Design and Verification of Adaptive Cache Coherence Protocols ...

Chapter 5
The Writer-Push Cache Coherence
Protocol
In Base, a reconcile operation on a clean cell forces the data to be purged from the cache before
the reconcile can complete. This may cause serious performance degradation for programs with
excessive use of reconcile operations. For example, this can happen for programs written under
release consistency that requires all addresses be indistinguishably reconciled after each acquire
operation. The Writer-Push (WP) protocol allows a reconcile operation on a clean cell to
complete without purging the data so that the data can be accessed by subsequent instructions
without causing a cache miss. It is ideal for programs with excessive use of reconcile operations,
for example, when some processors read a memory location many times using Reconcile and
Loadl instructions before the location is modi ed by another processor using Storel and Commit
instructions. A cache cell never needs to be purged from the cache unless the cache becomes
full or the memory location is modi ed by another processor.
The WP protocol ensures that if an address is cached in the Clean state, the cache cell
contains the same value as the memory cell. This is achieved by requiring that all clean copies
of an address be purged before the memory cell can be modi ed. As the name \Writer-Push"
suggests, the writer is responsible for informing potential readers to have their stale copies, if
any, purged in time. Therefore, a commit operation on a dirty cell can be a lengthy process
since it cannot complete before clean copies of the address are purged from all other caches.
Section 5.1 describes the system con guration of the WP protocol, which includes the
coherence states and protocol messages. We present the imperative rules of WP in Section 5.2
and the integrated rules of WP in Section 5.3. The soundness and liveness of WP are proved
in Sections 5.4 and 5.5, respectively. Section 5.6 demonstrates the use of voluntary rules via
an update protocol that is derived from WP. An alternative Writer-Push protocol is given in
Section 5.7.
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