[ros-users] Nodelets

[Tully Foote]

Hi Patrick,

onInit() is required to return.  See
http://ros-users.122217.n3.nabble.com/questions-about-nodelets-td1869563.html#a1871466for
past discussions.

It is true that if you want to block n threads you need to have more than n
threads available.  We have not set any rules as to what you can set the
number of threads, but the rule of >1 thread per worker is not necessarily
what you want if you are trying to optimize # threads vs # cpu cores, if you
thrash 10 threads across 2 cores you can significantly decrease
performance.  We thought about having separate thread pools for each nodelet
inside the manager but that too starts to get unreasonably complicated, and
adds overhead, plus potentially running out of threads. There ends up not
being any solution which is completely general and addresses all use cases.
Our decision was that if you're using the multithreaded interface you know
that you have to think about the number of threads available.

The NODELET_(INFO/DEBUG/...) setup a named logger such that you can
differentiate between the output of two of the same type nodelets running on
the same manager.  It also has the advantage that you can turn one specific
nodelet into debug, instead of all nodelets of a specific type.

Tully

On Wed, Dec 1, 2010 at 3:28 PM, Patrick Bouffard <bouffard at eecs.berkeley.edu
> wrote:

> After a bit of hacking around I think I found a solution, at least for
> now. Instead of doing the waitForService and service call in the
> onInit method of my nodelets (where it can block), I set up a one-shot
> timer which invokes a method that does the same thing. It also seems
> that upping the num_worker_threads parameter of the nodelet manager
> was required as well. In my case things start working as expected when
> I set it to 5 or higher (reliably, anyway--at lower numbers it might
> sometimes work, and I think that has to do with the unpredictable
> order of startup--if the 'client' nodelets grab all the threads first,
> then the 'server' nodelet is waiting forever for a thread on which to
> advertise its service). I'm loading a total of 6 nodelets for the time
> being but only 5 of them would be involved in the startup blocking
> problem, so I think this makes sense.
>
> Would it then be a good rule of thumb to set num_worker_threads to at
> least the number of nodelets being loaded into a manager? Is there a
> large performance penalty for having too many worker threads?
>
> Pat
>
> On Wed, Dec 1, 2010 at 12:54 PM, Patrick Bouffard
> <bouffard at eecs.berkeley.edu> wrote:
> > I'm trying to consolidate a number of separate nodes into a single
> > process using nodelets. I think I worked out the changes necessary to
> > go from ordinary nodes to nodelets that can be run standalone. But
> > when I try to take those same nodelets and load them all into a
> > nodelet manager, things start to break. I think this might have
> > something to do with threading/callback queues or something to that
> > effect.
> >
> > One of the things that my node(let)s do at startup is to wait for a
> > service to be advertised. The node that advertises that service is now
> > one of the nodelets, and I notice that when I load the nodelets that
> > are the service clients into the nodelet manager, the service doesn't
> > ever seem to start. I suspect this might be because when I call
> > ros::service::waitForService(), it's blocking all of the nodelets,
> > including the one that would be advertising the service--does that
> > make sense? What is the Right Thing to do here to get the illusion of
> > having multiple nodes but with them running in the same process to
> > take advantage of zero-copy message passing etc.?
> >
> > Initially I thought perhaps I just need to add ros::spin() in the
> > onInit method in each nodelet, but I saw in another thread (no pun
> > intended :-) that this is not correct. I'm guessing the answer lies in
> > threading but I'm not quite sure where to start. Is there anywhere in
> > the ROS ecosystem a package that has a similar structure to what I'm
> > trying to do, that I could look at as an example?
> >
> > Also, I'm wondering what the distinction between ROS_INFO and
> > NODELET_INFO is; should I always be using the latter in nodelets?
> >
> > Thanks,
> > Pat
> >
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-- 
Tully Foote
Systems Engineer
Willow Garage, Inc.
tfoote at willowgarage.com
(650) 475-2827
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