Given the limited memory on GPUs, I'm wondering why there are no socketed GDDR5 memory modules so that you can install more RAM.
The main challenge seems to be maintaining signal integrity since GDDR5 runs 2.5x faster than the fastest DDR4 (4 GHz vs. 1.6GHz, double pumped). Unlike DDR4, it needs to train links to find the phase offset between bits (to account for different PCB trace lengths) so that they can be sampled together. Also, they need to use CRC.
XDR 2 runs even faster at 800 MHz, x16 pumped! but uses differential signaling which means a lower bandwidth per pin than GDDR5.
So will putting GDDR5 onto DIMMs greatly increase wire length, signal attenuation, and negate the speed advantage?
Or is it lack of interest? There would hardly be a market for just GDDR5 upgrades for GPUs. CPU makers would need to support it too. AMD already has for their APU in the PS4.