[Development] [I/O, Core] Defer a emitting of signal. It is possible?

[Denis Shienkov]

Hi all.

In development of QtSerialPort faced an unpleasant thing - big loading 
of CPU in the process of I/O.
A short results can be see here: 
https://bugreports.qt-project.org/browse/QTBUG-36684

This loading very big in Linux (up to 30% on 115200 baud) even at using 
one instance of the serial port.
Of course, a lot of things depend on device driver, also maybe we missed 
something in implementation
of QtSerialPort, but anyway, a main the tendency is not good.

For comparing (see the link above) are given results with  use of 
Boost::asio which it is twice better
than with Qt.

It seems that select() which implicitly uses QSocketNotifier/Dispatcher 
is a bottleneck. But it is very strange
because the serial port is very slow device on the relation with others. 
Therefore in the theory the I/O with
the serial port shan't influence overall performance and the CPU loading.

So, I thought that the possible workaround is to use a "deferred" 
signals. E.g. the event from an defice
descriptor (fd) was be processed not immediately, but with some time 
delay, that can be expressed in
number of cycles of the event-loop. And this parameter can be configured 
into QObject::connect(...),
for example, or something. It would allow to accumulate more incoming 
data in the input FIFO buffer
of device driver, and to read them less often. And it would reduce load 
of Qt event-loop and CPU.

Or maybe, there are other solutions of a problem? Maybe someone can 
advise something?

Best regards,
Denis