PGI User Forum

[Youngsung_]

Hi,

After finding out that private clause in loop construct caused performance penalty, I had a question regarding PGI's private clause interpretation.

According to OpenACC standard v1.0, private clause is allowed on parallel construct and loop construct, but not on kernels construct. And if private clause is on loop construct, the variables in private clause are supposed to be created at every iteration. Here is my question regarding "kernels" and "private" usage: If I want to declare explicitly a list of variable as private within a gang, but do not want to create per every iteration with kernels construct, what is the correct way to use those constructs and clause?

Thanks,

Youngsung

[mkcolg]

Hi Youngsung,

Do you mean that you want to create a variable that is private to a gang but shared amongst the vectors in a gang?

[Youngsung_]

Hi Mat,

Thanks for your kind explanations. It is good to know to put private clause on loop gang construct for vectors to share variables.

However, my situation is a bit more complicated. Please see my code below:

1 !$acc kernels
2 !$acc loop gang(ngangs) vector(neblk)
3 do ie=1,nelem
4 !$acc loop vector(npts) private(s1,s2,i,j,k,l)
5 do ii=1,npts
6 ... computation using private variables and others

On line #4, I put private and it caused performance penalty.
On line #2, I have gang as well as vector. When I move private clause from line #4 to line #2, I saw approx. 10% performance improvement but had different computation result from previous one.

Actually, when I completely deleted private clause from source code, I was able to get the same result as well as 2X speed-ups. So, I am still confusing how PGI handles the private clause.

Thanks,

Youngsung

[mkcolg]

[Youngsung_]

I've got clear idea now how it works!!! Thanks a lot, Mat.