ltt/branches/poly/doc/developer/requests_servicing_schedulers.txt

   1 Linux Trace Toolkit
   2
   3 Requests Servicing Schedulers
   4
   5
   6 Mathieu Desnoyers, 07/06/2004
   7
   8
   9 In the LTT graphical interface, two main types of events requests may occur :
  10
  11 - events requests made by a viewer concerning a traceset for a ad hoc
  12   computation.
  13 - events requests made by a viewer concerning a trace for a precomputation.
  14
  15
  16 Ad Hoc Computation
  17
  18 The ad hoc computation must be serviced immediately : they are directly
  19 responding to events requests that must be serviced to complete the graphical
  20 widgets'data. This kind of computation may lead to incomplete result as long as
  21 precomputation are not finished. Once precomputation is over, the widgets will
  22 be redrawn if they needed such information. A ad hoc computation is done on a
  23 traceset : the workspace of a tab.
  24
  25 Precomputation
  26
  27 Traces are global objects. Only one instance of a trace is opened for all the
  28 program. Precomputation will append data to the traces attributes (states,
  29 statistics). It must inform the widgets which asked for such states or
  30 statistics of their availability. Only one precomputation must be launched for
  31 each trace and no duplication of precomputation must be done.
  32
  33
  34 Schedulers
  35
  36 There is one tracesetcontext per traceset. Each reference to a trace by a
  37 traceset also has its own tracecontext. Each trace, by itself, has its own
  38 tracecontext.
  39
  40 Let's define a scheduler as a g_idle events request servicing function.
  41
  42 There is one scheduler per traceset context (registered when there are requests
  43 to answer). There is also one scheduler per autonomous trace context (not
  44 related to any traceset context).
  45
  46 A scheduler processes requests for a specific traceset or trace by combining
  47 time intervals of the requests. It is interruptible by any GTK event. A
  48 precomputation scheduler has a lower priority than a ad hoc computation
  49 scheduler. That means that no precomputation will be performed until there is
  50 no more ad hoc compuation pending. When a scheduler is interrupted, it makes no
  51 assumption about the presence or absence of the current requests in its pool
  52 when it starts back.
  53
  54
  55 Foreground Scheduler
  56
  57 There can be one foreground scheduler per traceset (one traceset per tab). It
  58 simply calls the hooks given by the events requests of the viewers for the
  59 specified time intervals.
  60
  61
  62 Background Scheduler
  63
  64 Right now, to simplify the problem of the background scheduler, we assume that
  65 the module that loads the extended statistics hooks has been loaded before the
  66 data is requested and that it is not unloaded until the program stops. We will
  67 eventually have to deal with the requests removal based on module load/unload,
  68 but it complicates the problem quite a bit.
  69
  70 A background scheduler adds hooks located under a global attributes path
  71 (specified by the viewer who makes the request) to the trace's traceset
  72 context (the trace is specified by the viewer). Then, it processes the whole
  73 trace with this context (and hooks).
  74
  75 Typically, a module that extends statistics will register hooks in the global
  76 attributes tree under /computation/modulename/hook_name . A viewer
  77 that needs these statistics for a set of traces does a background computation
  78 request through a call to the main window API function. It must specify all
  79 types of hooks that must be called for the specified trace.
  80
  81 The background computation requests for a trace are queued. When the idle
  82 function kicks in to answer these requests, it add the hooks of all the requests
  83 toghether in the context and starts the read. It also keeps a list of the
  84 background requests currently serviced.
  85
  86 The read is done from start to end of the trace, calling all the hooks present
  87 in the context. Only when the read is over, the after_request hooks of the
  88 currently serviced requests are called and the requests are destroyed.
  89
  90 If there are requests in the waiting queue, they are all added to the current
  91 pool and processed. It is important to understand that, while a processing is in
  92 being done, no requests are added to the pool : they wait for their turn in the
  93 queue.
  94
  95 Every hook that are added to the context by the scheduler comes from global
  96 attributes, i.e.
  97 /traces/#
  98     in LttvTrace attributes : modulename/hook_name
  99
 100 They come with a flag telling either in_progress or ready. If the flag
 101 ready is set, a viewer knows that the data it needs is already ready and he
 102 doesn't have to make a request.
 103
 104 If the flag in_progress is set, that means that the data it needs is currently
 105 being serviced, and it must wait for the current servicing to be finished. It
 106 tells the lttvwindow API to call a hook when the actual servicing is over (there
 107 is a special function for this, as it requires to modify the pool of requests
 108 actually being serviced : we must make sure that no new reading hooks are
 109 added!).
 110
 111
 112
 113
 114
 115 New Global Attributes
 116
 117 /traces/#
 118     in LttvTrace attributes :
 119
 120 When a processing is fired, a variable
 121                               computation/modulename/in_progress is set.
 122
 123 When a processing finished, a variable
 124                               computation/modulename/in_progress is unset
 125                               computation/modulename/ready is set
 126
 127
 128
 129
 130
 131 Typical Use For a Viewer
 132
 133 When a viewer wants extended information, it must first check if it is ready.
 134 if not :
 135 Before a viewer makes a request, it must check the in_progress status of the
 136 hooks.
