Re: Problems with large records (GNAT) [continued]

[Dr Adrian Wrigley <amtw@linuxchip.demon.co.uk>]

Laurent Guerby wrote:
> 
> Dr Adrian Wrigley <amtw@linuxchip.demon.co.uk> writes:
> >    type Big_T is array (1 .. Size) of Float;
> >
> >    type Item_T is record
> >       First : Float;
> >       Item  : Big_T;
> >       Last  : Float;
> >    end record;
> 
> Why do you want to use a record here? Looks like 100% array here.

This example shows the problem.  Even if I use an simple array, I still can't
calculate the size using the 'Size attribute, since that always
raises an exception with the size of array used in some computations.
With a record, I can add whatever fields I need later, without recoding
stuff that already works.

> Also, if you use a clean abstraction to your data structure (private
> type), you can do all sorts of hacks behind the scene.

Actually, I tried to use a generic package which could map data
onto a file, hiding the implementation details.  What I want
is really a persistent object store, allowing different utilities
to access the same data.  It could be done with a standard
database binding (eg using SQL), but the performance is much
better with mmapped records, provided the 256M limit isn't broken.

I tried using different "hacks" to get it to work with big records
until I found this fundamental problem always caused the code
to break.  I have found no way to work out the size of the record
reliably.

> If you could describe more precisely the data structure you're trying
> to mmap, perhaps comp.lang.ada readers could help a bit more.

Since you ask, I have something like the following...
------------------------------------
   type PriceSummary_T is record
      OpenPrice   : Float;
      HighPrice   : Float;
      LowPrice    : Float;
      ClosePrice  : Float; -- Split and dividend corrected price
      UncorrectedClose : Float; -- Raw share price
      Volume      : Integer;
      Time        : Time_T;
   end record;

   type StockIndex_T is new Integer range 1 .. 3000;
   type TradingDay_T is new Integer range 0 .. 1860;

   type DailyArray_T is array (TradingDay_T, StockIndex_T) of PriceSummary_T;

   type BooleanDailyArray_T is array (TradingDay_T, StockIndex_T) of Boolean;
   pragma Pack (BooleanDailyArray_T);

   type Oracle_T is record
-- Various other record values may go here!
      Tradeable    : BooleanDailyArray_T;
      Daily        : DailyArray_T;
   end record;
------------------------------------
I then want to store Oracle_T representing 3000 stocks over seven years
in a single file for efficient access by various utility programs.

If I increase the number of stocks to 5000, things break and significant
changes are necessary because the 256M limit is exceeded.
Sometimes, the wrong answers are produced, which is particularly worrying.
--
Dr Adrian Wrigley