[SOLVED] Understanding memory usage

[Wesbat]

Hello.

I noticed that when I create multiple instances of a form, the memory usage climbs slightly each time. Also showing an already created form instance also increases memory slightly. And when these forms are closed with CloseAction := caFree it is not reclaimed back.

I noticed this only because my new hobby project requires opening multiple instances of a form.

I feel like my assumptions are wrong, or the metrics I'm seeing in Process explorer (also, Windows task manager) are misleading me.

I created a test project to demonstrate this. I enabled heaptrc and it reports no unfreed allocs.

As a learning experience I'd like to understand what is going on here, or how I should adjust my expectations 

[Leledumbo]

As far as I understood the documentation, your assumptions are correct. The memory manager does not immediately return deallocated memory to the OS upon call to Dispose/FreeMem, but instead put them into the freelist cache. The idea is that next allocation should search this cache first to avoid round trip to the OS.

[Handoko]

I downloaded the code, modified and tested on Ubuntu Mate 23.10. This is the test result:

- Starting the code the memory usage was 1.3 GB
- After clicking the button, the memory usage was 1.4 GB
- After I manually closing the forms, the memory usage was 1.3 GB

If your computer's memory usage didn't return to its previous size after closing the forms, probably because:
- OS related behavior
- You use too few samples

This is the code I added:

Code: Pascal  [Select][+][-]

1. procedure TForm1.Button1Click(Sender: TObject);
2. var
3.   NewForm: TForm2;
4.   i: Integer;
5. begin
6.   for i := 0 to 99 do
7.   begin
8.     NewForm := TForm2.Create(Self);
9.     NewForm.Color := Random($FFFFFF);
10.     NewForm.Show;
11.   end;
12. end;

------
Oops, I'm too late. Leledumbo already provided the answer.

[Wesbat]

As far as I understood the documentation, your assumptions are correct. The memory manager does not immediately return deallocated memory to the OS upon call to Dispose/FreeMem, but instead put them into the freelist cache. The idea is that next allocation should search this cache first to avoid round trip to the OS.

Thanks for the documentation link, that is good info to know Leledumbo.

If your computer's memory usage didn't return to its previous size after closing the forms, probably because:
- OS related behavior
- You use too few samples

Indeed, sample size is something I completely ignored - thanks for the good insight Handoko.

[marcov]

Code: [Select]

[quote author=Handoko link=topic=69364.msg538493#msg538493 date=1731914114]
    NewForm := TForm2.Create(Self);

Creating forms this way will assign "self" ((T)form1 instance)  as the owner of the newly created form2 instance. These will only be automatically freed when form1 is destroyed.

This is why at the end all memory is released.  For popups forms, assign NIL, and manually free them (isn't there a free on close property?), or remove them from the form1 list and free them.

[Wesbat]

You pose a good experiment marcov.

I tried instantiating the forms with both Self and Nil. In both cases I set CloseAction := caFree. In both cases the memory was released ... after waiting an undefined amount of time.

I assumed that without garbage collection, memory would be freed (nearly) immediately. I realize that is not the case - like the saying goes: assumption planted a feather and thought a chicken would grow 

[alpine]

I tried instantiating the forms with both Self and Nil. In both cases I set CloseAction := caFree. In both cases the memory was released ... after waiting an undefined amount of time.

I assumed that without garbage collection, memory would be freed (nearly) immediately. I realize that is not the case - like the saying goes: assumption planted a feather and thought a chicken would grow 

It is not so simple case with the widget-wrapper classes (as TForm). Actual reclaiming can occur at a later time depending on widgetset  internal processing - final messages exchange, GUI thread synchronization, etc.

AFAIK freeing a visual component with a non-Nil parent will extract it from the parents child list first, so it is also safe.

[Wesbat]

This was a good learning experience, thanks to everyone for your input!

I'm marking this topic as solved and bookmarking it for my own reference.