New Statistical Algorithms for the Analysis of Mass - FU Berlin, FB MI ...

132 CHAPTER 5. COMPUTER SCIENCE GRID STRATEGIES
currently running on this machine before returning to the worker thread
15 (see below for details).
Connection quality: The connection quality is measured with respect to
the RTT - the time a data packet needs to travel from the worker to the
platform server and back.
Each time a status update is performed a timestamp is automatically set to this
database entry. Therefore, the time of the latest update can be determined. If
the status has not been sent within the last 300 seconds a client is considered
lost and will be logged out.
Workload Determination
As stated above, we define the machine’s local load as the time needed by
the operating system to cycle through all processes currently running on this
machine excluding the worker process. In our worker reference implementation
we use the Java method Thread.yield() 16 that
“Causes the currently executing thread object to temporarily pause
and allow other threads to execute.” (SUNMicrosystems, 2006)
So, if a thread executes yield, it is suspended and the CPU is given to some
other runnable thread. It then sleeps until the CPU becomes available again.
Technically put, the executing thread is put back into the ready queue of the
processor and waits for its next turn.
This means, if the worker is the only (high priority) program (thread) on
a machine the time needed for the yield will be almost zero because there is
no other program that will consume time. The beauty of this approach is
that the CPU utilization can be well close to 100% but if caused exclusively
by the worker the local load is about zero, because the worker thread is the
only program running. On the other hand, if there are many CPU intensive
processes running on the host machine it will take a long time for the worker
thread to get back control. This time is measured and can be directly related
to the number of running other threads and their CPU consumption as can
be seen in Figure 5.4.9. Further, this tool gives us only the utilization of the
processor core we are really working on.
Other measure such as CPU utilization (Windows) or workload (Unix)
have the disadvantage that (a) it cannot be distinguished what process causes
the load (is it us or the others?) and (b) it is not entirely clear what is actually
measured. For example, the average load in the Linux world (which is often
mistakenly taken as the CPU utilization by many benchmarks) is actually an
exponentially-damped moving average of the total CPU queue length (see e.g.
(O’Reilly et al., 1997)).
15 A thread in computer science is short for a thread of execution. Threads are a way for a
program to fork (or split) itself into two or more simultaneously (or pseudo-simultaneously)
running tasks. Threads and processes differ from one operating system to another but, in
general, a thread is contained inside a process and different threads of the same process share
some resources while different processes do not. Multiple threads can be executed in parallel
on many computer systems. This multithreading generally occurs by time slicing (similar to
time-division multiplexing), wherein a single processor switches between different threads,
in which case the processing is not literally simultaneous, for the single processor is really
doing only one thing at a time.
16 The yield() method is available in almost all programming languages that provide the
thread concept.