caffe: What does the **group** param mean?

Question

I have read the documentation about the group param:

group (g) [default 1]: If g > 1, we restrict the connectivity of each filter to a su

Answer 1

And secondly, why would I use [grouping]?

This was originally presented as an optimization in the paper which sparked the current cycle of neural network popularity :

Alex Krizhevsky, Ilya Sutskever, and Geoffrey E. Hinton. "Imagenet classification with deep convolutional neural networks." In Advances in neural information processing systems, pp. 1097-1105. 2012.

Figure 2 shows how grouping was used for that work. The authors of caffe originally added this ability so they could replicate the AlexNet architecture. However grouping continues to show itself as beneficial in other scenarios.

For example both Facebook and Google have released papers which essentially show that grouping can dramatically reduce resource use while helping to preserve accuracy. The Facebook paper can be seen here:(ResNeXt) and the Google paper can be found here: (MobileNets)