Re: [Vala] Performance experiments with Vala, C and D-Bus

[Jürg Billeter <j bitron ch>]

Hi Borja,

On Mon, 2012-06-04 at 13:58 +0200, Borja Rodríguez Diliz wrote:
> I want to use Vala and Dbus for a project in which I need to exchange 
> information between objects about 4 Hz .  The final target is to run the 
> software on an embedded device (with a limited CPU / RAM)
> I performed some tests and I noticed that using Vala instead of plain C 
> requires significantly more CPU requirements.
>
> The test:
>   - A dbus server with 200 objects registered. Each object sends the 
> following signals:
>        signalA: a double value with a frequency of 4 Hz
>        signalB: a double value every 2 seconds
> - A client subscribed to 'signalA'
> - A client subscribed to 'signalB'

You use quite different approaches in the Vala version and the plain C
version. In the plain C server you start 400 threads (1 per object and
signal) and let them run/sleep in parallel. In the Vala server you use a
single application thread with an event loop and 400 timeout sources.

I'm not surprised that the two approaches have different performance
characteristics, however, that's not related to the chosen programming
language. It's possible that you hit the scalability of GMainLoop, but
I'm not sure whether that's where the overhead comes from.

You could easily create 400 threads in Vala and you would probably get
similar CPU usage in the Vala and C version. However, I'm not convinced
that creating 400 (+ another 400 in the clients) mostly sleeping threads
is the best approach given that the target has limited CPU/RAM.

Something you could try is to have only two timeout sources (one per
signal frequency) and use a loop in the timeout handler to send all
signals.

Regards,
Jürg