Memory mapped files and shared memory (Was: Newbie Needs Ada Advice)

[Jacob Sparre Andersen <sparre@nbi.dk>]

Markus E Leypold wrote:
> Jacob Sparre Andersen <sparre@nbi.dk> writes:

>> File names are slow.  Wouldn't it be more appropriate to keep the
>> currently relevant image(s) in a shared memory segment?
>
> Dependents on what you want to do. Probably not the whole 30 GB. My
> rule of thumb always is to do the elementary thing first and
> optimize later.

In general that is also what I do, but isn't it simpler to store the
whole sequence of images in an array of images?  Mapping that array
into a file/shared memory segment costs you around 20 lines of code
(even with my rather extravagant use of linebreaks).

There is one catch: I am not sure that the Ada interface to POSIX
systems is required to support memory maps as large as 30 Gb.
(Additionally I suspect that the original poster isn't using a POSIX
system.)

> In this case one could use the file storage (one use it anyway),
> introduce (when required) the shared memory for explicit caching (or
> perhaps play with memory mapping the file) and refine the
> get-the-image protocol appropriately. My gut feeling though is, to
> trust the file system cache of the machine to do the proper caching
> implicitely and there should be a good reason to think you can do
> better than that manually.

Using memory maps goes even further in leaving the caching/swapping
decisions to the operating systems, than copying individual images to
and from files.

Greetings,

Jacob
-- 
recursive, adj.; see recursive