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add19043 BP |
1 | Benjamin Poirier |
2 | benjamin.poirier@polymtl.ca | |
3 | 2009 | |
4 | ||
5 | + About time synchronization | |
6 | This framework performs offline time synchronization. This means that the | |
7 | synchronization is done after tracing is over. It is not the same as online | |
8 | synchronization like what is done by NTP. Nor is it directly influenced by it. | |
9 | ||
10 | Event timestamps are adjusted according to a clock correction function that | |
11 | palliates for initial offset and rate offset (ie. clocks that don't start out | |
12 | at the same value and clocks that don't run at the same speed). It can work on | |
13 | two or more traces. | |
14 | ||
15 | The synchronization is based on relations identified in network traffic | |
16 | between nodes. So, for it to work, there must be traffic exchanged between the | |
17 | nodes. At the moment, this must be TCP traffic. Any kind will do (ssh, http, | |
18 | ...) | |
19 | ||
20 | For scientific information about the algorithms used, see: | |
21 | * Duda, A., Harrus, G., Haddad, Y., and Bernard, G.: Estimating global time in | |
22 | distributed systems, Proc. 7th Int. Conf. on Distributed Computing Systems, | |
23 | Berlin, volume 18, 1987 | |
24 | * Ashton, P.: Algorithms for Off-line Clock Synchronisation, University of | |
25 | Canterbury, December 1995 | |
26 | http://www.cosc.canterbury.ac.nz/research/reports/TechReps/1995/tr_9512.pdf | |
27 | ||
28 | + Using time synchronization | |
29 | ++ Recording traces | |
30 | To use time synchronization you have to record traces on multiple nodes | |
31 | simultaneously with lttng (the tracer). While recording the traces, you have | |
32 | to make sure the following markers are enabled: | |
33 | * dev_receive | |
34 | * dev_xmit_extended | |
35 | * tcpv4_rcv_extended | |
36 | * udpv4_rcv_extended | |
9a9ca632 BP |
37 | You can use the 'ltt-armall' and 'ltt-armnetsync' scripts for this. |
38 | ||
add19043 BP |
39 | You also have to make sure there is some TCP traffic between the traced nodes. |
40 | ||
41 | ++ Viewing traces | |
42 | Afterwards, you have to make sure all the traces are accessible from a single | |
43 | machine, where lttv (the viewer) is run. | |
44 | ||
45 | Time synchronization is enabled and controlled via the following lttv options, | |
46 | as seen with "-h": | |
47 | --sync | |
48 | synchronize the time between the traces | |
49 | --sync-stats | |
50 | print statistics about the time synchronization | |
51 | --sync-null | |
52 | read the events but do not perform any processing, this | |
53 | is mostly for performance evaluation | |
54 | --sync-analysis - argument: chull, linreg | |
55 | specify the algorithm to use for event analysis | |
56 | --sync-graphs | |
57 | output gnuplot graph showing synchronization points | |
58 | --sync-graphs-dir - argument: DIRECTORY | |
59 | specify the directory where to store the graphs, by | |
60 | default in "graphs-<lttv-pid>" | |
61 | ||
62 | To enable synchronization, start lttv with the "--sync" option. It can be | |
63 | used in text mode or in GUI mode. You can add the traces one by one in the GUI | |
64 | but this will recompute the synchronization after every trace that is added. | |
65 | Instead, you can save some time by specifying all your traces on the command | |
66 | line (using -t). | |
67 | ||
68 | Example: | |
69 | lttv-gui -t traces/node1 -t traces/node2 --sync | |
70 | ||
71 | ++ Statistics | |
72 | The --sync-stats option is useful to make sure the synchronization algorithms | |
73 | worked. Here is an example output (with added comments) from a successful | |
74 | chull (one of the synchronization algorithms) run of two traces: | |
75 | LTTV processing stats: | |
76 | received frames: 452 | |
77 | received frames that are IP: 452 | |
78 | received and processed packets that are TCP: 268 | |
79 | sent packets that are TCP: 275 | |
80 | TCP matching stats: | |
81 | total input and output events matched together to form a packet: 240 | |
82 | Message traffic: | |
83 | 0 - 1 : sent 60 received 60 | |
84 | # Note that 60 + 60 < 240, this is because there was loopback traffic, which is | |
85 | # discarded. | |
86 | Convex hull analysis stats: | |
87 | out of order packets dropped from analysis: 0 | |
88 | Number of points in convex hulls: | |
89 | 0 - 1 : lower half-hull 7 upper half-hull 9 | |
90 | Individual synchronization factors: | |
91 | 0 - 1 : Middle a0= -1.33641e+08 a1= 1 - 4.5276e-08 accuracy 1.35355e-05 | |
92 | a0: -1.34095e+08 to -1.33187e+08 (delta= 907388) | |
93 | a1: 1 -6.81298e-06 to +6.72248e-06 (delta= 1.35355e-05) | |
94 | Resulting synchronization factors: | |
95 | trace 0 drift= 1 offset= 0 (0.000000) start time= 18.799023588 | |
96 | trace 1 drift= 1 offset= 1.33641e+08 (0.066818) start time= 19.090688494 | |
97 | Synchronization time: | |
98 | real time: 0.113308 | |
99 | user time: 0.112007 | |
100 | system time: 0.000000 | |
101 | ||
102 | ++ Algorithms | |
103 | The synchronization framework is extensible and already includes two | |
104 | algorithms: chull and linreg. You can choose which analysis algorithm to use | |
105 | with the --sync-analysis option. | |
106 | ||
107 | + Design | |
108 | This part describes the design of the synchronization framework. This is to | |
109 | help programmers interested in: | |
110 | * adding new synchronization algorithms (analysis part) | |
111 | There are already two analysis algorithms available: chull and linreg | |
112 | * using new types of events (processing and matching parts) | |
113 | * using time synchronization with another data source/tracer (processing part) | |
114 | There are already two data sources available: lttng and unittest | |
115 | ||
116 | ++ Sync chain | |
117 | This part is specific to the framework in use: the program doing | |
