Memory keeps increasing while FBX Import (modal) window is open in editor

Development Platform & Builds
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1

Hi,

I’m seeing a memory growth issue related to Slate modal windows.

Is this a known issue with FSlateApplication::AddModalWindow , and is there any recommended fix or workaround other than avoiding modal windows ?

If I set bSlowTaskWindowto true, the game thread keeps ticking, Slate gets flushed, and the memory growth issue goes away.

However, in our project this causes unwanted side effects because the game thread is ticking while the modal dialog is open, so we can’t use this workaround.

2

Steps to Reproduce
Repro steps (tested with UE 5.3.2 ~ UE 5.5):

  1. Open the Unreal Editor.
  2. Import any FBX file so that the FBX Import window (modal dialog) appears.
  3. Do nothing and just leave the FBX Import window open.
  4. Watch the editor process memory in Task Manager / Activity Monitor.

Result:

Memory usage keeps increasing steadily while the FBX Import modal window is open and idle.

If I avoid using modal windows (e.g., switch similar tools to modeless windows), memory stays stable and does not grow.

3

Hi,

We do have a bug report for this issue (UE-145672) but we’ve been unable to reproduce it, so it doesn’t appear to affect *all* modal windows. I’ll leave the engine running with the FBX import modal over the weekend and see if I hit the same issue.

Best,

Cody

4

Hi,

Running the editor over the weekend confirmed that we can reproduce this leak with the Import FBX dialogue. The report mentioned above is a bit stale and may not be the same exact issue, so I filed a new bug report (UE-357262) with the repro steps for the team to investigate.

I suspect we may be accumulating Stats messages that can’t be dispatched since we’re frozen on the same frame, so maybe the stats system just needs to be manually flushed somewhere as part of the modal loop. I’m not sure there’s a great workaround here at the moment since there seems to be more than one worker thread leaking messages, but I’ll reach out if we have any updates to share.

Best,

Cody