then, why not just using qualifiers ? from slowest to fastest. You might not know that, but you can develop bare metals solution for HPC that are used in several industries like telecommunication. Calculation based on cycles are totally accurate whether the number of cores...
mlyle|2 years ago
Because whether something is 5x slower or 5000x slower matters. Is it better to wait for 10 IOs, random access memory 10000x, or do a network transaction? We can figure out the cost of the memory/memory bandwidth, etc, but we also need to consider latency.
I've done plenty of work counting cycles; but it's a lot harder and less meaningful now. Too many of the things here happen in different clock domains. While it was a weekly way to look at problems for me a couple of decades ago, now I employ it for far less: perhaps once a year.
> Calculation based on cycles are totally accurate whether the number of cores...
No, they're not, because cores contend for resources. We contend for resources within a core (hyperthreading, L1 cache). We contend for resources within the package (L2+ cache lines and thermal management). And we contend for memory buses, I/O, and networks. These things can sometimes happen in parallel with other work, and sometimes we have to block for them, and often this is nondeterministic. In turn, the cycle counts for doing anything within the larger system are really nondeterministic.
Counting cycles works great to determine execution time on a small embedded system or a 1980s-1990s computer, or for a trivial single threaded loop running by itself on a 2020s computer. But most of the time now we need to think account for how much of some other scarce resource we're using (cache, memory bandwidth, network bandwidth, a lock, power dissipated in the package, etc), and think about how various kinds of latencies measured in different clock domains compose.
AlotOfReading|2 years ago