Q:

What is the use of task graphs in CUDA 10? 

CUDA 10 added runtime API calls for putting streams (= queues) in "capture mode", so that instead of executing, they are returned in a "graph".  These graphs can then be made to actually execute, or they can be cloned. 
But what is the rationale behind this feature?  Isn't it unlikely to execute the same "graph" twice?  After all, even if you do run the "same code", at least the data is different, i.e. the parameters the kernels take  likely change.  Or - am I missing something? 
PS - I skimmed this slide deck, but still didn't get it. 

A:

My experience with graphs is indeed that they are not so mutable.  You can change the parameters with 'cudaGraphHostNodeSetParams', but in order for the change of parameters to take effect, I had to rebuild the graph executable with 'cudaGraphInstantiate'.  This call takes so long that any gain of using graphs is lost (in my case).  Setting the parameters only worked for me when I build the graph manually.  When getting the graph through stream capture, I was not able to set the parameters of the nodes as you do not have the node pointers.  You would think the call 'cudaGraphGetNodes' on a stream captured graph would return you the nodes.  But the node pointer returned was NULL for me even though the 'numNodes' variable had the correct number.  The documentation explicitly mentions this as a possibility but fails to explain why. 