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

5.4. QAD GRID WORKER 133
Figure 5.4.9: Time needed for the OS to cycle through the active thread (yield) vs.
number of running threads. This shows that there exists an almost linear relationship.
Data was created on a 2.3GHz single-core machine (2GB RAM) running Debian Linux
and Windows Vista, respectively.
5.4.4 Worker Migration
So far, we presented a system that can collect and manage jobs that are
then requested by workers running on some client machine performing the
computation. At the moment a new job becomes available it is easy for a
client machine to decide whether to request or to skip it, given that a worker
has knowledge about the current system state (e.g. CPU utilization) of its
host computer. This concept is known as static scheduling, an early approach
being the Linda system (Carriero and Gelernter, 1986).
The problem now is that these conditions can (and mostly will) change
over time. So using static scheduling might be a good idea within a dedicated
cluster environment but as soon as user’s workstations are integrated
the following obvious problem arises: A user, say Alice, integrates her desktop
workstation into the Grid and goes away for some time. Another User, say
Bob, creates a job in the Grid system that is then requested by the worker
running on Alice’s machine. The trouble comes if Alice returns to her workstation
before Bob’s job has finished. If static scheduling is used, the options are
limited: (a) we can allow Bob’s job to finish which will make Alice unhappy
because her system will be slow or (b) terminate Bob’s job that will make Bob
unhappy because he loses work already done.
In the QAD Grid system it is of primary importance that an owner of a
workstation does not pay penalty for integrating its machine into the Grid, that
is running a worker on his/her machine. So, workers must have the ability to
vacate a workstation (or at least pause computation) if a users starts working
on that machine and/or wants the worker to quit. In the remaining of this
section we will describe a dynamic scheduling approach that is able to satisfy
both, Alice and Bob. The key idea is that when Alice returns and starts using
her machine Bob’s job is paused, transferred to another workstation that is idle
at that moment and continued from the point it was stopped. This concept
holds also for multi-core systems where the load of one of the cores might get
too high.
Worker migration (also known as Process Migration) is the process of freezing
a workers state, sending that state along with the worker to another idle
workstation, start that worker at that workstation initialized with the restored