PAPER 24 SETI@home is a massively distributed cycle donation system where data
collected by the Search for Extraterrestrial Intelligence project
distributes "work units" to client systems which are computer users who
donate their computers by installing the SETI@home screensaver.  The goal
is to identify a signal from an intelligent source by scanning frequency
ranges and varying many other parameters.  The dataset is very large (many
frequencies over a long period of time must be considered), but consists
of many independent "work units".  These work units each are small (350KB)
but require a great deal of computation (many parameter combinations must
be considered).  Clients request work units as cycles become available
using an HTTP-like protocol from a centralized server.  The work units are
redundantly computed to verify results.  If any positive result does come
through an additional out-of-band scientific protocol exists to further
verify the result.  In this way SETI@home was able to utilize an average
of 27.36TFLOPS.  SETI@home is technically simple, reliable results are
obtained through replicated computation, there is no trust measure.  But
SETI@home does introduce important social advances in public-cycle
sharing.  While incentives are not technically addressed by SETI@home,
they do exist out-of-band.  The client application acts as a screen-saver,
visualizing result computation.  While this may seem trival, developers
have found that this is an important incentive for users.  SETI@home also
maintains a website with rankings of groups donating cycles by amount of
cycles donated, so some accounting exists at the server.  This incentive
mechanism has been successfully attacked, but this poses no direct threat
to result reliability (through redundant computation) or throughput. 
SETI@home is considered a wild success, however the infrastructure
(technical and social) is single-purpose and cannot be harnessed by other
researchers seeking to take advantage of the resources available to
SETI@home.  Every such @home project must develop its own infrastructure (
technical and social).

Cluster Computing on the Fly is an architecture for cycle-sharing
applications.  in "Cluster Computing on the Fly: ..." Lo, Zappala, Zhou,
Liu and Zhao begin to address some of the problems faced by such an
architecture: scheduling, resource discovery, incentives, trust and
security.  Three classes of cycle-sharing applications are identified:
infinite workpile applications where an infinite stream of independent
work-units are available for client consumption (as in SETI@home),
workpile applications with deadlines where work-units must be computed by
a given deadline, tree-based search applications where clients interact to
some degree to help produce and bound future work-units, and
point-of-presence applications where client position in some space is
important (e.g. for measurement studies of the internet).  This paper goes
on to present an the Wave Scheduling algorithm seeking to provide
uninterrupted access to dedicated machines.  These machines are organized
in a CAN overlay organized by time-zone.  hosts are elected for dedication
by selecting the CAN zones which lie in the current off-time timezones
(e.g. night-time).  As time goes on some dedicated hosts are freed as
their time-zone becomes on-time (e.g. 8:00am) and new hosts are elected
when their zone becomes off-time (e.g. 5:00pm).  In this way dedicated
hosts are available to the scheduler without impact to the user (since his
or her machine is unused during off-time).  The SETI@home system seeks to
solve a similar problem: utilize cycles while the user's machine is not
needed.  However, SETI@home does this without an overlay by using a
screen-saver.  It's not clear that an overlay is needed for such a system.
 Although the fact of having an overlay identify likely un-used machines
is a nice property, this is not the main goal of Wave Scheduling.

