# Simple question: Is the official bandwidth 1-way or 2-way? **URL:** \ **Category:** CUDA Programming and Performance\ **Created:** [December 1, 2008, 6:29am UTC](https://forums.developer.nvidia.com/t/simple-question-is-the-official-bandwidth-1-way-or-2-way/6612 "2008-12-01T06:29:19Z")\ **Posts on this page:** 4\ **Page:** 1
**Author:** ![Oceanian](https://developer.download.nvidia.com/images/forums/profile-default-devtalk-84.png) [@Oceanian](https://forums.developer.nvidia.com/u/Oceanian)\ **Post date:** [December 1, 2008, 6:29am UTC](https://forums.developer.nvidia.com/t/simple-question-is-the-official-bandwidth-1-way-or-2-way/6612/1 "2008-12-01T06:29:19Z")
I mean the 86.4GB/s for 8800GTX, 141GB/s for GT280 data. Do I have to both read and write global memory in the same kernel to reach the peak bandwidth? 3X! ---
**Author:** ![alex\_dubinsky](https://developer.download.nvidia.com/images/forums/profile-default-devtalk-84.png) [@alex\_dubinsky](https://forums.developer.nvidia.com/u/alex_dubinsky)\ **Post date:** [December 1, 2008, 7:38pm UTC](https://forums.developer.nvidia.com/t/simple-question-is-the-official-bandwidth-1-way-or-2-way/6612/2 "2008-12-01T19:38:40Z")
The DRAM bus is half-duplex, so full bandwidth can be achieved 1-way. ---
**Author:** ![paulius](https://developer.download.nvidia.com/images/forums/profile-default-devtalk-84.png) [@paulius](https://forums.developer.nvidia.com/u/paulius)\ **Post date:** [December 3, 2008, 3:03am UTC](https://forums.developer.nvidia.com/t/simple-question-is-the-official-bandwidth-1-way-or-2-way/6612/3 "2008-12-03T03:03:45Z")
Correct. The numbers above are theoretical peaks for one direction. If your kernel reads and then writes (say, a memcopy kernel), then to measure the bandwidth you’re achieving you count each byte twice - once for reading, once for writing. Then you can compare it against the theoretical peak. In practice, you should be able to get to about 80% of the theoretical peak. Paulius ---
**Author:** ![alex\_dubinsky](https://developer.download.nvidia.com/images/forums/profile-default-devtalk-84.png) [@alex\_dubinsky](https://forums.developer.nvidia.com/u/alex_dubinsky)\ **Post date:** [December 3, 2008, 4:47am UTC](https://forums.developer.nvidia.com/t/simple-question-is-the-official-bandwidth-1-way-or-2-way/6612/4 "2008-12-03T04:47:58Z")
> [@](#): > > In practice, you should be able to get to about 80% of the theoretical peak. 93 if you try real hard.