Getting Started with QNX Neutrino - QNX Software Systems

Priority inheritance
© 2009, QNX Software Systems GmbH & Co. KG.
Magenta’s ND is 7
From magenta’s point of view, we’re ND 4
This says that on magenta, the node descriptor “4” refers to our node. (Notice the use
of the special constant ND_LOCAL_NODE, which is really zero, to indicate “this
node.”)
Now, recall that we said (in “Who sent the message?”) that the struct _msg_info
contains, among other things, two node descriptors:
struct _msg_info
{
int nd;
int srcnd;
...
};
We stated in the description for those two fields that:
• nd is the receiving node’s node descriptor for the transmitting node
• srcnd is the transmitting node’s node descriptor for the receiving node
So, for our example above, where wintermute is the local node and magenta is the
remote node, when magenta sends a message to us (wintermute), we’d expect that:
• nd would contain 7
Priority inheritance
• srcnd would contain 4.
One of the interesting issues in a realtime operating system is a phenomenon known as
priority inversion.
Priority inversion manifests itself as, for example, a low-priority thread consuming all
available CPU time, even though a higher-priority thread is ready to run.
Now you’re probably thinking, “Wait a minute! You said that a higher-priority thread
will always preempt a lower-priority thread! How can this be?”
This is true — a higher-priority thread will always preempt a lower-priority thread.
But something interesting can happen. Let’s look at a scenario where we have three
threads (in three different processes, just to keep things simple), “L” is our low-priority
thread, “H” is our high-priority thread, and “S” is a server. This diagram shows the
three threads and their priorities:
130 Chapter 2 • Message Passing April 30, 2009