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Linux Trace Toolkit

Requests Servicing Schedulers


Mathieu Desnoyers, 07/06/2004


In the LTT graphical interface, two main types of events requests may occur :

- events requests made by a viewer concerning a traceset for a ad hoc
  computation.
- events requests made by a viewer concerning a trace for a precomputation.


Ad Hoc Computation

The ad hoc computation must be serviced immediately : they are directly
responding to events requests that must be serviced to complete the graphical
widgets'data. This kind of computation may lead to incomplete result as long as
precomputation are not finished. Once precomputation is over, the widgets will
be redrawn if they needed such information. A ad hoc computation is done on a
traceset : the workspace of a tab.

Precomputation 

Traces are global objects. Only one instance of a trace is opened for all the
program. Precomputation will append data to the traces attributes (states,
statistics). It must inform the widgets which asked for such states or
statistics of their availability. Only one precomputation must be launched for
each trace and no duplication of precomputation must be done.


Schedulers

There is one tracesetcontext per traceset. Each reference to a trace by a
traceset also has its own tracecontext. Each trace, by itself, has its own
tracecontext.

Let's define a scheduler as a g_idle events request servicing function.

There is one scheduler per traceset context (registered when there are requests
to answer). There is also one scheduler per autonomous trace context (not
related to any traceset context).

A scheduler processes requests for a specific traceset or trace by combining
time intervals of the requests. It is interruptible by any GTK event. A
precomputation scheduler has a lower priority than a ad hoc computation
scheduler. That means that no precomputation will be performed until there is
no more ad hoc compuation pending. When a scheduler is interrupted, it makes no
assumption about the presence or absence of the current requests in its pool
when it starts back.


Foreground Scheduler

There can be one foreground scheduler per traceset (one traceset per tab). It
simply calls the hooks given by the events requests of the viewers for the
specified time intervals.


Background Scheduler

Right now, to simplify the problem of the background scheduler, we assume that
the module that loads the extended statistics hooks has been loaded before the
data is requested and that it is not unloaded until the program stops. We will
eventually have to deal with the requests removal based on module load/unload,
but it complicates the problem quite a bit.

A background scheduler adds hooks located under a global attributes path
(specified by the viewer who makes the request) to the trace's traceset
context (the trace is specified by the viewer). Then, it processes the whole
trace with this context (and hooks).

Typically, a module that extends statistics will register hooks in the global
attributes tree under /computation/modulename/hook_name . A viewer
that needs these statistics for a set of traces does a background computation
request through a call to the main window API function. It must specify all
types of hooks that must be called for the specified trace.

The background computation requests for a trace are queued. When the idle
function kicks in to answer these requests, it add the hooks of all the requests
toghether in the context and starts the read. It also keeps a list of the
background requests currently serviced.

The read is done from start to end of the trace, calling all the hooks present
in the context. Only when the read is over, the after_request hooks of the
currently serviced requests are called and the requests are destroyed.

If there are requests in the waiting queue, they are all added to the current
pool and processed. It is important to understand that, while a processing is in
being done, no requests are added to the pool : they wait for their turn in the
queue.

Every hook that are added to the context by the scheduler comes from global
attributes, i.e.
/traces/#
    in LttvTrace attributes : modulename/hook_name

They come with a flag telling either in_progress or ready. If the flag
ready is set, a viewer knows that the data it needs is already ready and he
doesn't have to make a request.

If the flag in_progress is set, that means that the data it needs is currently
being serviced, and it must wait for the current servicing to be finished. It
tells the lttvwindow API to call a hook when the actual servicing is over (there
is a special function for this, as it requires to modify the pool of requests
actually being serviced : we must make sure that no new reading hooks are
added!).





New Global Attributes

/traces/#
    in LttvTrace attributes :

When a processing is fired, a variable
                              computation/modulename/in_progress is set.

