I am reading William Stallings' Operating Systems: Internals and Design Principles (Seventh Edition), and a passage about memory hierarchy got me confused. What does the author mean when he mentions a "greater cost per bit"?
Here is what the relevant paragraph on page 24 says:
As might be expected, there is a trade-off among the three key characteristics of memory: namely, capacity, access time, and cost. A variety of technologies are used to implement memory systems, and across this spectrum of technologies, the following relationships hold:
- Faster access time, greater cost per bit
- Greater capacity, smaller cost per bit
- Greater capacity, slower access speed
I understand that the lower you go down the pyramid of the memory hierarchy the slower the access time.
Like, a magnetic disk or SSD card will have slower access times than the CPU registers or cache or the RAM.
But what does it mean for there to be a "greater cost per bit"? Does it slow down other processes to access this memory? That would seem counter-intuitive as the access times are supposed to be faster? Do they just take up more space and therefore there is less of it?