[Tcl-bugs] [ tcl-Bugs-3328635 ] Virtual Time broken on threaded unix

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Bugs item #3328635, was opened at 2011-06-24 22:37
Message generated for change (Comment added) made by ferrieux
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Category: 03. Timer Events
Group: None
Status: Open
Resolution: None
Priority: 7
Private: No
Submitted By: Alexandre Ferrieux (ferrieux)
Assigned to: Andreas Kupries (andreas_kupries)
Summary: Virtual Time broken on threaded unix

Initial Comment:
In threaded unix builds, timer events use pthread_cond_timedwait, which needs an absolute target time. The computation of this target is broken, because it adds the (correct) scaled==real delta to the (incorrect) virtual Tcl_GetTime. The fix is to use real time for the latter.

Moreover, in some unices, the clock used for pthread_cond_timedwait can be selected among several, with pthread_condattr_setclock at condvar creation time. One of them, CLOCK_MONOTONIC, has the extra advantage of being insensitive to system clock tweaks. I think that selecting it when possible can only improve things, since it allows:

     - usual, tip-302-sensitive [after] semantics unchanged, since the timer events are also stored as absolute epoch-time  targets by Tcl
     - fully working Virtual Time with clock_gettime(CLOCK_MONOTONIC) giving tip-302-insensitive [after].

I have a provisional patch for platforms with clock_gettime+CLOCK_MONOTONIC, designed to enable Joe Keen's code in http://groups.google.com/group/comp.lang.tcl/browse_frm/thread/8f820c68143850f9# . Some autogoo would be due before committing.

An alternative to 'select monotonic if possible, epoch if not' would be a runtime setting, like a ::tcl::unsupported::monotonicTime linked var. To be discussed.

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>Comment By: Alexandre Ferrieux (ferrieux)
Date: 2011-06-24 22:50

Message:
Actually, if we choose the explicit selection approach, the toggle should
rather be accessible at C level, by the extensions able to benefit from it:
Tcl_SetClockKind( 0==normal, 1==monotonic). This way, an extension can
enable the tweak-unsensitive [after] in a completely self-contained way. If
you like this method I'll write the patch shortly.

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