[systemd-devel] KDBus Signals and Timeouts

Justin Brown justin.brown at fandingo.org
Tue Feb 25 17:00:03 PST 2014

Kay and Lennart,

Thanks for taking the time to clarify everything. It's greatly appreciated.


On Tue, Feb 25, 2014 at 2:32 PM, Kay Sievers <kay at vrfy.org> wrote:
> On Tue, Feb 25, 2014 at 9:18 PM, Justin Brown <justin.brown at fandingo.org> wrote:
>> On Tue, Feb 25, 2014 at 12:12 PM, Lennart Poettering
>> <lennart at poettering.net> wrote:
>>> On Tue, 25.02.14 10:41, Justin Brown (justin.brown at fandingo.org) wrote:
>>> And broadcast signals should never be large datagrams, but
>>> only very short.
>> Could you elaborate on this point? One of the major points of emphasis
>> with kdbus seems to be performance. Why is that restricted to method
>> calls? I understand that there would be implementation complexities in
>> multiplexing memfds to all of the signal subscribers, but it seems
>> like a desirable feature. A single source application that sends
>> relatively large amounts of data (a few KB up to possibly several GB)
>> to several third-party sinks seems like a natural use of memfds.
> Broadcasting complex kernel objects like file descriptors inside the
> kernel to many connections at the same time is outside of the focus of
> kdbus. Things get really fragile very fast if we would allow such
> things.
>> As far as I can tell, the only way to accomplish this with kdbus would
>> be for the source application to have a subscribe() method, so it
>> could send methods to each sink. I apologize for asking such a
>> simplistic question (I don't have my kdbus VM up and running yet.),
>> but if I were to take this route, can the same memfd be used in
>> multiple methods or would a new one need to be created for each
>> subscriber?
> You can send any kind of file descriptor many times, therefore also
> memfds can be send many times.
> Memfds are special that they can be used to carry *payload* of a dbus
> message, not only be a naked fd. These memfds need to be sealed to get
> accepted as payload, but they can be used multiple times too.
> Kay

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