 137
 138 If the in_progress is unset, it makes the request.
 139
 140 If the in_progress is set, it makes a special request for being informed of the
 141 end of request.
 142
 143
 144
 145
 146 Hooks Lists
 147
 148 In order to answer the problems of background processing, we need to add a
 149 reference counter for each hook of a hook list. If the same hook is added twice,
 150 it will be called only once, but it will need two "remove" to be really removed
 151 from the list. Two hooks are identical if they have the same function pointer
 152 and hook_data.
 153
 154
 155
 156
 157
 158
 159 Implementation
 160
 161 Ad Hoc Computation
 162
 163 see lttvwindow_events_delivery.txt
 164
 165
 166 Hooks Lists
 167
 168 need new ref_count field with each hook
 169 lttv_hook_add and lttv_hook_add_list must compare addition with present and
 170 increment ref counter if already present.
 171
 172 lttv_hook_remove and remove_with_data must decrement ref_count is >1, or remove
 173 the element otherwise (==1).
 174
 175
 176
 177 Background Scheduler
 178
 179 Global traces
 180
 181 Two global attributes per trace :
 182 traces/#
 183   It is a pointer to the LttvTrace structure.
 184   In the LttvTrace attributes :
 185     state/
 186       saved_states/
 187     statistics/
 188       modes/
 189       cpu/
 190       processes/
 191       modulename1/
 192       modulename2/
 193       ...
 194     computation/  /* Trace specific background computation hooks status */
 195       state/
 196         in_progress
 197         ready
 198       statistics/
 199         in_progress
 200         ready
 201       modulename1/
 202         in_progress
 203         ready
 204     requests_queue/     /* Background computation requests */
 205     requests_current/   /* Type : BackgroundRequest */
 206     notify_queue/
 207     notify_current/
 208
 209 computation/      /* Global background computation hooks */
 210   state/
 211     before_chunk_traceset
 212     before_chunk_trace
 213     before_chunk_tracefile
 214     after_...
 215     before_request
 216     after_request
 217     event_hook
 218     event_hook_by_id
 219   statistics/
 220     ...
 221   modulename1/
 222     ...
 223
 224
 225 struct _LttvBackgroundComputation {
 226   GSList *events_requests;
 227  /* A GSList of events request background computation for a trace. */
 228   LttvTraceset *ts;
 229  /* A factice traceset that contains just one trace */
 230   LttvTracesetContext *tsc;
 231  /* The traceset context that reads this trace */
 232 }
 233
 234
 235
 236
 237 Modify Traceset
 238 Points to the global traces. Main window must open a new one only when no
 239 instance of the pathname exists.
 240
 241 Modify trace opening / close to make them create and destroy
 242 LttvBackgroundComputation (and call end requests hooks for servicing requests)
 243 and global trace info when references to the trace is zero.
 244
 245
 246
 247 EventsRequest Structure
 248
 249 This structure is the element of the events requests pools. The owner field is
 250 used as an ownership identifier. The viewer field is a pointer to the data
 251 structure upon which the action applies. Typically, both will be pointers to
 252 the viewer's data structure.
 253
 254 In a ad hoc events request, a pointer to the EventsRequest structure is used as
 255 hook_data in the hook lists : it must have been added by the viewers.
 256
 257
 258 Modify module load/unload
 259
 260 A module that registers global computation hooks in the global attributes upon
 261 load should unregister them when unloaded. Also, it must remove every background
 262 computation request for each trace that has its own string as gchar *.
 263
 264
 265
 266 Background Requests Servicing Algorithm (v1)
 267
 268
 269 list_in : currently serviced requests
 270 list_out : queue of requests waiting for processing
 271
 272 notification lists :
 273 notify_in : currently checked notifications
 274 notify_out : queue of notifications that comes along with next processing.
 275
 276
 277 1. Before processing
 278   if list_in is empty
 279     - Add all requests in list_out to list_in, empty list_out
 280     - for each request in list_in
 281       - add hooks to context
 282       - set hooks'in_progress flag to TRUE
 283     - seek trace to start
 284     - Move all notifications from notify_out to notify_in.
 285
 286 2. call process traceset middle for a chunk
 287   (assert list_in is not empty! : should not even be called in that case)
 288
 289 3. After the chunk
 290   3.1 call after_chunk hooks from list_in
 291   3.2 for each notify_in
 292     - if current time >= notify time, call notify and remove from notify_in
 293     - if current position >= notify position, call notify and remove from
 294       notify_in
 295   3.2 if end of trace reached
 296     - for each request in list_in
 297       - set hooks'in_progress flag to FALSE
 298       - set hooks'ready flag to TRUE
 299       - remove hooks from context
 300       - remove request
 301     - for each notifications in notify_in
 302       - call notify and remove from notify_in
 303     - return FALSE (scheduler stopped)
 304   3.3 else
 305     - return TRUE (scheduler still registered)
 306