[dolphin] [Bug 441384] New: Dolphin's UI responsiveness becomes slow while counting items inside subfolders in an NTFS partition

[Eduardo]

https://bugs.kde.org/show_bug.cgi?id=441384

            Bug ID: 441384
           Summary: Dolphin's UI responsiveness becomes slow while
                    counting items inside subfolders in an NTFS partition
           Product: dolphin
           Version: 21.08.0
          Platform: Neon Packages
                OS: Linux
            Status: REPORTED
          Severity: normal
          Priority: NOR
         Component: view-engine: details mode
          Assignee: dolphin-bugs-null at kde.org
          Reporter: zlotz at hotmail.com
                CC: kfm-devel at kde.org
  Target Milestone: ---

Created attachment 140954
  --> https://bugs.kde.org/attachment.cgi?id=140954&action=edit
Script to create a big folder structure

SUMMARY
Dolphin's UI responsiveness becomes slow while counting items inside subfolders
in an NTFS partition

STEPS TO REPRODUCE
1. Place the attached script on a writeable NTFS partition and run it. It will
create a folder named 'bigfolder' with 100 subfolders, and each of them with
100 empty files.
2. Open the newly created 'bigfolder' on Dolphin, making sure the "Details View
Mode" is active (Ctrl+3).
3. As soon as Dolphin opens the folder, try to resize the Dolphin window. Try
to move the scrollbar. Try to drag-select a bunch of subfolders.

OBSERVED RESULT
The above operations will all feel very slow and unresponsive: Dolphin window
will refresh itself at very slow fps. After around 5-10 seconds, it becomes
smoothly responsive again. Pressing F5 (refresh) will trigger the bug again.


EXPECTED RESULT
Dolphin's UI should remain perfectly responsive, even if there are background
tasks running, such as counting items inside subfolders.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: KDE Neon 5.22, Linux 5.11.0-27-generic (64-bit), Dolphin
21.08.0
KDE Plasma Version: 5.22.4
KDE Frameworks Version: 5.85.0
Qt Version: 5.15.13

ADDITIONAL INFORMATION
On my system the bug happens only on NTFS partitions, even if running on a fast
SSD.

Dolphin is counting the number of items inside each of the subfolders to
display in the "Size" column, which is great, but this task is slowing down the
UI thread.

I understand support for NTFS file system is not native to Linux, so it is
expected that the actual file system operations should not work as fast. But I
believe efforts can be made inside Dolphin to at least not lock the UI thread,
making sure the potentially slow tasks are running in a background thread,
keeping the UI responsive all the time.

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