Public bug reported:

I'm digging into unity8 performance problems on older generation Intel
GPUs - specifically i915 Atom GPUs.

I've tested a simple QML file with a single empty UbuntuShape in it:

import QtQuick 2.4
import Ubuntu.Components 1.3

Rectangle {
    width: 400
    height: 300
    color: "blue"

    UbuntuShape {}
}

Running it with INTEL_DEBUG=perf env var set, I get this output:

i915_program_error: Exceeded max ALU instructions (76 out of 64)
ENTER FALLBACK 10000: Program
Mapping a busy BO, causing a stall on the GPU.

which implies that MESA was unable to compile one of the UbuntuShape
shaders as it created more ALU instructions than the GPU could deal
with. MESA falls back to CPU rendering as a result.

The GPU stall message I guess is related to that.

** Affects: ubuntu-ui-toolkit (Ubuntu)
     Importance: Undecided
         Status: New

** Description changed:

  I'm digging into unity8 performance problems on older generation Intel
  GPUs - specifically i915 Atom GPUs.
  
  I've tested a simple QML file with a single empty UbuntuShape in it:
  
  import QtQuick 2.4
  import Ubuntu.Components 1.3
  
  Rectangle {
-     width: 400
-     height: 300
-     color: "blue"
+     width: 400
+     height: 300
+     color: "blue"
  
-     UbuntuShape {}
+     UbuntuShape {}
  }
  
  Running it with INTEL_DEBUG=perf env var set, I get this output:
  
  i915_program_error: Exceeded max ALU instructions (76 out of 64)
  ENTER FALLBACK 10000: Program
  Mapping a busy BO, causing a stall on the GPU.
  
  which implies that MESA was unable to compile one of the UbuntuShape
  shaders as it created more ALU instructions than the GPU could deal
- with.
+ with. MESA falls back to CPU rendering as a result.
  
  The GPU stall message I guess is related to that.

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https://bugs.launchpad.net/bugs/1585723

Title:
  UbuntuShape on intel i915 (Atom) uses fallback CPU rendering

Status in ubuntu-ui-toolkit package in Ubuntu:
  New

Bug description:
  I'm digging into unity8 performance problems on older generation Intel
  GPUs - specifically i915 Atom GPUs.

  I've tested a simple QML file with a single empty UbuntuShape in it:

  import QtQuick 2.4
  import Ubuntu.Components 1.3

  Rectangle {
      width: 400
      height: 300
      color: "blue"

      UbuntuShape {}
  }

  Running it with INTEL_DEBUG=perf env var set, I get this output:

  i915_program_error: Exceeded max ALU instructions (76 out of 64)
  ENTER FALLBACK 10000: Program
  Mapping a busy BO, causing a stall on the GPU.

  which implies that MESA was unable to compile one of the UbuntuShape
  shaders as it created more ALU instructions than the GPU could deal
  with. MESA falls back to CPU rendering as a result.

  The GPU stall message I guess is related to that.

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