as RECURSIVE. This forces all local variables onto the stack, meaning that each OpenMP thread which calls FUNSUB will have its own, private, set of local variables.

edited May 23, 2017 at 12:22

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answered Jun 23, 2012 at 16:30

Pedro

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3 Comments

drjrm3 Over a year ago

i knew it was because i was calling an external function, but i want to find a way around inlining it because FUNSUB is actually a function handle passed to the subroutine and may be a different subroutine on different calls.

Pedro Over a year ago

@Laurbert515: If your function is a function handle, then you're kind of stuck as OpenMP statements don't propagate to subroutines. How should the compiler know about that when compiling your subroutine? The only way around this is to either declare the variables in FUNSUB with the opposite of SAVE attribute, although I don't even know if this exists, or to pass them on as parameters to that function, which will be massively inefficient. There may be a compiler option that does the former.

Pedro Over a year ago

@Laurbert515: Scratch the previous comment, the RECURSIVE keyword should solve all your problems. I have updated my answer accordingly.

Vladimir F Героям слава · Accepted Answer · 2016-07-24 16:29:20Z

1

The full code of the FUNSUB would have to be investigated. Correct usage of an external function in an OpenMP region should not be a problem. There should be no reason and it is not possible to declare variables in scope of an external function as private.

Modern Fortran compilers will not create local variables of a function static, if you don't tell it to do so. This may be done using a compiler switch (big NO NO in a parallel code) or by declaring the variables as SAVE as has been save. Note that all initialized variables are SAVE implicitly. That means that:

   integer :: local = 0

is SAVE even without the SAVE keyword and will be shared among threads.

It is a good practice to have all functions called in parallel declared as PURE. I am even for having all non-recursive functions at least effectively PURE.

edited Jul 24, 2016 at 16:29

answered Jun 23, 2012 at 19:21

Vladimir F Героям слава

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4 Comments

drjrm3 Over a year ago

funsub is actually a function handle so any function could be passed in for it, thus it is not possible to post the code of funsub.

Pedro Over a year ago

Since FUNSUB actually seems to alter some of its input arguments, the keyword PURE will not work (see fourth point here).

Vladimir F Героям слава Over a year ago

In that case I am strongly against conszructing such procedures as functions and I know I am not alone. A subroutine should be used. If possible, a pure one

Vladimir F Героям слава Over a year ago

(i.e. see my last sentence of the answer)

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