Out of memory error (even when using destroy())

[Jordan Wilson via Digitalmars-d-learn]

On Friday, 26 May 2017 at 06:31:49 UTC, realhet wrote:
> Hi,
>
> I'm kinda new to the D language and I love it already. :D So 
> far I haven't got any serious problems but this one seems like 
> beyond me.
>
> import std.stdio;
> void main(){
>     foreach(i; 0..2000){
>         writeln(i);
>         auto st = new ubyte[500_000_000];
>         destroy(st); //<-this doesnt matter
>     }
> }
>
> Compiled with DMD 2.074.0 Win32 it produces the following 
> output:
> 0
> 1
> 2
> core.exception.OutOfMemoryError at src\core\exception.d(696): 
> Memory allocation failed
>
> It doesn't matter that I call destroy() or not. This is ok 
> because as I learned: destroy only calls the destructor and 
> marks the memory block as unused.
>
> But I also learned that GC will start to collect when it run 
> out of memory but in this time the following happens:
> 3x half GB of allocations and deallocations, and on the 4th the 
> system runs out of the 2GB
>  limit which is ok. At this point the GC already has 1.5GB of 
> free memory but instead of using that, it returns a Memory 
> Error. Why?
>
> Note: This is not a problem when I use smaller blocks (like 
> 50MB).
> But I want to use large blocks, without making a slow wrapper 
> that emulates a large block by using smaller GC allocated 
> blocks.
>
> Is there a solution to this?
>
> Thank You!

I believe the general solution would be to limit allocation 
within loops (given the issue Johnathan mentioned).

This I think achieves the spirit of your code, but without the 
memory exception:
     ubyte[] st;
     foreach(i; 0..2000){
         writeln(i);
         st.length=500_000_000; // auto = new ubyte[500_000_000];
         st.length=0; // destory(st)
         st.assumeSafeAppend;
// prevent allocation by assuming it's ok to overrwrite what's 
currently in st
     }