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08896533f2
I've found the cause of a long standing race condition in the parallel
runtime system. The fix is to properly design the engine state and
notification system in the parallel runtime system. I've worked on a design
and committed a description. I've started implementing the changes but found
a new different race condition. I don't currently have the time to complete
this change, therefore I will commit the documentation of the current
problem and proposed solution.
runtime/mercury_context.c:
As above, document the current race.
Also comment on a bug where an idle engine could be notified of a
context to execute. This notification could be dropped in some
situations causing the context to be lost.
runtime/notes/par_engine_state.txt:
runtime/notes/par_engine_state.dot:
As above, document the proposed new engine state and notification
design.
107 lines
2.4 KiB
Plaintext
107 lines
2.4 KiB
Plaintext
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// Copyright (C) The University of Melbourne 2012
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digraph G {
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// rankdir=LR;
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{ rank=source; startup };
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{ rank=same; context; context_advice; worksteal_advice };
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//{ rank=same; stealing; sleeping; };
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{ rank=sink; do_shutdown };
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{ rank=same; working; idle; stealing; };
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// { rank=same; working; idle; sleeping; };
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/*
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//subgraph cluster_working {
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//working;
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//idle;
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context_advice [label="context or worksteal advice" shape=rectangle];
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context_advice2 [label="context_advice" shape=rectangle];
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context [shape=rectangle];
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//worksteal [shape=rectangle];
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// label = "counted as working";
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//}
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*/
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/*
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subgraph cluster_idle {
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stealing;
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sleeping;
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soft_was_stealing [label="context or worksteal advice" shape=rectangle];
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label = "counted as idle";
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};
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*/
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// Styles for 'idle' states.
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sleeping [style=filled fillcolor=lightgrey];
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stealing [style=filled fillcolor=lightgrey];
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// Shapes for notifications
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shutdown [shape=rectangle];
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context [shape=rectangle];
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context_advice [shape=rectangle];
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worksteal_advice [shape=rectangle];
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// Illistrative only.
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worksteal_once [style=dashed];
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/*
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* Normal operation.
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*/
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// Primordial engine.
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startup -> working;
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// Worker engines.
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startup -> idle;
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// Finished work.
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working -> idle [style=bold];
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// Looking for work.
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idle -> working [style=bold];
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stealing -> working [style=bold];
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// Other transitions use CAS below.
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// Respond to notifications.
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shutdown -> do_shutdown;
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// We do the same action as idle does anyway.
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context_advice -> idle;
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worksteal_advice -> worksteal_once;
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worksteal_once -> working;
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worksteal_once -> stealing;
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context -> working;
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/* CAS Transitions */
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edge [color=blue];
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idle -> stealing [style=bold];
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stealing -> sleeping;
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/* Locked transitions from other engines */
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edge [color=red];
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sleeping -> context;
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sleeping -> context_advice;
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sleeping -> worksteal_advice;
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sleeping -> shutdown;
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/* CAS transitions from other engines */
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edge [color=green];
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idle -> context_advice;
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idle -> worksteal_advice;
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stealing -> context_advice;
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stealing -> worksteal_advice;
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// safe because there will be no other work
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idle -> shutdown;
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stealing -> shutdown;
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}
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