At Mon, 21 Nov 2016 15:57:47 -0500, Robert Haas <robertmh...@gmail.com> wrote 
in <ca+tgmobfjwcfeiq8j+fvh5cdxhdvjffmemnlq8mqfesg2+4...@mail.gmail.com>
> On Thu, Aug 11, 2016 at 5:47 PM, Robert Haas <robertmh...@gmail.com> wrote:
> > So, in my
> > implementation, a condition variable wait loop looks like this:
> >
> > for (;;)
> > {
> >     ConditionVariablePrepareToSleep(cv);
> >     if (condition for which we are waiting is satisfied)
> >         break;
> >     ConditionVariableSleep();
> > }
> > ConditionVariableCancelSleep();
> I have what I think is a better idea.  Let's get rid of
> ConditionVariablePrepareToSleep(cv) and instead tell users of this
> facility to write the loop this way:
> for (;;)
> {
>     if (condition for which we are waiting is satisfied)
>         break;
>     ConditionVariableSleep(cv);
> }
> ConditionVariableCancelSleep();

It seems rather a common way to wait on a condition variable, in

| while (condition for which we are waiting is *not* satisfied)
|     ConditionVariableSleep(cv);
| ConditionVariableCancelSleep();

> ConditionVariableSleep(cv) will check whether the current process is
> already on the condition variable's waitlist.  If so, it will sleep;
> if not, it will add the process and return without sleeping.
> It may seem odd that ConditionVariableSleep(cv) doesn't necessary
> sleep, but this design has a significant advantage: we avoid
> manipulating the wait-list altogether in the case where the condition
> is already satisfied when we enter the loop.  That's more like what we

The condition check is done far faster than maintaining the
wait-list for most cases, I believe.

> already do in lwlock.c: we try to grab the lock first; if we can't, we
> add ourselves to the wait-list and retry; if we then get the lock
> after all we have to recheck whether we can get the lock and remove
> ourselves from the wait-list if so.  Of course, there is some cost: if
> we do have to wait, we'll end up checking the condition twice before
> actually going to sleep.  However, it's probably smart to bet that
> actually needing to sleep is fairly infrequent, just as in lwlock.c.
> Thoughts?

FWIW, I agree to the assumption.


Kyotaro Horiguchi
NTT Open Source Software Center

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