minor bugfix

[lttv.git] / ltt / branches / poly / doc / developer / process_traceset_strict_boundaries.txt

Linux Trace Toolkit

Mathieu Desnoyers 17-05-2004


1. Read Requests Cases Study

The goal of this document is to describe the typical behavior of viewers when
they request data to a process traceset. After the implementation of three
viewers, with different needs, the idea of their need for a trace processing API
is getting clearer. We then describe a new API for process traceset that would
better suits the needs of those viewers.

They are splitted in two different categories : the first one is the one where
the viewers select the events they need by specifying a time interval in the
traceset and the second one is where the viewers specify a start event by its
position in the traceset and a certain amount of events it needs.

This is a simplified case study : we look at the direct interaction between
graphical viewers and process traceset, without the main window as a negociator.

Control Flow Viewer

This viewer, consisting in a two dimensions graph, shows the different processes
as its y axis and the time as x axis. It's clear that it needs to get the events
by specifying a start time and an end time, constituing a time interval.


Detailed Events List

This list has nothing to do with time : it shows the events one by one. It cares
about the quantity of events, not their time.

It would be simple to get the events one by one if we were reading only one
tracefile (one cpu), but the way events are read through each trace
(monothically increasing time) makes it a little bit more difficult to specify
how to increment event position. We will determine how it could be done simply.

Let's define an event position. It's a pointer to a position into each
tracefile. It's only meaningful when associated with a context. Comparisons
between positions are done by looking comparing saved positions for each
tracefile, until a difference is found.

A viewer could use a start time as a start event. It would specify a number of
events it needs. As a first call, it could ask for the start time of the
traceset. Afterward, it can save the position of the context after the last
event has been delivered in its after traceset function.

Now, let's see how process traceset could handle it. It would seek in the
traceset, searching the position number.
(need a new lttv_process_traceset_seek_position)

Then, the viewer could simply call a process traceset middle function
specifying a number of events to get.

The whole concept of numbering events would be hidden in the order in which the
process traceset gets the events in a monothically increasing time.



2. Architecture

API to seek/read traceset will be extended to fully support both start time,
start position, end time, end position and number of events as possible
boundaries for reading.

lttv_process_traceset_seek_time
lttv_process_traceset_seek_position

lttv_process_traceset_middle

It must be modified to end when it encounters the first criterion : number of
events to read reached, end time reached, end position reached.

lttv_traceset_context_position_save

The position_save saves a position that can be used later to seek back to this
exact same position, with event granularity. This implies that the
process_traceset must deliver events with the same timestamp in a deterministic
manner. This is actually done by using tracefile and trace numbers in the
context in the comparison function.



Description of new context API useage

1. seek
2. begin  -> add middle hooks
          -> call begin hooks by id
3. middle -> call middle hooks by id
4. end    -> call end hooks by id
          -> remove middle hooks

3. Impact on State

From now on, the state computation will be done in the middle hook call, with a
priority higher than default. We will define this priority as PRIO_STATE,
defined to -10.

If state has to be computed, lttv_process_traceset_begin has to be  called in
a first time. It adds the state hooks to the context. Then, the state
seek_closest will have to be used to restore the nearest state, plus a
process_traceset with no hooks present other than the state hooks will have to
be called to go from the closest state to the real time seeked.

The lttv_process_traceset_end will only need to be called if no further state
computation is needed.


4. Implementation in tracecontext.c


- Type LttvTracesetContextPosition

struct _LttvTracesetContextPosition {
  LttEventPosition *tracefile_position;  /* Position in each trace/tracefile */
  guint num_tracefiles;                   /* Number of tracefiles (check)     */
}

with interfaces :

lttv_traceset_context_position_save
(const LttvTracesetContext *context,
 LttvTracesetContextPosition *pos);


Dependencies :

- lttv_process_traceset_seek_position(LttvTracesetContext *self,
                                const LttvTracesetContextPosition *position);
    - ltt_tracefile_seek_position : already implemented

lttv_process_traceset_seek_position will seek each tracefile to the right
position. We keep information about number of tracefiles for extra integrity
checking when reloading the position in the context.



- lttv_process_traceset_middle
We modify lttv_process_traceset_middle so that it takes as arguments :
(LttvTracesetContext *self,
LttTime end,
unsigned num_events,
const LttvTracesetContextPosition *end_position)

This new version of process traceset middle will call the event hooks for
events until the first criterion is fulfilled : either the end time is passed,
the number of events requested is passed or the end position is reached. When
this function ends, the end position can be extracted from the context, the end
event is set as described below and the number of events read is returned.

The end event is a pointer to the last event the hooks has been called for.

- lttv_process_traceset_seek_time : already implemented

- lttv_process_traceset_begin(LttvTracesetContext *self,
                              LttvHooks       *before_traceset,
                              LttvHooks       *before_trace,
                              LttvHooks       *before_tracefile,
                              LttvHooks       *event,
                              LttvHooksById   *event_by_id)


- lttv_process_traceset_end(LttvTracesetContext *self,
                            LttvHooks           *after_traceset,
                            LttvHooks           *after_trace,
                            LttvHooks           *after_tracefile,
                            LttvHooks           *event,
                            LttvHooksById       *event_by_id)

- lttv_traceset_context_add_hooks and lttv_traceset_context_remove_hooks

These functions now become internal to tracecontext.c