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1 | Linux Trace Toolkit |
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3 | Mathieu Desnoyers 17-05-2004 |
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6 | 1. Read Requests Cases Study |
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8 | The goal of this document is to describe the typical behavior of viewers when |
9 | they request data to a process traceset. After the implementation of three |
10 | viewers, with different needs, the idea of their need for a trace processing API |
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11 | is getting clearer. We then describe a new API for process traceset that would |
12 | better suits the needs of those viewers. |
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13 | |
14 | They are splitted in two different categories : the first one is the one where |
15 | the viewers select the events they need by specifying a time interval in the |
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16 | traceset and the second one is where the viewers specify a start event by its |
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17 | position in the traceset and a certain amount of events it needs. |
18 | |
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19 | This is a simplified case study : we look at the direct interaction between |
20 | graphical viewers and process traceset, without the main window as a negociator. |
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21 | |
22 | Control Flow Viewer |
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24 | This viewer, consisting in a two dimensions graph, shows the different processes |
25 | as its y axis and the time as x axis. It's clear that it needs to get the events |
26 | by specifying a start time and an end time, constituing a time interval. |
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28 | |
29 | Detailed Events List |
30 | |
31 | This list has nothing to do with time : it shows the events one by one. It cares |
32 | about the quantity of events, not their time. |
33 | |
34 | It would be simple to get the events one by one if we were reading only one |
35 | tracefile (one cpu), but the way events are read through each trace |
36 | (monothically increasing time) makes it a little bit more difficult to specify |
37 | how to increment event position. We will determine how it could be done simply. |
38 | |
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39 | Let's define an event position. It's a pointer to a position into each |
40 | tracefile. It's only meaningful when associated with a context. Comparisons |
41 | between positions are done by looking comparing saved positions for each |
42 | tracefile, until a difference is found. |
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43 | |
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44 | A viewer could use a start time as a start event. It would specify a number of |
45 | events it needs. As a first call, it could ask for the start time of the |
46 | traceset. Afterward, it can save the position of the context after the last |
47 | event has been delivered in its after traceset function. |
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48 | |
49 | Now, let's see how process traceset could handle it. It would seek in the |
50 | traceset, searching the position number. |
51 | (need a new lttv_process_traceset_seek_position) |
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53 | Then, the viewer could simply call a process traceset middle function |
54 | specifying a number of events to get. |
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55 | |
56 | The whole concept of numbering events would be hidden in the order in which the |
57 | process traceset gets the events in a monothically increasing time. |
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59 | |
60 | |
61 | 2. Architecture |
62 | |
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63 | API to seek/read traceset will be extended to fully support both start time, |
64 | start position, end time, end position and number of events as possible |
65 | boundaries for reading. |
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66 | |
67 | lttv_process_traceset_seek_time |
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68 | lttv_process_traceset_seek_position |
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69 | |
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70 | lttv_process_traceset_middle |
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72 | It must be modified to end when it encounters the first criterion : number of |
73 | events to read reached, end time reached, end position reached. |
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74 | |
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75 | lttv_traceset_context_position_save |
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76 | |
77 | The position_save saves a position that can be used later to seek back to this |
78 | exact same position, with event granularity. This implies that the |
79 | process_traceset must deliver events with the same timestamp in a deterministic |
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80 | manner. This is actually done by using tracefile and trace numbers in the |
81 | context in the comparison function. |
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83 | |
84 | |
85 | Description of new context API useage |
86 | |
87 | 1. seek |
88 | 2. begin -> add middle hooks |
89 | -> call begin hooks by id |
90 | 3. middle -> call middle hooks by id |
91 | 4. end -> call end hooks by id |
92 | -> remove middle hooks |
93 | |
94 | 3. Impact on State |
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95 | |
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96 | From now on, the state computation will be done in the middle hook call, with a |
97 | priority higher than default. We will define this priority as PRIO_STATE, |
98 | defined to -10. |
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99 | |
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100 | If state has to be computed, lttv_process_traceset_begin is called to add state |
101 | hooks to the context. Then, the state seek_closest will have to be used to |
102 | restore the nearest state, plus a process_traceset with no hooks present other |
103 | than the state hooks will have to be called to go from the closest state to the |
104 | real time seeked. |
105 | |
106 | The lttv_process_traceset_end will only need to be called if no further state |
107 | computation is needed. |
108 | |
109 | |
110 | 4. Implementation in tracecontext.c |
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111 | |
112 | |
113 | - Type LttvTracesetContextPosition |
114 | |
115 | struct _LttvTracesetContextPosition { |
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116 | LttEventPosition *tracefile_position; /* Position in each trace/tracefile */ |
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117 | guint num_tracefiles; /* Number of tracefiles (check) */ |
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118 | } |
119 | |
120 | with interfaces : |
121 | |
122 | lttv_traceset_context_position_save |
123 | (const LttvTracesetContext *context, |
124 | LttvTracesetContextPosition *pos); |
125 | |
126 | |
127 | Dependencies : |
128 | |
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129 | - lttv_process_traceset_seek_position(LttvTracesetContext *self, |
130 | const LttvTracesetContextPosition *position); |
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131 | - ltt_tracefile_seek_position : already implemented |
132 | |
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133 | lttv_process_traceset_seek_position will seek each tracefile to the right |
134 | position. We keep information about number of tracefiles for extra integrity |
135 | checking when reloading the position in the context. |
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136 | |
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137 | |
138 | |
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139 | - lttv_process_traceset_middle |
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140 | We modify lttv_process_traceset_middle so that it takes as arguments : |
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141 | (LttvTracesetContext *self, |
142 | LttTime end, |
143 | unsigned num_events, |
144 | const LttvTracesetContextPosition *end_position) |
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146 | This new version of process traceset middle will call the middle hooks for |
147 | events until the first criterion is fulfilled : either the end time is passed, |
148 | the number of events requested is passed or the end position is reached. When |
149 | this function ends, the end position can be extracted from the context, the end |
150 | event is set as described below and the number of events read is returned. |
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151 | |
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152 | The end event is a pointer to the last event the hooks has been called for. |
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153 | |
154 | - lttv_process_traceset_seek_time : already implemented |
155 | |
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156 | - lttv_process_traceset_begin(LttvTracesetContext *self, |
157 | LttvHooksById *begin_hooks, |
158 | LttvHooksById *middle_hooks) |
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160 | |
161 | - lttv_process_traceset_end(LttvTracesetContext *self, |
162 | LttvHooksById *end_hooks, |
163 | LttvHooksById *middle_hooks) |
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165 | - lttv_traceset_context_add_hooks and lttv_traceset_context_remove_hooks |
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167 | These functions now become internal to tracecontext.c |
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