Even though ATI Radeon HD 4000 series graphics cards have been quite popular on the mainstream market, the family may have limited OpenCL perfromance, the company recently admits.

“There are known performance issues for ATI Radeon HD 4000-series of cards on OpenCL and there is currently no plan to focus exclusively on improving performance for that family. The ATI Radeon HD 4000-series was not designed for OpenCL, whereas the ATI Radeon HD 5000-series was. There will be performance improvements on this series because of improvements in the ATI Radeon HD 5000-series, so it will get better, but it is not our focus,” said Micah Villmow, an AMD spokesman, in AMD’s official software developer support forum.

Considering the fact that ATI Radeon HD 4000-series were designed back in 2006, it comes as no surprise that they show limited performance in case of OpenCL 1.0, which was finalized late in 2008.

“ATI Radeon HD 4000 just has to be programmed differently than the ATI Radeon HD 5000-series to get performance because of the lack of proper hardware local support. It is possible to get good performance, just not with a direct port from CUDA. [...] For example, if you are using local memory, they are all currently emulated in global memory. This can cause a fairly large performance hit if the application is memory bound. On the ATI Radeon HD 5000-series, local memory is mapped to hardware local and thus is many times faster than the ATI Radeon HD 4000-series,” explained Mr. Villmow.