Sculpt Performance Regression in 2.81 #71434

System Information
Operating system: Windows 10 64 bit
Graphics card: Nvidia GTX 750 Ti

Blender Version
Broken: blender-2.81 eefd806afc
Worked: blender-2.81 aff6446e06

Short description of error

Sculpting Performance is noticeably worse.
Especially switching brushes lags.

Exact steps for others to reproduce the error
slow_sculpt.blend

Open attached file.
Apply the modifiers on the mesh to get a dense mesh.
Enter sculpt mode.
Start sculpting and switch between brushes with keyboard shortcuts.
Experience lag with the newer version.

Added subscriber: @Foaly

#71457 was marked as duplicate of this issue

Added subscriber: @PabloDobarro

The PBVH rebuilds each time the brush radius changes and while modifying mesh properties. This is probably one of the reasons. 2019-11-09 01-03-16.mp4

I think the problem is in ##BKE_sculpt_update_object_before_eval##. Why is the mesh data updated when changing the brush radius?

Added subscriber: @Tyler-Foo

Added subscriber: @Thura

Added subscriber: @ebarranco

Added subscriber: @jfmatheu

As noted in D6219 this is caused by 79b703bb63.

Whats happening now is "technically" correct.

Brushes & UnifiedPaint request undo.
The radial control operator also is flagged to undo.
Before writing the memfile undo: sculpt data is flushed to the mesh,

otherwise memfile undo would store & load old mesh data, loosing the users sculpt.

Previously sculpt mode prevented all other undo systems from being used, so even when a change required memfile undo to be written, the undo step was written as a sculpt step which did nothing.

It seems users prefer undo to be disabled for brushes (#60640) so here is a patch that disables that D6240: Avoid undo pushes on brush & unified-paint data (addresses #71434).

From my own testing this works without storing undo steps when changing brush/scene/object settings.

Although operators that enable OPTYPE_UNDO and don't use sculpt-undo will write memfile steps.

Note that the fix for #69822 (Undo is making objects to disappear in sculpt mode) could be reverted, although this re-introduces errors selecting objects while sculpting which mixes up sculpt objects and their undo steps, crashing in some cases.

This issue was referenced by c7b7722254

Changed status from 'Open' to: 'Resolved'

Added subscriber: @Leha

This problem still exists when changing brush in sculpt mode. Changing brush size is good now.
Changing brush was fast in 2.80 and now its laggy
Exact steps for others to reproduce the error:
Open high poly mesh in sculpt mode and try changing brush

Committed changes so brush changes don't perform undo pushes bcff803712 9fff74f144

Thanks! Working good now! The only thing is the undo of the first stroke after entering sculpt mode is lagging. If you enter sculpt mode for high poly mesh and make one stroke and then undo is lagging. Don't know if its worth checking

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Sculpt Performance Regression in 2.81 #71434