Basically, we're never getting rid of the need for caches. You can't put enough of it close enough to the CPU because the muxes needed to select the data you need have too many FO4s[1] of fanning, and because you're limited by the speed of light, and the arrays you need are too big to get all of them close enough to the processor.
Another problem is write endurance. RRAM is expected to have be able to take many orders of magnitude more writes before failing than Flash, but its still limited. The fact that you have layers of SRAM cache between it and the processor buffering against repeated writes to the same location is why I'd still be comfortable using it as main memory. Otherwise you could get an infinite loop that could actually damage the memory.
Quite true, but far out speculation about the future of computer architectures doesn't really seem germane to talking about a new form of fast non-volatile memory.
It's just a popular sweet-nothing people have grown fond of parroting whenever memristors come up. It is impossible to really refute, because it speculates about the possibility of an architecture we haven't yet imagined. That plus how cool "brand new architectures, completely new ways of computing!" sounds to the layman, means you hear it every time the word "memristor" hits the headlines.
The joke, of course, is that (if memory serves) modern computers actually use the Harvard architecture- not Von Neumann.
When someone writes about the end of the Von Neumann architecture, I take it as dreaming that the poster's favorite language will someday be faster than C.
"You do realize that John von Neumann spent the last 10 years of his life singlehandedly developing a theory of computing based on cellular automata? The computer you're reading this blog rant on was his frigging prototype! He was going to throw it out and make a better one! And then he died of cancer, just like my brother Dave did, just like so many people with so much more to give and so much more life to live. And we're not making headway on cancer, either, because our computers and languages are such miserable crap."
With memristors it is not even remotely "far out". First, we can substantially modify the von neumann architecture by dramatically improving the capabilities of reconfigurable FPGA type devices, allowing them to run at the speeds of custom ASICs and reconfigure with a delay of only a few clock ticks. That alone is pretty astounding. Second, memristors could be used directly as logic and memory, making it possible to programatically transform blocks of memory into circuitry and back to memory, which would have implications beyond our current reckoning. And all of this is possible in the near-term within the next few decades.
Wow, sorry, that sentence I constructed there was really confusing. There are really two separate effects. On the one hand, you can only make a cell of this memory so small, so you can't pack it as densely as you'd like. The other is that you have one cell of memory you'd like to access, and an output line you'd like the data from it to go out on. And you don't want the data from any of the other N billion cells of memory to go out on that line. There are actually some structures you use to retrieve that particular bit, but the more bits you're selecting between the more delay is introduced. In most current CPU designs this is a actually a bigger deal than speed of light, at least for the various levels of on-chip cache.
Another problem is write endurance. RRAM is expected to have be able to take many orders of magnitude more writes before failing than Flash, but its still limited. The fact that you have layers of SRAM cache between it and the processor buffering against repeated writes to the same location is why I'd still be comfortable using it as main memory. Otherwise you could get an infinite loop that could actually damage the memory.
[1]http://en.wikipedia.org/wiki/FO4