std.container.Array/RefCounted(T) leaking memory?

[Steven Schveighoffer]

On Thu, 13 Jan 2011 12:40:04 -0500, Jesse Phillips  
<jessekphillips+D at gmail.com> wrote:

> Steven Schveighoffer Wrote:
>
>> But even so, malloc and free have the same property where they don't
>> always give back memory to the OS.  IIUC, Linux can only change the size
>> of memory it wants, it cannot free pages in the middle of the block.
>>
>> -Steve
>
> Disclaimer: I don't know what I am talking about.
>
> I think this is correct, the program isn't responsible for reclaiming  
> the memory, that is what the OS does. If you don't have an OS then you  
> don't have anything to return the memory to, so it just becomes free  
> memory. Modern operating systems aren't going to take their memory back  
> until it is needed (wast of cycles).
>
> What I observed using Linux and $ free; each section would result in a  
> reduction of free memory and an increase in buffered data. This suggests  
> to me that the OS doesn't want the memory yet.
>
> Tracking memory in a modern OS is not easy, and this is probably why no  
> one wanted to make a statement on what was really happening. As I said I  
> don't know if this is what is happening, but it usually isn't as  
> straight forward as checking memory usage.

I think all memory is allocated/deallocated from the OS via the sbrk/brk  
system call:


        brk()  and  sbrk()  change  the  location  of  the program break,  
which
        defines the end of the process's data segment (i.e., the program   
break
        is the first location after the end of the uninitialized data  
segment).
        Increasing the program break has the effect of allocating memory to  
the
        process; decreasing the break deallocates memory.

So you can only ever add to the *end* of memory, and you can only ever  
deallocate from the *end*.  And the OS doesn't ever just jump in and claim  
memory, you have to tell it that you are deallocating memory.

Which means, if you say allocated 100MB, and wanted to deallocate the  
first 99MB, you still couldn't release any back to the OS.

A moving GC would allow for more memory to be freed, but we aren't there  
yet.

Of course, I could be completely wrong about all this, I've never really  
used sbrk or brk :)

-Steve