Q:

When do I have to multiply by 2 to compute memory bandwidth

I learning to compute memory bandwidth by an  example, as follows,

I don't know what is the double data rate mean? which parameter of GPU decide this? 

PCle is Full Duplex?
Dual Slot?
the level of DDR, such as DDR3/DDR5?

or other?

A:

for double data rate : have you tried to google these keywords ? 
https://en.wikipedia.org/wiki/Double_data_rate
The answer is 3 : Its the type of memory of your GPU wich decides if you should multiply by 2 or not, a rule of dumb would be always multiply by two because the single data rate is not mainly used anymore.
if you see "DDR" in the memory type of your graphic card, multiply by two because this is an intrinsic property of your hardware (GDDR5 means Graphic Double Data Rate version 5, and DDR3 means Double Data Rate version 3)