118 | synchronization, the executable linking to the event_*.o | |
119 | eg. LTTV, unittest | |
120 | ||
121 | This reads parameters, creates SyncState and calls the processing init | |
122 | function. The "sync chain" is the set of event-* modules. At the moment there | |
123 | is only one module at each stage. However, as more module are added, it will | |
124 | become relevant to have many modules at the same stage simultaneously. This | |
125 | will require some modifications. I've kept this possibility at the back of my | |
126 | mind while designing. | |
127 | ||
128 | ++ Stage 1: Event processing | |
129 | Specific to the tracing data source. | |
130 | eg. LTTng, LTT userspace, libpcap | |
131 | ||
132 | Read the events from the trace and stuff them in an appropriate Event object. | |
133 | ||
134 | ++ Communication between stages 1 and 2: events | |
135 | Communication is done via objects specialized from Event. At the moment, all | |
136 | *Event are in data_structures.h. Specific event structures and functions could | |
137 | be in separate files. This way, adding a new set of modules would require | |
138 | shipping extra data_structures* files instead of modifying the existing one. | |
139 | For this to work, Event.type couldn't be an enum, it could be an int and use | |
f6691532 | 140 | #defines or constants defined in the specialized data_structures* files. |
add19043 BP |
141 | Event.event could be a void*. |
142 | ||
143 | ++ Stage 2: Event matching | |
144 | This stage and its modules are specific to the type of event. Event processing | |
145 | feeds the events one at a time but event analysis works on groups of events. | |
146 | Event matching is responsible for forming these groups. Generally speaking, | |
147 | these can have different types of relation ("one to one", "one to many", or a | |
148 | mix) and it will influence the overall behavior of the module. | |
149 | eg. TCP, UDP, MPI | |
150 | ||
f6691532 BP |
151 | matchEvent() takes an Event pointer. An actual matching module doesn't have to |
152 | be able to process every type of event. It will only be passed events of a | |
153 | type it can process (according to the .canMatch field of its MatchingModule | |
154 | struct). | |
add19043 BP |
155 | |
156 | ++ Communication between stages 2 and 3: event groups | |
157 | Communication consists of events grouped in Message, Exchange or Broadcast | |
158 | structs. | |
159 | ||
160 | About exchanges: | |
161 | If one event pair is a packet (more generally, something representable as a | |
162 | Message), an exchange is composed of at least two packets, one in each | |
163 | direction. There should be a non-negative minimum "round trip time" (RTT) | |
164 | between the first and last event of the exchange. This RTT should be as small | |
165 | as possible so these packets should be closely related in time like a data | |
166 | packet and an acknowledgement packet. If the events analyzed are such that the | |
167 | minimum RTT can be zero, there's nothing gained in analyzing exchanges beyond | |
168 | what can already be figured out by analyzing packets. | |
169 | ||
170 | An exchange can also consist of more than two packets, in case one packet | |
171 | single handedly acknowledges many data packets. In this case, it is best to | |
172 | use the last acknowledged packet. Assuming a linear clock, an acknowledged | |
173 | packet is as good as any other. However, since the linear clock assumption is | |
174 | further from reality as the interval grows longer, it is best to keep the | |
175 | interval between the two packets as short as possible. | |
176 | ||
177 | ++ Stage 3: Event analysis | |
178 | This stage and its modules are specific to the algorithm that analyzes events | |
179 | to deduce synchronization factors. | |
180 | eg. convex hull, linear regression, broadcast Maximum Likelihood Estimator | |
181 | ||
182 | Instead of having one analyzeEvents() function that can receive any sort of | |
183 | grouping of events, there are three prototypes: analyzeMessage(), | |
184 | analyzeExchange() and analyzeBroadcast(). A module implements only the | |
185 | relevant one(s) and sets the other function pointers to NULL in its | |
186 | AnalysisModule struct. | |
187 | ||
188 | The approach is different from matchEvent() where there is one point of entry | |
189 | no mather the type of event. The analyze*() approach has the advantage that | |
190 | there is no casting or type detection to do. It is also possible to deduce | |
191 | from the functions pointers which groupings of events a module can analyze. | |
192 | However, it means each analysis module will have to be modified if there is | |
193 | ever a new type of event grouping. | |
194 | ||
195 | I chose this approach because: | |
196 | 1) I thought it likely that there will be new types of events but not so | |
197 | likely that there will be new types of event groups. | |
198 | 2) all events share some members (time, traceNb, ...) but not event groups | |
199 | 3) we'll see which one of the two approaches works best and we can adapt | |
200 | later. | |
201 | ||
202 | ++ Data flow | |
203 | Data from traces flows "down" from processing to matching to analysis. Factors | |
204 | come back up. | |
205 | ||
206 | ++ Evolution and adaptation | |
207 | It is possible to change/add another sync chain and to add other event_* | |
208 | modules. It has been done. New types of events may need to be added to | |
209 | data_structures.h. This is only to link between Event-* modules. If the data | |
210 | does not have to be shared, data_structures.h does not have to be modified. | |
211 | ||
212 | At the moment there is some code duplication in the last steps of linreg and | |
213 | chull analysis: the code to propagate the factors when there are more than two | |
214 | nodes. Maybe there could be a Stage 4 that does that? |