Ask Slashdot: Is the Gap Between Data Access Speeds Widening Or Narrowing?

New submitter DidgetMaster writes: Everyone knows that CPU registers are much faster than level1, level2, and level3 caches. Likewise, those caches are much faster than RAM; and RAM in turn is much faster than disk (even SSD). But the past 30 years have seen tremendous improvements in data access speeds at all these levels. RAM today is much, much faster than RAM 10, 20, or 30 years ago. Disk accesses are also tremendously faster than previously as steady improvements in hard drive technology and the even more impressive gains in flash memory have occurred. Is the 'gap' between the fastest RAM and the fastest disks bigger or smaller now than the gap was 10 or 20 years ago? Are the gaps between all the various levels getting bigger or smaller? Anyone know of a definitive source that tracks these gaps over time?

Comments

Is the Gap Widening Or Narrowing?

[fleabay]

Yes, yes it is.

RAM latency is not getting much faster

[hkultala]

The latency of RAM is improving very slowly, only something like 2x-4x improvement in last 20 years.

Only the bandwidth of the memory is growing faster, and that's just because they have been putting more dram cells in parallel, always doing bigger data transfers and having faster memory bus.

Same is true for hard disk drive speed, the rotation speeds dictates the random access latency and the rotation speed of average hard disk has only gone up from 4200 or 5400 to 7200 rpm in the last 20 years, meaning only 1.7 or 1.33 times improvement in random access latency

  Though replacing hard disks with flash-based SSD storage has improved latency by a huge margin.

Calculating is easy

[guruevi]

You can look up the specs easy.

Back in the 80286 days, there was not even an L1 cache however the memory and ISA bus ran at CPU speed 8-20Mhz. Hard drive latency was ~65ms.

In the 80486 days L1 cache was introduced and L2 was sometimes available in (very) expensive modules. I remember buying 256kb for the same price as 16MB RAM. The L1 caches ran (if I remember correctly) at CPU speed, 1 cycle. However the bus speed started to slow down compared to the CPU. The VLB ran at CPU bus speed ("local" bus) and was often used for graphics but PCI (an inferior bus) ran at 33MHz so for anything over 33MHz, we started needing dividers. The RAM ran at 80-120ns so it started being slower than the CPU bus. Hard drive speeds were however up to ~30ms.

In the Pentium age memory slowed even farther compared to the CPU bus. Now it took several cycles to access memory, buses ran even slower (still PCI mostly, eventually PCI-X (133MHz?) until PCI-e (serial buses running) came along. Hard drive speeds went up to ~15ms

In modern age, L1 caches have slowed even further requiring 4 cycles for L1 cache and up to 30 for L3 caches. RAM is even slower access with bus speeds about a quarter of a single CPU but sometimes 16 CPU's need to share those lanes. Peripheral bus speeds however have gone up and PCIe 3.0 is now directly integrated into CPU 80486 VLB-style. Hard drives have latencies of 10ms (we have a mechanical issue there) still but even cheap SSD's can go down to ~1-2ms.