Add link integrity tester example.

A simple script which generates light but constant traffic across all links to
ensure no packets go missing due to link problems.

examples/link_integrity_test.py

@@ -0,0 +1,80 @@
+"""This network tester script attempts to check the link integrity of every
+link in a SpiNNaker system. It is intended to be run repeatedly for long
+durations and provides a steady (though not hugely intense) flow of traffic
+over all links. The script verifies that all packets sent arrived at their
+destinations ensuring that all routers and links are behaving correctly.
+
+The experiment takes two arguments on the commandline::
+
+    $ python link_integrity_test.py hostname duration
+
+The arguments specify the hostname of the machine whose links are to be tested
+and the duraton to run the experiment. In practice this script is typically
+executed repeatedly for 60-second runs. If the script finds any discrepancies,
+it will produce a descriptive error when it terminates, otherwise no output is
+produced.
+"""
+
+import sys
+
+from network_tester import Experiment
+
+
+# Take the hostname from the arguments
+e = Experiment(sys.argv[1])
+
+# Warn if anything is dead...
+if list(e.system_info.dead_links()):
+    print("Warning: Some chips are apprently dead: "
+          "{}".format(sorted(e.system_info.dead_chips())))
+if list(e.system_info.dead_links()):
+    print("Warning: Some links are apprently dead: "
+          "{}".format(sorted(e.system_info.dead_links())))
+    # Try using "dead" links anyway...
+    for x, y, link in e.system_info.dead_links():
+        e.system_info[(x, y)].working_links.add(link)
+
+# Create a core and flow to test tx/rx for each link
+cores = {(x, y, link): e.new_core(x, y)
+         for x, y, link in e.system_info.links()}
+for (x1, y1, link1), core in cores.items():
+    dx, dy = link1.to_vector()
+    x2 = (x1 + dx) % e.system_info.width
+    y2 = (y1 + dy) % e.system_info.height
+    link2 = link1.opposite
+    if (x2, y2, link2) in cores:
+        e.new_flow(core, cores[(x2, y2, link2)],
+                   name=str((x1, y1, link1)))
+
+# Run for a while...
+e.duration = float(sys.argv[2])
+e.timestep = 1e-5
+
+# Leave 0.1 seconds at the start and end to ensure we see all packets
+e.burst_period = e.duration
+e.burst_duty = 1.0 - (2.0 / e.duration)
+e.burst_phase = 1.0 - (1.0 / e.duration)
+
+# Record both the packet generator/consumer counters and also the router
+# dropping counters so that if there is a mismatch between packets sent and
+# received it can be checked whether this was due to a router dropping the
+# packets or due to some other malfunction.
+e.record_sent = True
+e.record_received = True
+e.record_dropped_multicast = True
+
+results = e.run()
+
+# Report any mismatches in sent/received packet counts
+for result in results.flow_totals():
+    flow = result["flow"]
+    sent = result["sent"]
+    received = result["received"]
+    if sent != received:
+        print("Error: Sent {} and received {} across link {}".format(
+            sent, received, flow.name))
+
+# Report any dropped packets
+num_dropped = sum(results.totals()["dropped_multicast"])
+if num_dropped != 0:
+    print("Error: {} packets dropped by a router.".format(num_dropped))