Why is QtConcurrent so slow?

**ArlexBee-871RBO** · 28th December 2009, 18:54

Originally Posted by wysota

What does QThread::idealThreadCount() return?

It returns 2, and I'm running Linux on a dual-core Intel.

There is some kind of system overhead when running more than one thread, and that causes multi-threaded version of all examples I've tried to run drastically slower than the single-threaded version. I just can't figure out what it is.

**wysota** · 28th December 2009, 18:59

Is your system doing anything heavy in the background? Does your system have free memory or is it swapping?

**ArlexBee-871RBO** · 28th December 2009, 20:46

Originally Posted by wysota

Is your system doing anything heavy in the background? Does your system have free memory or is it swapping?

No, my system isn't doing anything heavy, and I have more than enough ram on it. 2GB total, and i'm not running anything else that would exhaust the CPU or memory.

Question: Does the multi-threaded version (using QtConcurrent::map or mapped) run faster than the single-threaded version on your system??

**numbat** · 29th December 2009, 10:37

rand() is not thread-safe*. Changing to rand_r and re-running, the multi-threaded version takes slightly less real time, although about 50% extra user time, so there is reasonable overhead using QtConcurrent for very short tasks.

*Or maybe rand is thread safe and the extra time is waiting for a lock on the state information kept by rand.