| 1 | Linux Trace Toolkit |
| 2 | |
| 3 | Mathieu Desnoyers 17-05-2004 |
| 4 | |
| 5 | |
| 6 | This document explains how the lttvwindow API could process the event requests |
| 7 | of the viewers, merging event requests and hook lists to benefit from the fact |
| 8 | that process_traceset can call multiple hooks for the same event. |
| 9 | |
| 10 | First, we will explain the detailed process of event delivery in the current |
| 11 | framework. We will then study its strengths and weaknesses. |
| 12 | |
| 13 | In a second time, a framework where the events requests are dealt by the main |
| 14 | window with fine granularity will be described. We will then discussed the |
| 15 | advantages and inconvenients over the first framework. |
| 16 | |
| 17 | |
| 18 | 1. (Actual) Boundaryless event reading |
| 19 | |
| 20 | Actually, viewers request events in a time interval from the main window. They |
| 21 | also specify a (not so) maximum number of events to be delivered. In fact, the |
| 22 | number of events to read only gives a stop point, from where only events with |
| 23 | the same timestamp will be delivered. |
| 24 | |
| 25 | Viewers register hooks themselves in the traceset context. When merging read |
| 26 | requests in the main window, all hooks registered by viewers will be called for |
| 27 | the union of all the read requests, because the main window has no control on |
| 28 | hook registration. |
| 29 | |
| 30 | The main window calls process_traceset on its own for all the intervals |
| 31 | requested by all the viewers. It must not duplicate a read of the same time |
| 32 | interval : it could be very hard to filter by viewers. So, in order to achieve |
| 33 | this, time requests are sorted by start time, and process_traceset is called for |
| 34 | each time request. We keep the last event time between each read : if the start |
| 35 | time of the next read is lower than the time reached, we continue the reading |
| 36 | from the actual position. |
| 37 | |
| 38 | We deal with specific number of events requests (infinite end time) by |
| 39 | garantying that, starting from the time start of the request, at least that |
| 40 | number of events will be read. As we can't do it efficiently without interacting |
| 41 | very closely with process_traceset, we always read the specified number of |
| 42 | events requested starting from the current position when we answer to a request |
| 43 | based on the number of events. |
| 44 | |
| 45 | The viewers have to filter events delivered by traceset reading, because they |
| 46 | can be asked by another viewer for a totally (or partially) different time |
| 47 | interval. |
| 48 | |
| 49 | |
| 50 | Weaknesses |
| 51 | |
| 52 | - process_middle does not guarantee the number of events read |
| 53 | |
| 54 | First of all, a viewer that requests events to process_traceset has no garantee |
| 55 | that it will get exactly what it asked for. For example, a direct call to |
| 56 | traceset_middle for a specific number of events will delived _at least_ that |
| 57 | quantity of events, plus the ones that have the same timestamp that the last one |
| 58 | has. |
| 59 | |
| 60 | - Border effects |
| 61 | |
| 62 | Viewer's writers will have to deal with a lot of border effects caused by the |
| 63 | particularities of the reading. They will be required to select the information |
| 64 | they need from their input by filtering. |
| 65 | |
| 66 | - Lack of encapsulation and difficulty of testing |
| 67 | |
| 68 | The viewer's writer will have to take into account all the border effects caused |
| 69 | by the interaction with other modules. This means that event if a viewer works |
| 70 | well alone or with another viewer, it's possible that new bugs arises when a new |
| 71 | viewer comes around. So, even if a perfect testbench works well for a viewer, it |
| 72 | does not confirm that no new bug will arise when another viewer is loaded at the |
| 73 | same moment asking for different time intervals. |
| 74 | |
| 75 | |
| 76 | - Duplication of the work |
| 77 | |
| 78 | Time based filters and counters of events will have to be implemented at the |
| 79 | viewer's side, which is a duplication of the functionnalities that would |
| 80 | normally be expected from the tracecontext API. |
