Re: condition variables

On Thu, Aug 11, 2016 at 2:47 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> Another approach to the problem is to use a latch wait loop. That
> almost works. Interrupts can be serviced, and you can recheck shared
> memory to see whether the condition for proceeding is satisfied after
> each iteration of the loop. There's only one problem: when you do
> something that might cause the condition to be satisfied for other
> waiting backends, you need to set their latch - but you don't have an
> easy way to know exactly which processes are waiting, so how do you
> call SetLatch?

That's what I ended up doing with parallel CREATE INDEX. It worked,
but it would be nice to have a general purpose facility for waiting
for conditions to change.

> https://en.wikipedia.org/wiki/Monitor_(synchronization)#Condition_variables_2
>
> Basically, a condition variable has three operations: you can wait for
> the condition variable; you can signal the condition variable to wake
> up one waiter; or you can broadcast on the condition variable to wake
> up all waiters. Atomically with entering the wait, you must be able
> to check whether the condition is satisfied. 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();
>
> To wake up one waiter, another backend can call
> ConditionVariableSignal(cv); to wake up all waiters,
> ConditionVariableBroadcast(cv).

This seems convenient.

I notice that you acquire a spinlock within the implementation of
condition variables. Is it worth any effort to consolidate the number
of spinlock acquisitions? In other words, maybe the most common idioms
should be baked into the ConditionVariable interface, which could save
callers from having to use their own mutex variable.

--
Peter Geoghegan