Re: gtkmm Monitoring IO example

[Chris Vine <chris cvine freeserve co uk>]

On Fri, 25 Sep 2015 15:13:48 +0200
Kjell Ahlstedt <kjell ahlstedt bredband net> wrote:
> When I tested the input example program on my dual-core PC, I got the 
> same result: The CPU load on one of the cores is 100% after something
> is written to the fifo. Most of the relevant code is in glib. I don't
> know all details, but this is what I found:
>
> Before anything is written to the fifo, the program waits at
>     read_fd = open("testfifo", O_RDONLY);
> without using much CPU time (perhaps no time at all).
> Once the fifo contains data to read, open() returns, and the program 
> enters the main loop at
>     app->run();
> After that, the fifo is monitored by polling. Polling is inefficient. 
> Probably it's the only possible way that the OS offers. It does seem
> a bit extreme to use all available CPU time for polling, but probably
> it's normal behaviour.

Assuming we are talking about a unix-like system, it certainly isn't.
glib will either call poll() or select() and block in a separate i/o
thread, and I think it is poll() which it uses.  Neither should busy
wait, assuming the waiting input is correctly extracted from the file
descriptor which has become ready.

I suspect there is something wrong with the example code, possibly
to do with the way Glib::IOChannel is used with the version of
Glib::SignalIO::connect() which takes a file descriptor rather than the
IOChannel object itself - it would be worth trying the version of
Glib::signal_io().connect which takes the IOChannel object.  Possibly
something goes amiss if you don't do that, but that's a guess.

As an aside, I have always found Glib::IOChannel and GIOChannel quite
difficult to use and I avoid them.  The Gio namespace offers better
functionality.  Where Glib::SignalIO::connect() comes into use is if
you have a raw file descriptor which you want to read when it comes
available using something like unix read(): then, the version of
Glib::SignalIO::connect() which takes a descriptor can be very useful.

Chris