that this communication would take `0.81` seconds to complete according to a
simple latency-plus-size-divided-by-bandwidth model (0.01 + 8e5/1e6 = 0.81).
However, the default TCP model of SimGrid is a bit more complex than that. It
-accounts for three phenomena that directly impacts the simulation time even
+accounts for three phenomena that directly impact the simulation time even
on such a simple example:
- The size of a message at the application level (i.e., 100kB in this
- example) is not the size of what will actually be transferred over the
- network. To mimic the fact that TCP and IP header are added to each packet of
+ example) is not the size that will actually be transferred over the
+ network. To mimic the fact that TCP and IP headers are added to each packet of
the original payload, the TCP model of SimGrid empirically considers that
- `only 97% of the nominal bandwidth` are available. In other works, the
+ `only 97% of the nominal bandwidth` are available. In other words, the
size of your message is increased by a few percents, whatever this size be.
- In the real world, the TCP protocol is not able to fully exploit the
bandwidth of a link from the emission of the first packet. To reflect this
`slow start` phenomenon, the latency declared in the platform file is
multiplied by `a factor of 13.01`. Here again, this is an empirically
- determined value that may not correspond to every TCP implementation on
+ determined value that may not correspond to every TCP implementations on
every networks. It can be tuned when more realistic simulated times for
short messages are needed though.
- When data is transferred from A to B, some TCP ACK messages travel in the
opposite direction. To reflect the impact of this `cross-traffic`, SimGrid
simulates a flow from B to A that represents an additional bandwidth
- consumption of `0.05`. The bandwidth share allocated to the flow from A to
- B is then the available bandwidth (i.e., 97% of the nominal one) divided by
- 1.05 (i.e., the total consumption). This feature, activated by default, can be
- disabled by adding the `--cfg=network/crosstraffic:0` flag to command line.
+ consumption of `0.05`. The route from B to A is implicity declared in the
+ platfrom file and uses the same link `link1` as if the two hosts were
+ connected through a communication bus. The bandwidth share allocated to the
+ flow from A to B is then the available bandwidth of `link1` (i.e., 97% of
+ the nominal bandwidth of 1Mb/s) divided by 1.05 (i.e., the total consumption).
+ This feature, activated by default, can be disabled by adding the
+ `--cfg=network/crosstraffic:0` flag to command line.
-As a consequence, the simulated time of a transfer of 100kB from A to B is not
-0.81 seconds but
+As a consequence, the time to transfer 100kB from A to B as simulated by the
+default TCP model of SimGrid is not 0.81 seconds but
.. code-block:: python
- 0.01 * 13.01 + 800000 / ((0.97 * 1e6) / 1.05) = 0.996079 seconds
+ 0.01 * 13.01 + 800000 / ((0.97 * 1e6) / 1.05) = 0.996079 seconds.
