Why is the hyper threading absent in AMD processors?
Oh they do have Hyperthreading. Sort of.
Just like Roger Shepherd said in his answer.
Hyperthreading is just Intel's moniker for Simultaneous Multi Threading (SMT). Well AMD has its own version called CMT - Clustered Multi Threading. Or as I like to call it - Clusterfuck. SMT is essentially time sharing of resources by two threads. CMT is creating two seperate partitions for two threads.
In the image, the one you see on the left is AMD's CMT and the one on the right is Intel's SMT. In multi-threading as general, the CPU core poses to the Software as two separate cores and fetches two programs in parallel, just like two separate cores would. Now these two programs are fed into the core to make the most of the unused assets in the chip. In the early 2000's, this was a cheap way to make the most out of wide CPU designs which are seldom leveraged by simple (or highly branched) software. Some designs such as the UltraSPARCs went as far as 4 way multithreading. That is 4 threads for every core, which worked very well for server workloads.
Over the years, multithreading lost its appeal as CPU designs got exceptionally good at extracting parallelism from software code. Now CPUs housed more execution units, fetched more instructions per clock, and executed more per cycle, thanks to the complex control circuits. And just when things got too predictable AMD came up with CMT in Bulldozer family. Instead of two threads being fed into one execution cluster, this architecture had two separate clusters of integer ALUs for the two threads in each core. This means lesser execution resources per thread and overall less complexity of control circuit(s). While this DID save in chip complexity and costs, it also reduced the maximum performance for single threaded applications.
All this in early 2010's when Intel was improving its single thread performance by leaps and bounds. This left AMD in the dust with an out of place architecture. Too radical for its time and too slow to be good. Hence they did the next best thing - marketed it as Double cores instead of CMT.
So the 8 core FX 8350 is not actually 8 cores. It is 8 integer clusters in 4 cores with CMT.
Which is sort of hyperthreading. That would be like Intel marketing its hyperhtreaded quad core i7 as octa core just because it runs 8 threads. But wait, AMD Zen is coming out this year. It will have genuine Hyperthreading (simultaneous multi threading).
So keep calm and wait for Zen.
