[systemd-devel] feedback on sd-bus

[Umut Tezduyar Lindskog]

Hi,

We recently have reimplemented a central component of our software
stack with sd-bus and the results are very satisfactory. Though, we
have some feedback to discuss.

The reimplemented component is a glib based application. For this
reason, we have integrated GMain with sd-event (using Tom's sample
code). We are aware of the fact that sd-bus is relatively lower level
than gdbus.

- Auto generation tool coming with gdbus (gio) is priceless. Without
the tool, packing/upacking messages are time consuming.

- Reference counting on the bus messages needs to be handled by the
API user which is not that difficult but error prone. Again, maybe
some kind of auto generation tool can help us out. Which brings up
another question which is why are cleanup macros not public?

- A simple proxy object is needed. Simple proxy is needed to figure
out when proxy is online/offline. The current properties of the proxy
and getting events when the properties are changed. By default
creating gdbus proxy is expensive in a way that it starts the proxy,
loads properties etc (All of this can be turned off with flags
though). Maybe systemd's proxy can do all of it in a on-demand way by
default.

- Lennart's sd-bus introduction blog recommends the usage of gdbus if
the application is a glib application. I couldn't understand the
reason for this. We have all the hooks to integrate sd-event with
glib.

- Previously we opened a ticket about the under performing glib
(https://bugzilla.gnome.org/show_bug.cgi?id=749533). There are two
biggies mentioned in the ticket for the under performance. AFAIK,
sd-bus is also using gvariant, so that is not relevant. The other
issue is extra context switch on gdbus which doesn't seem to be
something easy to get rid of. Where does this leave us? A) Use high
level bus implementation with performance penalty, B) Use low level
bus implementation with the penalty of going lower level. Isn't it
possible to either get gdbus close to sd-bus or sd-bus close to gdbus?

Feedback is appreciated,
Umut