Fortran - array of pointers

Author

array of pointers

Millena

2005-09-26, 7:00 pm

Hi...

I am using an array of pointers in FORTRAN 90 where those points store
the address of a linked list. I Would like to know if when I use an
array of pointers is neccesary more RAM memory to store these datas. Is
possible to know how much percent it increase ??

here is an example of an array of pointers , where the array eul_pts
store the current pointer for each cell of an adapative mesh... is
correct the notation?

Do i = ig-nbc,ig+nx-1+nbc
Do j = jg-nbc,jg+ny-1+nbc
eul_pts(i,j)%em_pt(i,j)=curr_aux
end do
end do

Millena

Steven G. Kargl

2005-09-26, 7:00 pm

In article <1127758349.567198.214370@z14g2000cwz.googlegroups.com>,
"Millena" <millena.villar@gmail.com> writes:

> I am using an array of pointers in FORTRAN 90 where those points store
> the address of a linked list. I Would like to know if when I use an
> array of pointers is neccesary more RAM memory to store these datas. Is
> possible to know how much percent it increase ??
>
>
> here is an example of an array of pointers , where the array eul_pts
> store the current pointer for each cell of an adapative mesh... is
> correct the notation?
>
> Do i = ig-nbc,ig+nx-1+nbc
> Do j = jg-nbc,jg+ny-1+nbc
> eul_pts(i,j)%em_pt(i,j)=curr_aux
> end do
> end do
>

You need to provide more information. In particular, we
need to know the definition of your derived type.

--
Steve
http://troutmask.apl.washington.edu/~kargl/

Richard E Maine

2005-09-26, 7:00 pm

In article <1127758349.567198.214370@z14g2000cwz.googlegroups.com>,
"Millena" <millena.villar@gmail.com> wrote:

> I Would like to know if when I use an
> array of pointers is neccesary more RAM memory to store these datas. Is
> possible to know how much percent it increase ??

Well, for a start, there is no such thing as an array of pointers in
Fortran. The closest thing is an array of derived type, where the
derived type has a pointer component. From your example, this is almost
certainly what you are talking about.

If you want precise size numbers, that is compiler-specific. But as a
rough approximation, a pointer to an array is going to need to have room
to store the base address of the array, plus the lower bound, size, and
stride for each rank. Some compilers might plausibly also store the
upper bound for each rank, although that is at least largely redundant.

Note that the additional size used by a pointer has nothing at all to do
with the size of the array - only with its rank. So it is pretty much
meaningless to ask about a percentage.

On the whole, unless you have an awful lot of small arrays, the extra
space used for pointers is negligible. That's because the extra size for
the pointer is not proportional to the array size.

Since your sample arrays are of rank 2, assuming 3 values per rank, plus
a base address, that is about 7 values per pointer. Round up a bit to
about 10 in case the particular implementation stores some extra data.
Assume 8 bytes (64-bits) per value; many implementations will use 4
bytes, but assume 8 to be pessimistic, as some use that much. That adds
up to about 80 bytes per array. If you have a million of these arrays,
that's still only about 80 megabytes, which is a fair amount of storage,
but if you have a million arrays, odds are high that your application is
using a lot more than 80 megabytes anyway.

It is *POSSIBLE* to have cases where the extra storage for pointers is
important, but that is an unusual edge case. Other efficiency issues
than space are much more likely to be pertinent.

--
Richard Maine | Good judgment comes from experience;
email: my first.last at org.domain | experience comes from bad judgment.
org: nasa, domain: gov | -- Mark Twain

Millena

2005-09-27, 7:57 am

Hi Steve,
below there are the derived type that I am using, it is a little
extensive and confuse... but the important thing is that my pointer
store 5 or 6 informations and I will have a hundred of arrays of
pointers in my code. I hope that it show what I am implementing...

thanks in advance
Millena

Type grid_components
Integer :: ix ,iy ,mx ,my , iu, ip
Integer:: iCoord
End Type grid_components

Type level_components
Double precision:: hx ,hy
Type(grid_components) :: grid
Type(level_components), Pointer :: next
End Type level_components

Type EL_pointer
Type(level_components),Dimension(:,:),Po
inter::em_pt
End type EL_pointer
Type(EL_pointer),allocatable,dimension(:
),save::ptarray_EL

Subroutine self_address(Sx,Sy,ig,jg,nx,ny,curr_aux,
eul_pts)
Implicit None
Integer::ig,jg,nx,ny,i,j,&
Sx,Sy

Type(level_components),Pointer::curr_aux

Type(EL_pointer),dimension(ig-nbc:ig+nx-1+nbc+Sx,jg-nbc:jg+ny-1+nbc+Sy)::eul_pts

Do i = ig,ig+nx-1
Do j = jg,jg+ny-1
eul_pts(i,j)%em_pt(i,j) = curr_aux
End do
End do

End Subroutine self_address

Millena

2005-09-27, 7:02 pm

Hi Richard,

my concern is that when I implemented the array of pointers was
necessary to obtain more RAM memory. Now I really can't imagine what
is leading this increase in the merory mainly if I am taking care in
store the datas dinamicaly.

after the array of pointers I was runing my code in a machine with 512M
now I have to use one with 1.4G. Perhaps the dinamic allocation of a
array of pointers can demand more memory... Is possible ??

thank you
Millena

Jugoslav Dujic

2005-09-27, 7:02 pm

Millena wrote:
| Hi...
|
| I am using an array of pointers in FORTRAN 90 where those points store
| the address of a linked list. I Would like to know if when I use an
| array of pointers is neccesary more RAM memory to store these datas. Is
| possible to know how much percent it increase ??

For what it's worth, Visual Fortran pointer descriptor format (on 32-bit
systems) is:

- 4 bytes for a pointer-to-scalar (same as C void*)
- for a pointer-to-array, 5 (Compaq) or 6 (Intel) dwords header,
plus 3 dwords per each dimension, meaning 32 (48) bytes for
a 2-D array.

You can find for yourself the figures for your compiler using:

type foo
integer, pointer:: p(:) !or p(:,:)
end type foo

type(foo):: f
write(*,*) size(transfer(f,(/" "/)))

--
Jugoslav
___________
www.xeffort.com

Please reply to the newsgroup.
You can find my real e-mail on my home page above.

Richard Maine

2005-09-27, 7:02 pm

Millena wrote:

> after the array of pointers I was runing my code in a machine with 512M
> now I have to use one with 1.4G. Perhaps the dinamic allocation of a
> array of pointers can demand more memory... Is possible ??

From another of your posts, I saw a mention of "hundreds" of pointers.
That is not within several orders of magnitude of plausible for a cause
of your memory problems. Let me round your "hundreds" up to a thousand,
and my size estimate for pointer overhead up to 100 bytes. That gets up
to about 1 megabyte. No. This just is not even close to plausible. You
are looking in the wrong direction if you are looking there.

I would find it much more plausible that you have a memory leak - where
you repeatedly allocate pointers and fail to deallocate them. That is a
very common problem with pointers. There are also lots of other common
problems with using pointers, but the overhead of pointer storage just
is not one except in the most unusual of edge cases, which is clearly
*NOT* what you have - not if you only have "hundreds" of arrays. Not
even if you have thousands. You need to have more like millions before
it would be plausible as an issue.

--
Richard Maine | Good judgment comes from experience;
email: my first.last at org.domain | experience comes from bad judgment.
org: nasa, domain: gov | -- Mark Twain

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