Multiprocessing: why doesn't a single thread just use more cpu?

Question

I'm learning about multiprocessing and it seems to be applicable in one of two scenarios:

• our program is waitng for some I/O, so it makes sense to go do something else while waiting;
• we break our program up so that individual parts of it can run "in parellel", in an attempt to take full advantage of the cpu

My confusion is about the second case. I'm probably just lacking in my understanding of how cpus really work: but if our single thread process is only using 1% of the cpu and it therefore makes sense to get more threads going, then why wouldn't we just (somehow?) speed up that single process so that it uses more cpu and finishes faster?

Answer 1

but if our single thread process is only using 1% of the cpu and it therefore makes sense to get more threads going, then why wouldn't we just (somehow?) speed up that single process so that it uses more cpu and finishes faster?

We don't know how to. There seem to be fundamental limitations to how fast we can do things that we haven't quite figured out how to get around. So instead, we do more than one thing at a time.

It takes a woman 9 months to make a baby. So if you want lots of babies, you get lots of women. You don't try to get one woman to go faster.

Say you want to raise 7 to the twenty-millionth power and also raise 11 to the twenty-millionth power. Each of these two operations can be reduced in the number of steps, but you will reach a limit. Say each operation takes N sequential steps (each requiring the output from the previous step as its input) and the fastest we can do a single step is Q nanoseconds. With one thread, it will take at least 2NQ nanoseconds to perform all the operations. With two threads, can do one step from each of the two operations at the same time, reducing the time minimum to N*Q nanoseconds.

That's a big win.

Answer 2

I might be wrong, but when we split things into threads, we want to make use of multi-core architecture of our CPUs.

We mostly think CPUs being a single unit, but you must've heard about how i5 is a quad-core processor, meaning it has 4 cores-- or 4 cores make a CPU, i3 is a dual core processor-- i.e, it only has two cores.

So the aggregate CPU utilization for quad-core would be 100% split into 4x25. There's a difference b/w concurrency and parallelism. Parallel means each thread runs on a separate core, making full use of it. Now you have 4 people doing one job-- or a better analogy would be there are 4 printers in the office, and 4 people can go ahead and get the copies that they want. This is parallelism.

Using that same analogy let's extend it to just one copier/printer and 4 people want to make copies, what we do is make use concurrency, we print each requested copy but only 25% of it, then we switch to the next person, then the next and then the next, this will take 4 iterations for all the copies to get printed. Even though we utilized 100% of the copier's capability, still our guys had to wait-- this waiting time also depends on what was the length of the document they wanted to print-- so we use something like pre-emption, you can only execute/print for a certain amount of time, before we start printing for the next guy.

Speeding up a single process-- allocating it 100% of the CPU is not a problem [although we want to run bunch of other stuff like GUI, play music, system services etc, but 85% is doable], the execution time becomes 1/4th when it's distributed b/w the CPUs. Imagine you have to print a book, with 4 copiers, book is 400pages long-- you use 4 copiers to print 100pages each. Will be faster right?

I hope I made some sense, Going to sleep.