Getting Started with QNX Neutrino - QNX Software Systems

Using message passing
© 2009, QNX Software Systems GmbH & Co. KG.
Master
Worker 1
Worker 2 Worker 3 Worker 4 Worker 5
One master, multiple workers.
Multi-threaded server
Using message passing
The solution to this problem is to have the worker programs start up, and ask the
master program if there’s any work to do by sending it a message. Once again, we’ve
used the direction of the arrows in the diagram to indicate the direction of the send.
Now the worker programs are waiting for the master to reply. When something tells
the master program to do some work, it replies to one or more of the workers, which
causes them to go off and do the work. This lets the workers go about their business;
the master program can still respond to new requests (it’s not blocked waiting for a
reply from one of the workers).
Multi-threaded servers are indistinguishable from single-threaded servers from the
client’s point of view. In fact, the designer of a server can just “turn on”
multi-threading by starting another thread.
In any event, the server can still make use of multiple CPUs in an SMP configuration,
even if it is servicing only one “client.” What does that mean? Let’s revisit the fractal
graphics example. When a subserver gets a request from the server to “compute,”
there’s absolutely nothing stopping the subserver from starting up multiple threads on
multiple CPUs to service the one request. In fact, to make the application scale better
across networks that have some SMP boxes and some single-CPU boxes, the server
and subserver can initially exchange a message whereby the subserver tells the server
how many CPUs it has — this lets it know how many requests it can service
simultaneously. The server would then queue up more requests for SMP boxes,
allowing the SMP boxes to do more work than single-CPU boxes.
Now that we’ve seen the basic concepts involved in message passing, and learned that
even common everyday things like the C library use it, let’s take a look at some of the
details.
90 Chapter 2 • Message Passing April 30, 2009