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packet (
GridSimTags.EMPTY_PKT
), with the last packet containing the
actual data (
IO_data
). If the maximum transmission unit (MTU) was 1500
on all elements between the source and the destination, sending a 10 MB
i le would result in approximately 34,952 packets being generated. In
GridSim, each packet is represented by a
NetPacket
Java object, thus
creating a considerable amount of overhead for large data transfers. This
can lead to lengthy simulation execution times for data or network-
dependent simulations. The magnitude of this overhead will be quanti-
i ed later in this chapter. In the next section, we describe the new l ow
networking implementation that seeks to minimize the overhead of
network-dependent simulations.
15.4 Flow Networking Concepts
Rather than modeling each network transfer using packets, we wish to
consider a network l ow model that captures the steady-state behavior of
network transfers. For convenience we will denote our Grid topology (such
as that depicted in
Figure 15.2
)
as a graph,
G
= (
V
,
E
) where
V
is the set of
vertices and
E
is the set of edges, consisting of two-element subsets of
V.
For
instance, if vertices
x
and
y
are connected, then {
x
,
y
}
E
. In the system there
exists l ows
f
= 1, 2, . . . ,
F
, with each l ow
f
having a source and destination.
Each l ow
f
describes a simple path of length
k
represented by a set of edges
{(
v
1
,
v
2
), . . . , (
v
k
,
v
k
+1
)}. The number of bytes in each l ow
f
is denoted as
SIZE
f
.
Let us consider a simple topology where the two entities, node
u
and
node
v
, are
directly
connected by an edge (
u
,
v
), with available bandwidth
BW
u
,
v
(in bytes per second) and latency
BW
u
,
v
(in seconds). Calculating the
duration of a single network l ow
f
with size
SIZE
f
from
u
to
v
can be
trivially computed as follows:
∈
T
f
=
LAT
u
,
v
+
SIZE
f
_____
BW
u,v
(15.1)
As an interesting aside, the above equation can be tested in a rudimentary
fashion by utilizing the i rst networking example in the GridSim distribu-
tion (NetEx01).* An extremely coarse approximation of basic l ow network-
ing can be achieved with the current packet-level network framework in
GridSim by setting the MTU to equal the size of the network l ow to be
transferred, causing only a single
NetPacket
to be generated, which is held
at the
Output
of the
NetUser
GridSim entity for the appropriate duration.
However, this does not model bandwidth sharing on the links in any way.
*
Available at http://www.gridbus.org/gridsim/example/net_index.html.
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