| 81 | |
| 82 | - Lack of control over the data input |
| 83 | |
| 84 | As we expect module's writers to prefer to be as close as possible from the raw |
| 85 | datas, making them interact with a lower level library that gives them a data |
| 86 | input that they only control by further filtering of the input is not |
| 87 | appropriated. We should expect some reluctancy from them about using this API |
| 88 | because of this lack of control on the input. |
| 89 | |
| 90 | - Speed cost |
| 91 | |
| 92 | All hooks of all viewers will be called for all the time intervals. So, if we |
| 93 | have a detailed events list and a control flow view, asking both for different |
| 94 | time intervals, the detailed events list will have to filter all the events |
| 95 | delivered originally to the control flow view. This can be a case occuring quite |
| 96 | often. |
| 97 | |
| 98 | |
| 99 | |
| 100 | Strengths |
| 101 | |
| 102 | - Simple concatenation of time intervals at the main window level. |
| 103 | |
| 104 | Having the opportunity of delivering more events than necessary to the viewers |
| 105 | means that we can concatenate time intervals and number of events requested |
| 106 | fairly easily, while being hard to determine if some specific cases will be |
| 107 | wrong, in depth testing being impossible. |
| 108 | |
| 109 | - No duplication of the tracecontext API |
| 110 | |
| 111 | Viewers deal directly with the tracecontext API for registering hooks, removing |
| 112 | a layer of encapsulation. |
| 113 | |
| 114 | |
| 115 | |
| 116 | |
| 117 | |
| 118 | 2. (Proposed) Strict boundaries events reading |
| 119 | |
| 120 | The idea behind this method is to provide exactly the events requested by the |
| 121 | viewers to them, no more, no less. |
| 122 | |
| 123 | It uses the new API for process traceset suggested in the document |
| 124 | process_traceset_strict_boundaries.txt. |
| 125 | |
| 126 | It also means that the lttvwindow API will have to deal with viewer's hooks. |
| 127 | Those will not be allowed to add them directly in the context. They will give |
| 128 | them to the lttvwindow API, along with the time interval or the position and |
| 129 | number of events. The lttvwindow API will have to take care of adding and |
| 130 | removing hooks for the different time intervals requested. That means that hooks |
| 131 | insertion and removal will be done between each traceset processing based on |
| 132 | the time intervals and event positions related to each hook. We must therefore |
| 133 | provide a simple interface for hooks passing between the viewers and the main |
| 134 | window, make them easier to manage from the main window. A modification to the |
| 135 | LttvHooks type solves this problem. |
| 136 | |
| 137 | |
| 138 | Architecture |
| 139 | |
| 140 | Added to the lttvwindow API : |
| 141 | |
| 142 | |
| 143 | void lttvwindow_events_request |
| 144 | ( MainWindow *main_win, |
| 145 | EventsRequest events_request); |
| 146 | |
| 147 | |
| 148 | Internal functions : |
| 149 | |
| 150 | - lttvwindow_process_pending_requests |
| 151 | |
| 152 | |
| 153 | |
| 154 | Implementation |
| 155 | |
| 156 | |
| 157 | - Type LttvHooks |
| 158 | |
| 159 | see hook_prio.txt |
| 160 | |
| 161 | The viewers will just have to pass hooks to the main window through this type, |
| 162 | using the hook.h interface to manipulate it. Then, the main window will add |
| 163 | them and remove them from the context to deliver exactly the events requested by |
| 164 | each viewer through process traceset. |
| 165 | |
| 166 | |
| 167 | - lttvwindow_events_request |
| 168 | |
| 169 | It adds the EventsRequest struct to the array of time requests pending and |
| 170 | registers a pending request for the next g_idle if none is registered. |
| 171 | |
| 172 | typedef struct _EventsRequest { |
| 173 | LttTime start_time, /* Unset : { 0, 0 } */ |
| 174 | LttvTracesetContextPosition start_position, /* Unset : num_traces = 0 */ |
| 175 | LttTime end_time, /* Unset : { 0, 0 } */ |
| 176 | guint num_events, /* Unset : G_MAXUINT */ |
| 177 | LttvTracesetContextPosition end_position, /* Unset : num_traces = 0 */ |
| 178 | LttvHooksById *before_traceset, /* Unset : NULL */ |
| 179 | LttvHooksById *before_trace, /* Unset : NULL */ |
