Design and Verification of Adaptive Cache Coherence Protocols ...

7.1.1 Putting Things Together
It is easy to see that di erent addresses can employ di erent micro-protocols without any in-
terference. The primary objective of Cachet is to integrate the micro-protocols in a seamless
fashion in that di erent caches can use di erent micro-protocols on the same address simulta-
neously, and a cache can dynamically switch from one micro-protocol to another. For example,
when something is known about the access pattern for a speci c memory region, a cache can
employ an appropriate micro-protocol for that region.
Di erent micro-protocols have di erent implications on the execution of memory instruc-
tions. The micro-protocols form an access privilege hierarchy. The Migratory protocol has the
most privilege in that both commit and reconcile operations have no impact on the cache cell,
while the Base protocol has the least privilege in that both commit and reconcile operations
may require proper actions to be taken on the cache cell. The WP protocol has less privilege
than Migratory but more privilege than Base. It allows reconcile operations to complete in the
presence of a clean copy, but requires a dirty copy to be written back before commit operations
on that address can complete.
With appropriate handling, the Base protocol can coexist with either WP or Migratory on
the same address. The Base protocol requires that a dirty be written back to the memory on a
commit, and a clean copy be purged on a reconcile so that the subsequent load operation must
retrieve the data from the memory. This gives the memory an opportunity totake appropriate
actions whenever necessary, regardless of how the address is cached in other caches at the time.
In contrast, WP and Migratory cannot coexist with each other on the same address.
Since di erent micro-protocols have di erent treatment for Commit and Reconcile instruc-
tions, a cache must be able to tell which micro-protocol is in use for each cache cell. We can
annotate a cache state with a subscript to represent the operational micro-protocol: Cleanb
and Dirtyb are Base states, Cleanw and Dirtyw are WP states, and Cleanm and Dirtym are
Migratory states. The Cachet protocol draws no distinction between di erent micro-protocols
for an uncached address, or an address cached in a transient state. We can also use subscripts
to distinguish protocol messages whenever necessary. For example, the memory can supply a
Base, WP or Migratory copy to a cache via a Cacheb, Cachew or Cachem message. A cache
can write a dirty copy back to the memory via a Wbb or Wbw message, depending on whether
Base or WP is in use on the address.
7.1.2 Dynamic Micro-protocol Switch
The Cachet protocol provides inter-protocol adaptivity viadowngrade and upgrade operations.
A downgrade operation switches a cache cell to a less privileged micro-protocol, while an up-
grade operation switches a cache cell to a more privileged micro-protocol. Figure 7.2 shows
the cache state transitions caused by downgrade and upgrade operations (associated with each
transition is the corresponding protocol message that is generated or received at the cache site).
135