问题
I have 16000 jobs to perform.
Each job is independent. There is no shared memory, no interprocess communication, no lock or mutex.
I am on ubuntu 16.06. c++11. Intel® Core™ i7-8550U CPU @ 1.80GHz × 8
I use std::async to split jobs between cores.
If I split the jobs into 8 (2000 per core), computation time is 145. If I split the jobs into 4 (4000 per core), computation time is 60.
Output after reduce is the same in both case.
If I monitor the CPU during computation (just using htop), things happen as expected (8 cores are used at 100% in first case, only 4 cores are used 100% in second case).
I am very confused why 4 cores would process much faster than 8.
回答1:
The i7-8550U has 4 cores and 8 threads.
What is the difference? Quoting How-To Geek:
Hyper-threading was Intel’s first attempt to bring parallel computation to consumer PCs. It debuted on desktop CPUs with the Pentium 4 HT back in 2002. The Pentium 4’s of the day featured just a single CPU core, so it could really only perform one task at a time—even if it was able to switch between tasks quickly enough that it seemed like multitasking. Hyper-threading attempted to make up for that.
A single physical CPU core with hyper-threading appears as two logical CPUs to an operating system. The CPU is still a single CPU, so it’s a little bit of a cheat. While the operating system sees two CPUs for each core, the actual CPU hardware only has a single set of execution resources for each core. The CPU pretends it has more cores than it does, and it uses its own logic to speed up program execution. In other words, the operating system is tricked into seeing two CPUs for each actual CPU core.
Hyper-threading allows the two logical CPU cores to share physical execution resources. This can speed things up somewhat—if one virtual CPU is stalled and waiting, the other virtual CPU can borrow its execution resources. Hyper-threading can help speed your system up, but it’s nowhere near as good as having actual additional cores.
By splitting the jobs to more cores than available - you are paying a big penalty.
来源：https://stackoverflow.com/questions/47989178/c-stdasync-faster-on-4-cores-compared-to-8-cores