Event counts and sequencers

 

There are three operations available on an event count e and two on a sequencer s. These are:

read(e)
This returns the current value of the event count e. More strictly this returns some value of the event count between the start of this operation and its termination.
await(e,v)
This operation blocks the calling thread until the event count e reaches or exceeds the value v.
advance(e,n)
This operation increments the value of event count e by the amount n. This may cause other threads to become runnable.
read(s)
This returns the current value of the sequencer s. More strictly this returns some value of the sequencer between the start of this operation and its termination.
ticket(s)
This returns the current member of a monotonically increasing sequence and guarantees that any subsequent calls to either ticket or read will return a higher value.

In fact there is little difference between the underlying semantics of sequencers and event counts; the difference is that the ticket operation does not need to consider awaking threads, whereas the advance operation does (therefore it is wrong for a thread to await on a sequencer). The initial value for sequencers and event counts is zero; this may be altered immediately after creation using the above primitives. An additional operation await_until(e,v,t) is supported, which waits until event e has value v or until time has value t.

By convention an advance on an outbound event will cause the new value to be propagated by issuing the send() system call. Only read and await should be used on incoming events as their value may be overwritten at any time.

In this way, both local and interdomain synchronization can be achieved using the same interface and, unless required, a user level thread need not concern itself with the difference.