DBus-Daemon Optimizations

[Thiago Macieira]

Schmottlach, Glenn wrote:
>I was wondering if the reference DBus Daemon actually
>marshals and unmarshals the messages that pass through it. Does the
>daemon attempt to validate each message that passes through it rather
>than just decoding the message header so it knows where to route the
>message? Any insights into easy optimization approaches would be
>appreciated.

You're correct: the daemon unmarshalls, validates and remarshalls the 
messages passing through it. If the incoming message is invalid in any 
shape or form, it will disconnect the sender and discard the message.

The rationale is that an invalid message could cause a crash in the 
receiver application(s) or worse. Remember that the same D-Bus daemon is 
meant to be used in the system bus, where applications can communicate at 
different privilege levels.

Also note that the same check is done once again in the receiver 
application, to ensure that the message is valid (and since it's the same 
library, it's also the same check). The reason there is that the bus could 
be a different library or that could be a peer-to-peer connection.

I don't have your device to test with, so the best I can provide is my own 
laptop (Linux, Core2 Duo, 1.6 GHz, not fully idle). In that case, my 
performance tests give me 713 round-trip method calls in 500 ms where the 
call and the reply contain a 1 kB byte-array payload. That's 701 µs/call 
in a dual-core system. If I disable one of the cores, I get 683 calls 
under the same conditions. 

Note that the test includes marshalling and demarshalling the message 
entirely, so there's no caching trick.

For comparison, the same code can do 12158 calls in 500 ms if the data 
isn't marshalled and sent to another process (the QtDBus loopback call 
hack).

-- 
  Thiago Macieira  -  thiago (AT) macieira.info - thiago (AT) kde.org
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