RPC parameters

[Norman Feske]

Hello,

On 14.01.2016 11:52, Bilal ma wrote:
> Thank you for the examples, does it make difference if i  don't pass my
> input-output POD struct by reference  since it need to be copied to the
> server side, and later from the server to the client. 
> from the point view of  performance  which one is the best if we add the
> option of placing my POD in shared memory. 

the RPC mechanism always copies data. It does not use shared memory.
Please review Section 8.12.2. "Transferable argument types" to get a
picture how different argument types are handled.

If a reference is passed as argument, the following steps are taken:

1. The argument is copied into the message to be sent to the server.

2. The server instantiates a temporary object of the argument type
   and copies the argument from the message to the temporary object.

3. The server-side RPC method is called with a reference to the
   temporary object as argument.

4. Once the server-side RPC method returns, the new content of the
   temporary object is copied to the reply message buffer.

By using the temporary object instance, it looks like you pass a
reference. But under the hood, the server operates on a different object.

You mentioned performance. If you need to transfer large amounts of bulk
data between components, you may consider shared memory. For a
description of how to set up shared memory between components, please
refer to Section 3.6.3. "Shared memory" in [1].

That said, I recommend you to think twice about introducing a new
session interface. Maybe an existing interface would do the job just
fine when looking at your problem from a slightly different angle? To
cite the intro of Chapter 4:

 "The versatility of a component-based system does not come from
  the existence of many components alone.  Even more important is
  the composability of components. Components can be combined only
  if their interfaces match. To maximize composability, the number
  of interfaces throughout the system should be as low as possible,
  and all interfaces should be largely orthogonal to each other."

Whereas it is a nice exercise to design a custom RPC interface, I mostly
advise against it. In particular, if your interface is just a bunch of
getters and setters, the existing ROM and Report session interfaces may
accommodate you quite well.

[1] http://genode.org/documentation/genode-foundations-15-05.pdf

Cheers
Norman

-- 
Dr.-Ing. Norman Feske
Genode Labs

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