[Qt-interest] Multithreading - yield function

[Patric]

Hi, msleep(1) is doing what yield should do from my point of view.
Very tricky, though...


----- Original Message ----- 
From: "Thomas Dähling" <t.daehling at googlemail.com>
To: "Patric" <userqt at gmail.com>
Cc: <qt-interest at trolltech.com>
Sent: Monday, August 31, 2009 11:46 PM
Subject: Re: [Qt-interest] Multithreading - yield function


> Someone please correct me if this is inherently wrong, but this is how I 
> understood and used yielding so far ;-)
>
> Roughly speaking, yield() is useful in a few situations only, and mainly 
> used to tell the operating system: "Hey scheduler, I am done with the work 
> that I am supposed to do, you might want to give someone else execution 
> time now, if you think there is someone who needs it more than me".
>
> So a typical use-case would be a system where you want to process at least 
> X iterations of something within a thread (note: this is of course no 
> guaranteed unless you provide some sort of blocking mechanism). There 
> might be situations where said thread gets a time slice assigned to do X+N 
> iterations, in that case a yield() after X iterations might be useful to 
> return not-necessarily-required computation time to the scheduler. 
> However, there is no guarantee that it will actually make another thread 
> run - and of course there is no guarantee that you will get the X 
> iterations done (since both parameters are almost completely out of your 
> control).
>
> /Thomas
>
> Patric schrieb:
>> I will consider that, I don't have much experience with threads.
>>
>> One thing I don't understand, why there would be a function like yield 
>> that don't always work. What is the reason such a function to exist if 
>> it's not good to use it.
>> Sorry if my questions are obvious or something like that. :)
>>
>> Regards,
>> Patric
>>
>>
>> ----- Original Message ----- 
>> From: "Andreas Pakulat" <apaku at gmx.de>
>> To: <qt-interest at trolltech.com>
>> Sent: Monday, August 31, 2009 9:42 PM
>> Subject: Re: [Qt-interest] Multithreading - yield function
>>
>>
>>> On 31.08.09 21:09:12, Patric wrote:
>>>> And how do you know that it didn't yield?
>>>>
>>>> Yielding execution means allowing other threads to execute, while the
>>>> calling
>>>> thread is suspended. Eventually, however, the calling thread will 
>>>> resume
>>>> working. So the "some other code" line *will* be executed, eventually.
>>>>
>>>> Anyway, whether the yielding actually does anything depends on the 
>>>> operating
>>>> system and the scheduling priorities of the runnable threads. Like 
>>>> Thomas
>>>> said, if there are no threads waiting to be executed, there will be no
>>>> yielding. But even if there are other threads waiting, they may be of 
>>>> lower
>>>> priority, so yielding will do nothing. You may have to exhaust your
>>>> timeslice
>>>> instead.
>>>>
>>>> Finally, why do you need to yield? It's just a hint to the operating 
>>>> system.
>>>> It may decide to ignore it.
>>>>
>>>>
>>>> ----------------------------------------------------------------------------------
>>>> Hi Thiago,
>>>> I'm using the debugger with multiple breakpoints and because of that I 
>>>> know
>>>> if it's yielding or not. By default all threads priorities should be 
>>>> equal.
>>>> I'm sure there is one thread that is waiting.
>>>>
>>>> I need to yield it because my logic requires it. I'm using queued
>>>> connections here and it's very important to know that the destination 
>>>> slot
>>>> is emitted before the thread continues.
>>> Then either don't use queued connections but direct ones, or use the
>>> usual thread-communication mechanisms to make your thread stop at that
>>> point until the receiving thread is done with the slot. You could do
>>> this for example with a QWaitCondition.
>>>
>>> Andreas
>>>
>>> -- 
>>> You will become rich and famous unless you don't.
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