[Interest] Qt signal overhead (same thread, direct connection)

[Nuno Santos]

Hey,

It is definitely taking to much time, enough to cause buffer drops in this situation. 

I have changed to a callback calling mechanism and the problem is now fixed. 

I now have this in mind when I program sensitive parts of my apps. 

Regards,

Nuno

> On 22 Apr 2016, at 17:34, Alejandro Exojo <suy at badopi.org> wrote:
> 
> El Friday 22 April 2016, Nuno Santos escribió:
>> Then I made an experiment. Arpeggiator was using a signal to send the
>> noteOn to the audio render engine. I decided to call the function directly
>> on the synth render engine. Guess what, the problem disappears.
>> 
>> Is the signal/slot mechanism tied to the event loop? How much does it take
>> for a signal/slot to be called? It might not be significant on a desktop
>> computer but it certainly is on a ARM device specially in this kind of
>> performance sensitive contexts.
> 
> This paragraph in the documentation (same link Benjamin sent) is also pretty 
> enlightening:
> 
>> Compared to callbacks, signals and slots are slightly slower because of the
>> increased flexibility they provide, although the difference for real
>> applications is insignificant. In general, emitting a signal that is
>> connected to some slots, is approximately ten times slower than calling
>> the receivers directly, with non-virtual function calls. This is the
>> overhead required to locate the connection object, to safely iterate over
>> all connections (i.e. checking that subsequent receivers have not been
>> destroyed during the emission), and to marshall any parameters in a
>> generic fashion. While ten non-virtual function calls may sound like a
>> lot, it's much less overhead than any new or delete operation, for
>> example. As soon as you perform a string, vector or list operation that
>> behind the scene requires new or delete, the signals and slots overhead is
>> only responsible for a very small proportion of the complete function call
>> costs. The same is true whenever you do a system call in a slot; or
>> indirectly call more than ten functions. The simplicity and flexibility of
>> the signals and slots mechanism is well worth the overhead, which your
>> users won't even notice.
> 
> That was probably written long ago. I've read people mentioning that it might 
> be that signal emission would be slower nowadays.
> 
> Also, what type are you passing in the signal? The value is copied, so if it's 
> expensive to copy, that might be a factor as well. I believe that is one of 
> the reasons why APIs like QIODevice don't send the received data through the 
> signal, but just the notification of the presence of new data. Later on you 
> have to retrieve it.
> 
> -- 
> Alex (a.k.a. suy) | GPG ID 0x0B8B0BC2
> http://barnacity.net/ | http://disperso.net
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