[PD-dev] [ pure-data-Patches-3322973 ] make clocks thread safe

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Mon Jun 27 15:20:39 CEST 2011


Patches item #3322973, was opened at 2011-06-20 11:59
Message generated for change (Settings changed) made by zmoelnig
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>Category: puredata-dev
Group: None
Status: Open
Resolution: None
Priority: 5
Private: No
Submitted By: IOhannes m zmölnig (zmoelnig)
Assigned to: Miller Puckette (millerpuckette)
Summary: make clocks thread safe

Initial Comment:
when writing threaded externals, i often find myself wanting Pd to call a callback from the main thread, depending on the "other" thread's state.

e.g. when doing asynchronous data acquisition, i want to notify Pd when new data has arrived, so it can be delivered in the main thread.

the obvious way is to use t_clock, and registers a new clock_delay() event from within the helper-thread; Pd will then callback in the next tick.
unfortunately the clock_set(),... methods are not thread safe, possibly leading to corruption of the clock_setlist when accessing them simultaneously.

one workaround is to use the global sys_lock(), but i find this to terribly slow down the entire Pd process, esp. when i have to call sys_lock() multiple times before ticks. that's most likely because sys_lock() locks about everything.
an alternative i used in the past, is to have a special (mutex-protected) flag, indicating whether i already have a clock running that is waiting to be called back from the main thread.

however, i find this quite complicated

therefore, i propose to make the clock_delay/clock_set/clock_unset methods threadsafe, by internally maintaining a separate mutex that solely protects the clock_setlist.



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Comment By: IOhannes m zmölnig (zmoelnig)
Date: 2011-06-20 13:24

Message:
just a few final words:

the problem i describe can well be solved with sys_lock()
however this creates a "Big Kernel Lock", potentially affecting
performance.

my proposal is to use finer grained locking, to avoid problems with the
BIG syslock.


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Comment By: IOhannes m zmölnig (zmoelnig)
Date: 2011-06-20 13:19

Message:
i have also written a small external that illustrates the problem.
find it in the "threadclock.tgz" archive, and compile it.

the object will create a number of threads (use the argument to specify
how many threads you want);
each thread will sleep for a short time (100ms or so), and then call
clock_delay() to wake up the object from the main thread.

sooner or later, this will crash Pd; using my path it won't

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Comment By: IOhannes m zmölnig (zmoelnig)
Date: 2011-06-20 12:05

Message:
attached is a first draft (against todays git master)

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