[Qt-interest] Multithreading - yield function

[Sean Harmer]

Hi,

On Tuesday 01 Sep 2009 10:56:52 Patric wrote:
> Also when I'm debugging and it's clear that another thread is waiting for
> execution, after yield it's just not jumping to the other thread.
How do you *know* the other thread is waiting for execution? This is up to the 
kernel's scheduler to decide not you (unless you use explicit synchronisation 
methods).

> >It may get suspended but then woken up again before the other thread
> >finishes execution.
>
> you are telling me that if I have two threads that are running, I really
> don't know when the second one will start executing in the middle of the
> code ? And stop also.
Correct. This is why things like QMutex and QSemaphore exist. Please read up 
on their use. It is also why multi-threaded programming is hard.

> This don't sound very cool.
It is perfectly reasonable but you need to understand it. Consider the case of 
a dual core system. In this case *both* of your threads could be running at 
exactly the same time - one on each core. However even in this case the kernel 
has to share the CPU cores and other resources between your applications 
threads and all other applications and their threads and the kernel's own 
threads. Not to mention respond to interrupt events. 

Just let the kernel do its job. It is written by very good people who know 
what they are doing. If you need to have some sort of specific synchronisation 
between your threads then the yield function is not the way to go as this is 
just a *hint* to the kernel. To force synchronisation look at QMutex and 
QWaitCondition for example.

Good luck,

Sean