On Saturday 06 October 2018 at 17:43:06 -0400, Daniel Eischen wrote:
> > On Oct 6, 2018, at 3:00 PM, Mike Crowe <theopengr...@mac.mcrowe.com> wrote:
> > 
> > Thank you for your detailed responses and patch links. I'm sorry it's taken
> > me so long to respond. I was waiting to see if there were any other 
> > responses.
> Aside from what may be needed by the implementation for the proposed C++
> standard functionality, I don't understand why initializing a mutex or
> condition variable (via the attribute) with the desired clock is not
> sufficient.  I can see adding an analogue to pthread_condattr_setclock()
> for a mutex attribute, but what is the real need to provide both
> real-time and monotonic sleeps on the same mutex or condition variable?
> Initialize them once with the clock you want, and be done with it.

For application code written to talk directly to the pthread functions, I
agree. But for a library, like the C++ standard library, that wishes to
make the clock to use part of the timeout then this is not possible.

If the choice of clock is distant from the wait (perhaps the condition
variable is passed into or out of library code) then there is a higher risk
of the wrong clock being accidentally used, and there is no way to detect
that automatically at compile time or run time[1]. Libraries, such as the
C++ library, completely avoid that class of bugs at compile time using
their type system. A C library could too, if it so wished, but it would
require separate functions to accept the different clock types.

> I'd almost rather see the timespec changed (controversial much?!) to add
> the clock id rather than keep adding new interfaces to support
> waits/sleeps on different clocks.

I think that if we were starting from scratch then that wouldn't be a bad
plan, and it would support the C++ library use case. I can't see it
happening now though.

> Or maybe to provide a hint to the implementation as to what clock to use
> on a per-thread basis via something like pthread_attr_setpreferredclock()
> and pthread_setpreferredclock().  If you had these, I suppose you could
> implement the C++ functionality by saving the current thread's preferred
> clock, setting it to the desired clock, calling the mutex lock or CV wait
> function, then restoring the thread's clock before returning to the
> caller.

That all seems like a lot of overhead of messing about with TLS just to
influence a single flag passed to the syscall, that may not even end up
needing to be called. I imagine that the overhead of passing a single extra
parameter to a function is tiny compared to that, especially when the
thread's preferred clock is not already in the data cache.



[1] Well, I suppose on platforms that have 64-bit time_t, a high bit could
always be set for CLOCK_MONOTONIC which would allow detection at run time.

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