Static Memory allocation

**Elysia** · 12-20-2008

Originally Posted by kermit

I am not quite following you here. Where do you get the 1MB from? (Not trying to nitpick, but rather trying to learn

)

Because a typical stack size is 1 MB. It's the default under Windows.
Not to say it is limited to one MB. It can be larger (or smaller), but I just did not want to type that out.

Originally Posted by itCbitC

Considering that a double is 8 bytes on most machines, the storage requirements of those arrays would be "13824*3*3*3*2 + 576*12*2*1*9*9*2 + 576*2" times 8 bytes which is about 6Mb plus the size of the arrays defined in the other subroutines.

Which would probably be too much for the stack.
Yes. This is probably the main cause for the problems.

Physical memory on your machine is about 3 Gb and swap is about 2 Gb; so total virtual memory is 5 Gb not 0.5 Gb; not sure how you get 0.5 Gb.

There is one computer (laptop) with 3 GB of memory, and one machine (university) with 0.5 GB of memory.

**p3rry** · 12-21-2008

Hey yesterday the message board didn't work...but thank to your advices I got the solution of my problem.
The fact is that since I started to program in c nobody told me that I could declare global variables... now I declare the arrays outside the main and I got all the memory I need. Thank you very much...bye

**Elysia** · 12-21-2008

Originally Posted by p3rry

The fact is that since I started to program in c nobody told me that I could declare global variables... now I declare the arrays outside the main and I got all the memory I need. Thank you very much...bye

You should not do that!
Use dynamic memory instead!

**p3rry** · 12-22-2008

Why? I heard that dynamic allocation slows down the program and I need much statistic, so I'd prefer a faster simulation.

**tabstop** · 12-22-2008

You can make dynamic allocation very slow, if you want to. If you want it fast, then don't do that. (I.e., calling malloc once in the program should not slow your program down to any appreciable degree. Calling malloc once for every element in your array will make your program very slow indeed.)

**zacs7** · 12-22-2008

> Why? I heard that dynamic allocation slows down the program and I need much statistic, so I'd prefer a faster simulation.
And using global memory for such a thing is stupid. Take your pick, "slow" or stupid?

Well depending on how you do it, there may be no slow down or close to none (~0%) [a].

[a] For example grow in chunks of 1 MB as you require memory -- or 10MB

**Elysia** · 12-22-2008

Originally Posted by p3rry

Why? I heard that dynamic allocation slows down the program and I need much statistic, so I'd prefer a faster simulation.

That is a myth. Regardless if dynamic or static, the OS still needs to allocate it, and it usually goes to the same system call in the end anyway.
Dynamic memory can be slow if you use is incorrectly. But if you simply need one big block of memory, it should be equal in speed.

**whiteflags** · 12-22-2008

i.e. most people who care about performance will operate on their assumptions, which is worse than actually measuring, or in fact, not caring at all.

Memory allocation is not instantaneous, true, but it is not relevant to user time. Ever.

**C_ntua** · 12-22-2008

Originally Posted by Elysia

That is a myth. Regardless if dynamic or static, the OS still needs to allocate it, and it usually goes to the same system call in the end anyway.
Dynamic memory can be slow if you use is incorrectly. But if you simply need one big block of memory, it should be equal in speed.

Well said.
The "dynamic memory allocation can slow the program" refers to situations that you have no other choice. For example:

for(alot){
do stuf
re-allocate memory
}

The reallocation may slow a lot the program compared to "do stuff". But there is no other way to achieve this. So you are warned that allocation isn't free.

If you just allocate in the beginning of your program memory then it shouldn't really matter (a lot) where you allocate memory. If the program runs main and allocates memory, or pre-allocates memory for globals and afterward calls main should make small difference

**p3rry** · 12-23-2008

Ok... but there's any problem in using global arrays?
(Sorry but my basic knowledge is very poor)

**laserlight** · 12-23-2008

Originally Posted by p3rry

Ok... but there's any problem in using global arrays?

There would be the problems associated with global variables in general (we have discussed this in recents threads, so you might want to do a search).

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