3 ms·
The speed is not limited by propagation delay, but by signal integrity. It is non trivial to have an external parallel bus operating at low Ghz speed. On chip
by jdsully 7y ago
The speed is not limited by propagation delay, but by signal integrity. It is non trivial to have an external parallel bus operating at low Ghz speed. On chip signals can be much faster because they don’t have the capacitive load.
On top of that is power requirements which are again orders of magnitude higher than on chip signals.
Optical has a chance to fix that for the same reasons it works so well for longer distance networking.
- adwn 7y agoWhat you're saying applies to throughput, not latency. > On chip signals can be much faster because they don’t have the capacitive load. You can routinely achieve > 30 Gb/s off-chip in copper cables over distances > 1 m using differential signaling [1]. Capacitive load is only a limiting factor if you directly drive the gate of a transistor. [1] For example, high-end Xilinx FPGAs provide several of those transceivers. They really operate at > 30 GHz.
- jdsully 7y agoTrue it won't affect latency. Yet caches are pretty good at ensuring most DRAM requests take advantage of the hardware parallelization available. That’s why despite DRAM having pretty much constant latency for the last 20 years bus speeds and bank counts have been consistently increasing. Optical interconnects will help immensely. We may also see things like off chip SRAM come back into vogue once its feasible to take advantage of their performance.