OpenCL - 大字符串和内存性能？

I'm an OpenCL newbie but I've had good success with many of the examples I've found online. My application is very basic bioinformatics so I'm concerned with very long strings of "ACTCGAAAGGTA...." perhaps 1-3MB long. A simple calculation would be to count the number of A,T,C,G elements and that seems very suited for OpenCL. So if I start with the canonical "add two vectors" example found I'm an OpenCL newbie but I've had good success