Design and Verification of Adaptive Cache Coherence Protocols ...

arbitrarily in an outgoing queue (thus any outgoing message can be brought to the front of the
queue). FIFO message passing, on the other hand, allows outgoing messages to be reordered
only when they have di erent destinations or addresses. It is worth emphasizing that FIFO
message passing does not guarantee the ordering for messages with di erent addresses, even
if they have the same source and destination. The rational behind is that memory cells of
di erent addresses may physically reside in di erent sites in DSM systems (that is, the home
`H' may represent more than one site in the implementation).
Non-FIFO: msg1 msg2 msg2 msg1
FIFO: msg1 msg2 msg2 msg1
if Dest(msg1) 6= Dest(msg2) _ Addr(msg1) 6= Addr(msg2)
The message passing rules are mandatory rules in the sense that each outgoing message
must be delivered to its destination in nite time. Unless otherwise speci ed, FIFO message
passing is assumed for all the protocols described in this thesis.
Bu er Management Ideally we would like to treat incoming queues as FIFOs and process
incoming messages in the order in which theyarereceived. However, this may cause deadlock
or livelock unless messages that cannot be processed temporarily are properly bu ered so that
following messages can still be processed. Conceptually, this can be modeled by allowing in-
coming messages to be reordered under appropriate conditions so that stalled messages do not
block other messages.
Di erent protocols have di erent bu ering requirements that can be speci ed by di erent
reordering conditions. For example, by allowing incoming messages with di erent sources or
addresses to be reordered arbitrarily, wecantreat each incoming queue as a set of FIFO sub-
queues. This prevents messages with di erent sources or addresses to block eachother.
Sub-queues for di erent sources or addresses:
msg1 msg2 msg2 msg1
if Src(msg1) 6=Src(msg2) _ Addr(msg1) 6=Addr(msg2)
Exposing bu er management at early design stages is inappropriate, since it could give rise
to a bloated set of rules and dramatically complicate the protocol veri cation. Instead, we rst
model bu er management via message reordering in incoming queues, and then determine what
must be done in practice in order to satisfy the reordering requirements. This strategy can lead
to more concise speci cations and simplify protocol design and veri cation.
We use outgoing queues to model FIFO and non-FIFO networks, and incoming queues
to model message bu ering at destinations. For outgoing queues, it is desirable to have less
stringent reordering conditions since this implies less requirements on message ordering that
must be guaranteed by the network. For incoming queues, it is desirable to have more stringent
reordering conditions since this implies less e ort for message bu ering.
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