[ros-users] Nodelet image transfer performance

[Josh Faust]

Intraprocess publishing is only significantly faster if you're publishing a
shared_ptr to the message, otherwise it still has to serialize/deserialize
as usual.

So instead of keeping an image as part of the class, filling that image, and
then publishing it, you need to allocate the image each time you publish:
sensor_msgs::ImagePtr image(new sensor_msgs::Image);
sensor_msgs::fillImage(*image, "bgr8", height, width, 3 * width, data);
pub_.publish(image);

It doesn't look like this is explained anywhere on the wiki other than the
cturtle release notes:
http://www.ros.org/wiki/ROS/ChangeList/1.2/roscpp_changes#Publish.2BAC8-Subscribe_Changes,
I'll remedy that tomorrow.

Josh

On Fri, Sep 17, 2010 at 1:01 AM, Cartwright, Joel J <J.J.Cartwright at hw.ac.uk
> wrote:

>  Hi all,
>
> I've been running some rough performance tests on transferring images
> using nodelets, and have found the overhead to be higher than I
> expected. For reference, I'm running ROS cturtle on Ubuntu 10.04 64 bit
> (stock kernel 2.6.32-24-generic) on a 2 GHz Intel Core 2 Duo laptop. CPU
> use monitored approximately using htop.
>
> Exchanging one 1920x1280 bgr8 image at 10 fps incurs ~30% CPU use for
> the nodelet manager. As expected, this is better than when the nodelets
> are run standalone (then ~25% CPU each), but I wanted to check if I'm
> doing something wrong that might affect nodelet performance. I've been
> using raw Image publishing rather than image_transport, to keep the test
> as simple as possible.
>
> The image 'source' is just an uninitialised unsigned char array of the
> correct size. As a comparison, I created a node that merely copies the
> source array a number of times, and found that 5 memcpy operations
> produce about the same CPU load as the best nodelet case, around 30%.
> Of course CPU caching may improve the speed of those 5 consecutive
> memcpy operations, so I'm not saying "ROS is copying the data 5 times".
>
> I've attached the relevant source files. The question is: Is this the
> best we can do at present for image transfer, and if not, are there
> modifications on the horizon that will improve performance?
>
> Joel
> --
> Research Assistant
> Ocean Systems Laboratory
> Heriot-Watt University, UK
>
> ------------------------------
>  Heriot-Watt University is a Scottish charity registered under charity
> number SC000278.
>
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