[ros-users] need help: set_parameters service callback very sluggish with nodelet-based camera1394 driver

[Blaise Gassend]

Hi Jack,

> The idea is to keep the lock, but add a bool class member for when the
> reconfig callback needs to interrupt the poll() thread. The poll()
> loop is almost the same:

I guess your method is a small improvement over doing the sleep each
time, as it sometimes avoids the sleep. It isn't clear to me that
there is a benefit over doing the sleep every time. Retrospectively, I
probably shouldn't have dissed the "sleep each time" method in my
initial reply. It really solves the problem with maximum simplicity.
There is really nothing wrong introducing a few hundred milliseconds
of sleep in a loop is just going to waits on hardware for tens of ms.

The benefit of my method is that it guarantees that once the configure
thread starts waiting, the other thread will never start a new
iteration before configuration completes. And I agree that in your
case that guarantee is completely unnecessary. (My case was a bit more
complex as the two threads needed to rendez-vous.)

> I've thought about portability to machines with weak storage ordering,
> but I think the scoped_lock should always invoke any memory fences
> required for the underlying processor architecture, so it should be
> OK.

I think that technically you should mark your flag as volatile,
otherwise the compiler can legitimately assume that it does not change
in other threads, and assume that the condition in the while loop is
constant. In practice this is probably a non-issue as you are calling
a function in a different module from the while loop, so the compiler
can't be sure that the function does not change the flag. However, if
the function being called was in the same file, the compiler might
inline it and consider that the condition in the while does not
change.

Cheers,
Blaise