14.2.3 Variables Can Have More Than One Value
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14.2.3 Variables Can Have More Than One Value

It is natural to think of a variable as taking on a well-defined sequence of values in a well-defined, global order. Unfortunately, it is time to say ``goodbye'' to this sort of comforting fiction.

To see this, consider the program fragment shown in Figure . This code fragment is executed in parallel by several CPUs. Line 1 sets a shared variable to the current CPU's ID, line 2 initializes several variables from a gettb() function that delivers the value of fine-grained hardware ``timebase'' counter that is synchronized among all CPUs (not available from all CPU architectures, unfortunately!), and the loop from lines 3-8 records the length of time that the variable retains the value that this CPU assigned to it. Of course, one of the CPUs will ``win'', and would thus never exit the loop if not for the check on lines 7-8.

Quick Quiz 14.3: What assumption is the code fragment in Figure making that might not be valid on real hardware? End Quick Quiz

Figure: Software Logic Analyzer

Upon exit from the loop, firsttb will hold a timestamp taken shortly after the assignment and lasttb will hold a timestamp taken before the last sampling of the shared variable that still retained the assigned value, or a value equal to firsttb if the shared variable had changed before entry into the loop. This allows us to plot each CPU's view of the value of state.variable over a 532-nanosecond time period, as shown in Figure . This data was collected on 1.5GHz POWER5 system with 8 cores, each containing a pair of hardware threads. CPUs 1, 2, 3, and 4 recorded the values, while CPU 0 controlled the test. The timebase counter period was about 5.32ns, sufficiently fine-grained to allow observations of intermediate cache states.

Figure: A Variable With Multiple Simultaneous Values

Each horizontal bar represents the observations of a given CPU over time, with the black regions to the left indicating the time before the corresponding CPU's first measurement. During the first 5ns, only CPU 3 has an opinion about the value of the variable. During the next 10ns, CPUs 2 and 3 disagree on the value of the variable, but thereafter agree that the value is ``2'', which is in fact the final agreed-upon value. However, CPU 1 believes that the value is ``1'' for almost 300ns, and CPU 4 believes that the value is ``4'' for almost 500ns.

Quick Quiz 14.4: How could CPUs possibly have different views of the value of a single variable at the same time? End Quick Quiz

Quick Quiz 14.5: Why do CPUs 2 and 3 come to agreement so quickly, when it takes so long for CPUs 1 and 4 to come to the party? End Quick Quiz

We have entered a regime where we must bade a fond farewell to comfortable intuitions about values of variables and the passage of time. This is the regime where memory barriers are needed.