About performance of D-Bus

[tian tian]

Many thanks for all these advices.

I did some more tests today. As Havoc's advice, I changed the code:
    mode = DBUS_VALIDATION_MODE_DATA_IS_UNTRUSTED;
to
    mode = DBUS_VALIDATION_MODE_WE_TRUST_THIS_DATA_ABSOLUTELY;
, then re-compile D-Bus V1.0.2 with:
    ./configure --host=arm-linux
                --with-xml=expat
                --without-x
                --disable-tests
                --disable-verbose-mode
                --disable-asserts
                --disable-checks
    make install

And the test program is:
    client
        gettimeofday()  /* -> timer1 */
            dbus_message_new_method_call()
            dbus_message_append_args()
            dbus_message_set_no_reply()
            dbus_connection_send()
            dbus_connection_flush()
        gettimeofday()  /* -> timer2 */

        gettimeofday()  /* -> timer3 */
            dbus_message_new_method_call()
            dbus_message_append_args()
            dbus_connection_send_with_reply_and_block()
        gettimeofday()  /* -> timer4 */

    server
        dbus_filter()
        {
            gettimeofday()  /* -> timer5 */
        }

Results(data unit is "byte", time unit is "microsecond"):
    input   output  timer1  timer2  timer3  timer4  timer5
---------------------------------------------------------------------------------------
    0         0         0         2260    -          -          1336
    0         0         -          -          0        462       1987
    100      0         0        2741     -         -           1701
    100      0         -         -           0        573       1991
    100     100       0        2826    -          -           1547
    100     100       -         -          0         462       3647

    I'am glad to see it works much better. Nevertheless, I want it can
complete a synchronous remote call in several hundreds microseconds(at least
< 1500 microseconds). And it is more slower(about 1.3x) if I use the
bus-daemon.

    I have noticed that D-Bus is designed to enable asynchronous
communication and avoid round trips, but I hope to use this useful message
bus system in my embeded system. In this system, there are synchronous
remote calls as many as asynchronous calls. That's why I care about the
performance of D-Bus greadtly.
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