[Lazarus] custom thread reading file does not respond properly

el es el.es.cr at gmail.com
Wed Sep 6 16:45:39 CEST 2017

On 06/09/17 15:26, el es wrote:
> On 06/09/17 10:31, Andrea Mauri via Lazarus wrote:
>> Il 05/09/2017 22:51, Sven Barth via Lazarus ha scritto:
>>> It is however solved if you add a "Yield;" after the Synchronize
>>> call. So my suspicion is that the scheduling of Mac OS X is somehow
>>> messing things up. Either the scheduling is favoring the one thread
>>> while it shouldn't (though even reducing the priority doesn't help)
>>> or the UI framework of Mac OS X (and whatever else is involved in
>>> the main thread) is so heavy weight that the timeslice of the
>>> mainthread is used up more often than not thus leading to the read
>>> thread being scheduled much more often. By using Yield (or also
>>> Sleep(), but Yield is without any pause) the thread is explicitly
>>> giving up its timeslice and that seems to be enough here...
>> I changed the code following your hints and I added Yield. This is
>> not enough, it seems that the application is a little bit more
>> responding (I can click on Tedit and after seconds the text is
>> highlighted) but not enough to be usable. Actually for me it works
>> only if I add at least Sleep(1). It is a pity that there is not a
>> unique solution.
> Can you try ThreadSwitch after Sleep ?
> while (not Terminated) do
>     begin
>       if eof(f) then
>         reset(f);
>       ReadLn(f, newStatus);
>       if NewStatus <> fStatusText then
>       begin
>         fStatusText := newStatus;
>         //Queue(@ShowStatus);
>         Synchronize(@Showstatus);
>         Sleep(5);
>         ThreadSwitch; /// <<<< 
>       end
>       else  // also you need to cover what happens in ANY OTHER outcome of any test, e.g.
>         ThreadSwitch;
>       // because if NewStatus = fStatusText, you will IMMEDIATELY jump to the first instruction of the thread
>       // so adding the ThreadSwitch at the very end of the loop, you give the rest of the application (the main thread) a chance to
>       // actually do anything
>     end;
> Thread.Execute will execute ALL THE TIME if you don't yield/threadswitch in some code path, and WILL saturate your CPU.
> -l,

In addition to why this isn't a problem on Windows or Linux: they have supposedly better schedulers...

but if not all codepaths on a thread are covered to give some time to main thread, try to imagine what happens:

- could the un-yielded code path lead to over-zealous enqueuing of the ShowStatus routine?
- if yes, what would happen in such case?

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