Why TList uses a lot more memory than TArray?
I'm loading a lot of data from files, so I was testing the difference between TList and TArray memory consumption. And it seems TList uses a lot more memory.
This is my simple test:
procedure TForm8.Button1Click(Sender: TObject);
type
TDataRec = record
ID: integer;
Name: string;
end;
var vDataRec: TDataRec;
vDataList: TList<TDataRec>;
vArray: TArray<TDataRec>;
i, vMax: integer;
begin
vMax := 10000000;
vDataList := TList<TDataRec>.Create;
try
for i := 1 to vMax do
begin
vDataRec := Default(TDataRec);
vDataRec.ID := i;
vDataRec.Name := 'X';
vDataList.Add(vDataRec);
end;
finally
vDataList.Free;
end;
SetLength(vArray, vMax);
for i := 0 to vMax-1 do
begin
vArray[i].ID := i;
vArray[i].Name := 'X';
end;
vArray := nil;
end;
And here are results as I see them in Task Manager in Memory (active private working set) column - I believe this is the one that can be monitored when/if Out of memory error occurs.
TList<TDataRec>:
Starting memory: 3,716 K
Filled list: 369,488 K
Consumption: 365,772 K
TArray<TDataRec>:
Starting memory: 5,548 K
Filled list: 316,588 K
Consumption: 311,040 K
TList<TDataRec> vs TArray<TDataRec> = 54,732 K = 17%
17% is not extreme, but it is significant difference.
But! If the TDataRec only has 1 integer:
TDataRec = record
ID: integer;
end;
Now the difference is unacceptable:
TList<TDataRec>:
Starting memory: 3,720 K
Filled list: 69,700 K
Consumption: 65,980 K
TArray<TDataRec>:
Starting memory: 4,208 K
Filled list: 43,272 K
Consumption: 39,064 K
TList<TDataRec> vs TArray<TDataRec> = 26,916 K = 68%
68% seems extreme difference!
Is this normal, or am I missing something very obvious?
Like others said, TList has a capacity growing strategy.
The second, and more important thing is, that the memory manager doesn't release the memory back to windows.
So what you might getting is this:
when you add items to the list, and it tries to resize, it might happen, that there is not sufficient memory in the current place to just in place resize. so the memory manager copies the whole array to a new location, that can hold the size of the new array.
but the old memory space is still kept reserved by the memory manager.
So when you use the task manager, windows sees both.
Andd BTW, the memory manager doesn't request the exact size either. Like TList, it has a growth strategy, too.
So what you might getting is this:
when you add items to the list, and it tries to resize, it might happen, that there is not sufficient memory in the current place to just in place resize. so the memory manager copies the whole array to a new location, that can hold the size of the new array.
but the old memory space is still kept reserved by the memory manager.
So when you use the task manager, windows sees both.
Andd BTW, the memory manager doesn't request the exact size either. Like TList, it has a growth strategy, too.
