Why is QtConcurrent so slow?

[ArlexBee-871RBO]

wysota, yes I've been testing with the same exact code. It does run and I started getting errors when I raised the SIZE. I fixed the errors by replacing the last three lines with this:

QFuture<void> res = QtConcurrent::map(data, Func);
  res.waitForFinished();
 
return 0;

QFuture<void> res = QtConcurrent::map(data, Func);
  res.waitForFinished();
 
return 0;

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I do monitor it during run-time with top, and both cores are used, but they never rise above 50%. They usually hover around 30-40%. The single-threaded version runs on one core, but at 100%.

numbat, I removed the if-else block just so there are no errors in the code, but it still runs several times slower than the single-threaded version. About ten times slower to be exact.

Tanuki-no, QThread::idealThreadCount() returns 2 for my system, which is correct.

[ArlexBee-871RBO]

I just tested a simple pthread program that creates two threads. I get similar results with it as I did with Qt. The single-threaded version runs about 10 times faster than the multi-threaded version. Does this mean there is something wrong with my system?

[wysota]

What does QThread::idealThreadCount() return?

[ArlexBee-871RBO]

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]

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

[ArlexBee-871RBO]

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]

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.