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125 lines
5.2 KiB
Text
125 lines
5.2 KiB
Text
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- GStreamer Scheduling / Synchronization (incsched) Notes -
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-----------------------------------------------------------
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These notes describe deadlock scenarios and proposed solutions for
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GStreamer. This will be implemented in the INCSCHED1 branch.
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I. Miscelaneous proposals
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II. Liveness problems (sometimes deadlock ;) and propsed solutions
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III. State transition approach and responsibility
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MattH.
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--------------------------------
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- I. Miscalenous proposals -
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--------------------------------
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1. Change the names of GstThread and GstQueue to GstPtThread and GstPtQueue
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for pthread versions of Thread and Queue.
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2. Change GstPtQueue to check its pads' peers' managers and make sure
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they are different. If not, fail and generate error message. (This
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ensures a GstPtQueue straddles a pthread boundary.)
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3. Change state transitions to NULL <-> READY <-> PAUSED <-> PLAYING.
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---------------------------------------------------
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- II. Deadlock Scenarios and Proposed Solutions -
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- (in the order they will be implemented) -
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---------------------------------------------------
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1. A downstream element "waits" for a buffer from its upstream element,
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a state change happens and "pauses" the upstream element -- the
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downstream element is blocked and cannot execute its change_state.
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Note that this can only happen within a single GstPtQueue! Either a
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downstream element calls Pull, finds no buffer, and does a
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wait_cond(new buffer) or an upstream element calls Push, finds no
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room, and does a wait_cond(new room). Thus, GstPtQueue contains all
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the cond_wait / signal code.
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=> The managing container (thread, pipeline) "wakes" up any sleep
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conditions of its "bottom half". (In the scenario described, it
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wakes the blocked downstream element's call to Pull.) The GstPtQueue
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cond_wait section determines that it woke up due to a pending state
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change and does a cothread_switch(0) to return to the main loop,
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which then executes the state transition.
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Note that a managing container will have only one sleep condition
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in its "bottom half."
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2. Element "blocked" on getting I/O and cannot execute its change_state.
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=> We will provide an I/O library for the elements to use that does
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not actually block. (A retry-loop with timeout or select() on
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2 -- or more -- file descriptors: one the one you want I/O from,
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the other one that GStreamer uses to "wake" everyone up.) The
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I/O library determines that it was woken due to a pending state
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change and does a cothread_switch(0) to return to the main loop,
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which then executes the state transition.
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Note that a managing container will have only one elements in
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the middle of doing blocking I/O.
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3. Element using a library (code out of its control) which blocks for
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some reason (e.g., using real blocking I/O) so main loop never gets
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run to execute change_state.
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=> Build in some timeout in the manging container (the "top half")
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when waiting for bottom half to respond to pending state. If
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managing container times out, kill the element's thread with a
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signal (or series of signals -- escalating priority). This
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requires that the element (the "bottom half") have matching
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signal handler(s) that execute(s) the state-transition.
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--------------------------------------------------------
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- III. State-transition Approach and Responsibility -
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--------------------------------------------------------
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A. The "top half" context of the managing container. (This is likely the
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context of the application.)
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Call change_state on the managing container (GstPipeline, GstPtThread).
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If its "bottom half" (main_loop) is asleep, signal the condition to
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wake it up. Then do a cond_wait for the "bottom half" to execute the
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state transition and return (once the state has been changed).
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B. The main_loop (the "bottom half") of the managing container.
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Needs to check for pending state transition after every switch back from
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one of its elements. If a pending state is found, it calls change_state
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on each of its elements, signals the "top half" that the state has been
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changed, then continues executing the plan (if Playing) or puts itself
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to sleep (Paused, Ready).
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C. Element.
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Implement a change_state function to make transition for that element.
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The elements' change_state is what actually changes the state variable
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and notifies the scheduler that the state was changed. This function
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may also do things like close or open resources.
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NOTE: when an element goes through certain state transitions (e.g., from
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Paused to Ready) its state (stack) will be wiped out. If it wants to
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preserve any state or data, it needs to store the information in a safe
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place.
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D. Cothread Scheduler.
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Gets notified of state transition by elements' change_state functions
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then (re)set the plan accordingly. Assuming
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NULL <-> READY <-> PAUSED <-> PLAYING, some would be
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+ Paused -> Playing: jump back where you were in the Plan and continue
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its execution
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+ Ready -> Paused: reset the cothread pointers foreach cothread in the
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Plan (don't run)
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