Memory usage

[Zhanghong_T_]

Hi all,

I found a strange thing when I debugging the following code:

implicit none
integer::i,j,n
real*8,allocatable::a(:)
n=100000000
allocate(a(n))

1 &
do j=1,n
a(j)=0.d0
enddo

2 &
end

I set two breakpoints at the label 1 and 2 respectively. When the program runs to the label 1, the memory usage is (observed from Windows Task Manager):

Mem Usage: 2,488k VM Size: 783,516k

However, after it runs to the label 2, the memory usage changes greatly:

Mem Usage: 785,264k VM Size: 783,516k

Could anyone explain the changes of memory usage?

Thanks,

Zhanghong Tang

[Steven_L_Intel1]

"Mem Usage" here is most likely the number of physical pages of RAM in your process working set. Until you actually touched the elements of the allocated array, they existed only as reservations in a page table and not RAM pages assigned to your process.

[Zhanghong_T_]

Oh, that's a new concept for me. Then how to 'measure' the memory usage of a program? Based on the allocated space or the space which is actually touched? Is there any program to 'measure' the memory usage of a subroutine?

Thanks,

Zhanghong Tang

[Steven_L_Intel1]

It's hard to measure the usage of a subroutine - especialy as once memory is allocated, your virtual address space size doesn't shrink. I suppose it depends on what it is you want the information for.

Windows will tell you the total virtual address space assigned to your process, the maximum available and how many physical memory pages you are using. Depending on why you care, you may want any or all of these.