Find performance bottlenecks

gDEBugger User Guide

Find performance bottlenecks

Graphic Pipeline Bottlenecks

The graphics system generates images through a pipelined sequence of operations. A pipeline runs only as fast as its slowest stage. The slowest stage is often called the pipeline bottleneck. A single graphic primitive (for example, a triangle) has a single graphic pipeline bottleneck. However, the bottleneck may change when rendering a graphic frame that contains multiple primitives. For example, if the application first renders a group of lines and afterwards a group of lit and shaded triangles, we can expect the bottleneck to change.

The first step in optimizing graphical applications performance is to identify the slowest graphics pipeline stage. gDEBugger provides tools that enable you to identify graphic pipeline bottlenecks.

The OpenGL Pipeline

The OpenGL pipeline is an abstraction of the graphics system pipeline.

GPU pipeline image

Some of the pipeline stages are executed on the CPU; other stages are executed on the GPU. Operations executed on top of the GPU may be executed in parallel.

Application û the graphical application, executed on the CPU and calls OpenGL API functions.
Driver û the graphics system driver, runs on the CPU and translates OpenGL API calls into actions executed on either the CPU or the GPU.
Geometric operations û the operations required to calculate vertices attributes and position within the rendered image 2D space: multiplying vertices by the model view and projection matrices, calculating vertices lighting values, executing vertex shaders, etc.
Raster operations û operations operating on fragments / screen pixels: reading and writing color components, reading and writing depth and stencil buffers, performing alpha blending, executing fragment shaders, etc.
Frame buffer û a memory area holding the rendered 2D image.

Optimizing Graphical Application Performance

The process for optimizing graphical application performance contains the following stages:

Identifying the bottleneck û finding the graphic pipeline stage that is the current performance bottleneck.
Optimizing û reducing the workload done in the bottleneck stage until performance stops improving or you have achieved the desired performance level.

Steps 1 and 2 should be repeated until the desired performance level is reached.