frame rendering time measurement anomaly - kotlin

Question

I draw 2d content on GlSurfaceView using VBO/IBO with following function:
override fun onDrawFrame(gl: GL10?) {
val renderTime= measureTimeMillis {
GLES20.glClearColor(bgComps[0], bgComps[1], bgComps[2], 1f)
GLES20.glClear(GLES20.GL_DEPTH_BUFFER_BIT or GLES20.GL_COLOR_BUFFER_BIT)
bindAttributes()
GLES20.glBindBuffer(GLES20.GL_ELEMENT_ARRAY_BUFFER, ibo)
//draw half of the polygons
val pieces = GameDataLoader.GameData.piecesByDiff(gameData.diff)
piecesProgram!!.setUniforms(projMatrix,imageTextureId,maskTextureId,PointF(4f,4f),0f)
GLES20.glDrawElements(
GLES20.GL_TRIANGLE_STRIP,
pieces * MeshBuilder.INDICES_PER_PIECE,
GLES20.GL_UNSIGNED_SHORT,
0
)
//draw another half
piecesProgram!!.setUniforms(projMatrix,imageTextureId,maskTextureId,PointF(10f,10f),sin(frame.toFloat() / 60 * 2 * PI).toFloat())
GLES20.glDrawElements(
GLES20.GL_TRIANGLE_STRIP,
pieces * MeshBuilder.INDICES_PER_PIECE,
GLES20.GL_UNSIGNED_SHORT,
pieces * MeshBuilder.INDICES_PER_PIECE * MeshBuilder.BYTES_PER_SHORT
)
GLES20.glBindBuffer(GLES20.GL_ARRAY_BUFFER, 0)
GLES20.glBindBuffer(GLES20.GL_ELEMENT_ARRAY_BUFFER, 0)
}
val newFrameTS=System.currentTimeMillis()
if (lastFrameTS!=-1L)
println("frame time:${newFrameTS - lastFrameTS},render time:$renderTime")
lastFrameTS=newFrameTS
frame++
}
There are about 5.6K polygons on the screen.When I run this code I see in the console frame time: ~33ms,render time: 0-2ms(!).In most cases render time=0.If I comment everything except glClearColor and glClear render time jumps to 30ms.How can it be that the short code executes faster than long one?
UPD:The question relates to android OS.I observe such behavior on both emulator and real device.
The question is not why onDrawFrame call occurs too often or too seldom(in fact time between 2 last onDraw calls is measured ok.The question is how can I measure time for opengl calls within onDrawFrame function?How can these calls take 0ms?

Answer 1

This is usually caused by V-sync. The driver waits at the end of every frame until the next image can be presented on the monitor and delays execution of your program.
Depending on your hardware, you can force enable/disable this. Most PC hardware allows the application to choose if it wants to wait. On others, especially on mobile devices, you cannot.