RE: [sv-ec] Strict FIFO ordering of mailboxes try_put/try_get

> Is there a big problem with the rule being that a try_get() 
> succeeds if and only if BOTH the mailbox queue size is
> greater than 0 AND the queue of processes suspended on
> a blocking get has a size of 0?

Either try_get() jumps the queue of waiters if it 
succeeds (Dave Rich's proposal), or it loses its 
place in the queue if it fails (Mark's proposal).  
Neither behaviour is likely to satisfy all users.

Consider a doughnut stall.  There is a FIFO queue
of customers, and FIFO delivery of doughnuts to
customers as they are prepared.  Now I arrive
at the doughnut stall.  If I see no queue of 
customers, and no doughnuts, I may choose to 
go away and return later (try_get()).  If I see
a queue of customers, I do not go away to return
later, as I thereby lose my place in the queue. 
Instead I put my friend in the queue as a proxy,
and occasionally take a look to see how far down 
the queue my friend has progressed.  When my
friend is served, he will stand nearby and I
can pick up my doughnut whenever I choose.  My
try_get() waits on the proxy (my friend) and 
not on the main queue of customers.

To implement this kind of mechanism we need some
additional information that's not available in
mailboxes right now.  The failing return value 
from try_get() needs to distinguish between 
"queue of customers" and "no doughnuts", so that 
the caller can make a sensible choice of whether 
to put its friend (proxy process) in the queue.
If we follow Mark's suggestion, we *nearly* have
this already - if num() is nonzero and try_get()
fails, then there is a queue of customers - but
obviously it would be better to have a single call
to test both things atomically.

Mutatis mutandis for try_put(), of course.
-- 
Jonathan Bromley, Consultant

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