Re: Help needed - Upper Bound of an array - Question.

[adacrypt <austin.obyrne@hotmail.com>]

On Feb 7, 3:08 pm, Ludovic Brenta <ludo...@ludovic-brenta.org> wrote:
> adacrypt wrote on comp.lang.ada:
>
>
>
>
>
> > I am not an expert programmer in Ada but I have taught myself enough
> > Ada-95 to be able to write some difficult cryptography programs that I
> > post in another group.
>
> > My problem is this.  I need to test the frequency of the ciphertext
> > which is sometimes a long string of large positive integers of 7 or 8
> > digits in magnitude and to do that I need an array that will
> > accommodate up to 10,000,000 elements ideally.
>
> > I have already found out that I cannot go more than 500,000 elements
> > in the array size.  My computer has 32-bit architecture.
>
> > What I need to know from some kind person is this - Is the array size
> > a property of the Ada-95 language or the computer?
>
> > I need to know this before resorting to a 64-bit computer which might
> > not solve the problem and be an expensive mistake.
>
> > Your help would be greatly appreciated.
>
> Both.
>
> The computer (and its operating system) limit the size of a single
> object in memory and also the maximum combined sizes of all objects
> you allocate.  On modern 32-bit operating systems, this limit is
> usually 2, 3 or 4 gibibytes (1 gibibyte = 2**30 bytes) per process.
>
> However there is also a limit imposed by the compiler and linker: the
> stack size. The compiler and linker emit code that will allocate a
> stack for each thread. This stack will have a limited size, typically
> 2 mebibytes (2 * 2**20 bytes) per thread.  If you declare Ada tasks,
> you can tell the compiler how large the stack must be for each but you
> must use linker options to control the size of the stack for the
> environment task (the task where your main program executes).
>
> Your observation that 500_000 elements (presumably 32-bit integers) is
> the maximum you can achieve suggests that you are allocating your
> array on the stack of the environment task (500_000 * 32bits = 2
> million bytes, i.e. almost 2 mebibytes).  If you would allocate your
> array from the heap, you would be able to use all your physical and
> virtual memory.
>
> Stack allocation looks like:
>
> declare
>    My_Array : array (1 .. 500_000) of Integer; -- 2_000_000 bytes on
> the stack
> begin
>    ...
> end;
>
> Heap allocation looks like:
>
> declare
>    type Big_Array is array (Positive range <>) of Integer;
>    type Big_Array_Access is access Big_Array;
>    procedure Free is
>      new Ada.Uncheched_Deallocation (Big_Array, Big_Array_Access);
>    My_Array : Big_Array_Access is new Big_Array (1 .. 10_000_000);
>    -- 40_000_000 bytes on the heap
> begin
>    ...
>    Free (My_Array); -- optional, if your program just exits at this
> point, the operating system will reclaim memory
> end;
>
> Note that allocating such a large array from the heap presumes a
> large, *contiguous* range of free addresses in your virtual address
> space.  Such a range normally exists when your program starts but will
> probably be fragmented into insufficiently large free ranges if you
> allocate and deallocate lots of smaller objects.  So, you should
> allocate your large array as early as possible in the execution of
> your program.  Alternatively, you should consider replacing the array
> with a rope (seehttp://en.wikipedia.org/wiki/Rope_%28computer_science%29).
>
> HTH
>
> --
> Ludovic Brenta.- Hide quoted text -
>
> - Show quoted text -

Many thanks to everyone - your'e a bit over my head technically but I
get the gist of it from each one of you.

This is my declaration.

 SUBTYPE Index_30 IS Integer RANGE -500000 .. 500000;
  TYPE I_CoefficientsNumArray IS ARRAY(Index_30) OF Integer;
  I_Num : I_CoefficientsNumArray;
  -- counts the occurences of the j coefficients of the ciphertext.

Question : can I use your 'big array' declaration procedure in Ada-95
just as you have typed here ? that would be great - will it work just
by replacing the present declaration with this? do I type in as a
straight crib just like that, regards - adacrypt