When a processing finished, a variable
                              computation/modulename/in_progress is unset
                              computation/modulename/ready is set





Typical Use For a Viewer

When a viewer wants extended information, it must first check if it is ready.
if not :
Before a viewer makes a request, it must check the in_progress status of the
hooks.

If the in_progress is unset, it makes the request.

If the in_progress is set, it makes a special request for being informed of the
end of request.




Hooks Lists

In order to answer the problems of background processing, we need to add a
reference counter for each hook of a hook list. If the same hook is added twice,
it will be called only once, but it will need two "remove" to be really removed
from the list. Two hooks are identical if they have the same function pointer
and hook_data.






Implementation

Ad Hoc Computation

see lttvwindow_events_delivery.txt


Hooks Lists

need new ref_count field with each hook
lttv_hook_add and lttv_hook_add_list must compare addition with present and
increment ref counter if already present.

lttv_hook_remove and remove_with_data must decrement ref_count is >1, or remove
the element otherwise (==1).



Background Scheduler

Global traces

Two global attributes per trace : 
traces/#
  It is a pointer to the LttvTrace structure.
  In the LttvTrace attributes :
    state/
      saved_states/
    statistics/
      modes/
      cpu/
      processes/
      modulename1/
      modulename2/
      ...
    computation/  /* Trace specific background computation hooks status */
      state/
        in_progress
        ready
      statistics/
        in_progress
        ready
      modulename1/
        in_progress
        ready
    requests_queue/     /* Background computation requests */
    requests_current/   /* Type : BackgroundRequest */
    notify_queue/
    notify_current/

computation/      /* Global background computation hooks */
  state/
    before_chunk_traceset
    before_chunk_trace
    before_chunk_tracefile
    after_...
    before_request
    after_request
    event_hook
    event_hook_by_id
  statistics/
    ...
  modulename1/
    ...


struct _LttvBackgroundComputation {
  GSList *events_requests;
 /* A GSList of events request background computation for a trace. */
  LttvTraceset *ts;
 /* A factice traceset that contains just one trace */
  LttvTracesetContext *tsc;
 /* The traceset context that reads this trace */
}




Modify Traceset
Points to the global traces. Main window must open a new one only when no
instance of the pathname exists.

Modify trace opening / close to make them create and destroy
LttvBackgroundComputation (and call end requests hooks for servicing requests)
and global trace info when references to the trace is zero.



EventsRequest Structure

This structure is the element of the events requests pools. The owner field is
used as an ownership identifier. The viewer field is a pointer to the data
structure upon which the action applies. Typically, both will be pointers to
the viewer's data structure.

In a ad hoc events request, a pointer to the EventsRequest structure is used as
hook_data in the hook lists : it must have been added by the viewers.


Modify module load/unload

A module that registers global computation hooks in the global attributes upon
load should unregister them when unloaded. Also, it must remove every background
computation request for each trace that has its own string as gchar *.



Background Requests Servicing Algorithm (v1)


list_in : currently serviced requests
list_out : queue of requests waiting for processing

notification lists :
notify_in : currently checked notifications
notify_out : queue of notifications that comes along with next processing.


1. Before processing
  if list_in is empty
    - Add all requests in list_out to list_in, empty list_out
    - for each request in list_in
      - add hooks to context
      - set hooks'in_progress flag to TRUE
    - seek trace to start
    - Move all notifications from notify_out to notify_in.

2. call process traceset middle for a chunk
  (assert list_in is not empty! : should not even be called in that case)

3. After the chunk
  3.1 call after_chunk hooks from list_in
  3.2 for each notify_in
    - if current time >= notify time, call notify and remove from notify_in
    - if current position >= notify position, call notify and remove from
      notify_in
  3.2 if end of trace reached
    - for each request in list_in
      - set hooks'in_progress flag to FALSE
      - set hooks'ready flag to TRUE
      - remove hooks from context
      - remove request
    - for each notifications in notify_in
      - call notify and remove from notify_in
    - return FALSE (scheduler stopped)
  3.3 else
    - return TRUE (scheduler still registered)