如何在 C 中对多个内核进行编程

Question

I have a program and want to increase the runtime performance more.

let a = 1;
let b = 2;

let c = a + b;
let d = c + 2;

let e = 3;

let f = c + d;
let g = a + e;

Step 1: Because a, b and e are independent so I want to execute them in parallel (different cores).

Step 2: Because c is depended on a and b; g is depended on a and e, but c and g are independent each other so execute c and g after step 1 but in parallel.

Step 3: Because d is depended on c so they are executed after step 2.

Step 4: Because f is depended on c and d, so it is executed after step 3.

Can we achieve this one with C or any programing language support this natively?

Answer 1

Multi-threading is clearly not suited for your problem. The synchronization/data-movement time is far bigger than the time to compute an addition of two native-typed values (eg. floating-point number, integers, etc.). Indeed, adding two integers take about 1 cycle on mainstream x86-64 processors while the time to move data from one cache of a core to another one takes at least several dozens of cycles (if not hundreds regarding the target architecture). Thus, using multiple cores will actually slow down massively the code. Multi-threading only worth it for a relatively heavy grained computation (at least few microseconds and generally even a bit more).

Fortunately, modern processors can execute multiple instructions in parallel per cycle (see Instruction-level parallelism and Superscalar processor ). For example, an Intel Skylake can execute 4 addition per cycle. It can also execute instructions in an out-of-order way. A processor can detect dependencies for you so you do not need to do much. You just need to ensure instructions are independent so they can be executed in parallel.

Answer 2

The concept you are looking for is "multi-threading". Most modern progressing languages have built-in support for this concept (for example C++'s std::thread , C is a bit ancient but multi-threading support was added in the 2011 standard update (known as C11) using the threads.h header. The new API is optional and while many common C implementations do offer support, the one you use may not.

Luckily, threading has been used extensively with C, before the 2011 standard update, and there are multiple non-standard APIs for threading that you can use as libraries. For example, check out pthread as a common cross platform threading library for C.