| 180 | LttvHooksById *before_tracefile, /* Unset : NULL */ |
| 181 | LttvHooksById *middle, /* Unset : NULL */ |
| 182 | LttvHooksById *after_tracefile, /* Unset : NULL */ |
| 183 | LttvHooksById *after_trace, /* Unset : NULL */ |
| 184 | LttvHooksById *after_traceset /* Unset : NULL */ |
| 185 | } EventsRequest; |
| 186 | |
| 187 | |
| 188 | - lttvwindow_process_pending_requests |
| 189 | |
| 190 | This internal function gets called by g_idle, taking care of the pending |
| 191 | requests. It is responsible for concatenation of time intervals and position |
| 192 | requests. It does it with the following algorithm organizing process traceset |
| 193 | calls. Here is the detailed description of the way it works : |
| 194 | |
| 195 | |
| 196 | - Events Requests Servicing Algorithm |
| 197 | |
| 198 | Data structures necessary : |
| 199 | |
| 200 | List of requests added to context : list_in |
| 201 | List of requests not added to context : list_out |
| 202 | |
| 203 | Initial state : |
| 204 | |
| 205 | list_in : empty |
| 206 | list_out : many events requests |
| 207 | |
| 208 | |
| 209 | While list_in !empty and list_out !empty |
| 210 | 1. If list_in is empty (need a seek) |
| 211 | 1.1 Add requests to list_in |
| 212 | 1.1.1 Find all time requests with the lowest start time in list_out |
| 213 | (ltime) |
| 214 | 1.1.2 Find all position requests with the lowest position in list_out |
| 215 | (lpos) |
| 216 | 1.1.3 If lpos.start time < ltime |
| 217 | - Add lpos to list_in, remove them from list_out |
| 218 | 1.1.4 Else, (lpos.start time >= ltime) |
| 219 | - Add ltime to list_in, remove them from list_out |
| 220 | 1.2 Seek |
| 221 | 1.2.1 If first request in list_in is a time request |
| 222 | 1.2.1.1 Seek to that time |
| 223 | 1.2.2 Else, the first request in list_in is a position request |
| 224 | 1.2.2.1 Seek to that position |
| 225 | 1.3 Call begin for all list_in members |
| 226 | (1.3.1 begin hooks called) |
| 227 | (1.3.2 middle hooks added) |
| 228 | 2. Else, list_in is not empty, we continue a read |
| 229 | 2.1 For each req of list_out |
| 230 | - if req.start time == current context time |
| 231 | - Add to list_in, remove from list_out |
| 232 | - Call begin |
| 233 | - if req.start position == current position |
| 234 | - Add to list_in, remove from list_out |
| 235 | - Call begin |
| 236 | |
| 237 | 3. Find end criterions |
| 238 | 3.1 End time |
| 239 | 3.1.1 Find lowest end time in list_in |
| 240 | 3.1.2 Find lowest start time in list_out |
| 241 | 3.1.3 Use lowest of both as end time |
| 242 | 3.2 Number of events |
| 243 | 3.2.1 Find lowest number of events in list_in |
| 244 | 3.3 End position |
| 245 | 3.3.1 Find lowest end position in list_in |
| 246 | 3.3.2 Find lowest start position in list_out |
| 247 | 3.3.3 Use lowest of both as end position |
| 248 | |
| 249 | 4. Call process traceset middle |
| 250 | 4.1 Call process traceset middle (Use end criterion found in 3) |
| 251 | 5. After process traceset middle |
| 252 | 5.1 For each req in list_in |
| 253 | - req.num -= count |
| 254 | - if req.num == 0 |
| 255 | - Call end for req |
| 256 | - remove req from list_in |
| 257 | - if current context time > req.end time |
| 258 | - Call end for req |
| 259 | - remove req from list_in |
| 260 | - if req.end pos == current pos |
| 261 | - Call end for req |
| 262 | - remove req from list_in |
| 263 | |
| 264 | |
| 265 | |
| 266 | Notes : |
| 267 | End criterions for process traceset middle : |
| 268 | If the criterion is reached, event is out of boundaries and we return. |
| 269 | Current time > End time |
| 270 | Event count > Number of events |
| 271 | Current position >= End position |
| 272 | |
| 273 | The >= for position is necessary to make ensure consistency between start time |
| 274 | requests and positions requests that happens to be at the exact same start time |
| 275 | and position. |
| 276 | |
| 277 | |
| 278 | |
| 279 | Weaknesses |
| 280 | |
| 281 | - None (nearly?) :) |
| 282 | |
| 283 | |
| 284 | Strengths |
| 285 | |
| 286 | - Removes the need for filtering of information supplied to the viewers. |
| 287 | |
| 288 | - Viewers have a better control on their data input. |
| 289 | |
| 290 | - Solves all the weaknesses idenfied in the actual boundaryless traceset |
| 291 | reading. |