Are you an EPFL student looking for a semester project?
Work with us on data science and visualisation projects, and deploy your project as an app on top of Graph Search.
Concept
Bus snooping
Summary
Bus snooping or bus sniffing is a scheme by which a coherency controller (snooper) in a cache (a snoopy cache) monitors or snoops the bus transactions, and its goal is to maintain a cache coherency in distributed shared memory systems. This scheme was introduced by Ravishankar and Goodman in 1983, under the name "write-once" cache coherency. A cache containing a coherency controller (snooper) is called a snoopy cache.
When specific data is shared by several caches and a processor modifies the value of the shared data, the change must be propagated to all the other caches which have a copy of the data. This change propagation prevents the system from violating cache coherency. The notification of data change can be done by bus snooping. All the snoopers monitor every transaction on a bus. If a transaction modifying a shared cache block appears on a bus, all the snoopers check whether their caches have the same copy of the shared block. If a cache has a copy of the shared block, the corresponding snooper performs an action to ensure cache coherency. The action can be a flush or an invalidation of the cache block. It also involves a change of cache block state depending on the cache coherence protocol.
There are two kinds of snooping protocols depending on the way to manage a local copy of a write operation:
When a processor writes on a shared cache block, all the shared copies in the other caches are invalidated through bus snooping. This method ensures that only one copy of a datum can be exclusively read and written by a processor. All the other copies in other caches are invalidated. This is the most commonly used snooping protocol. MSI, MESI, MOSI, MOESI, and MESIF protocols belong to this category.
When a processor writes on a shared cache block, all the shared copies of the other caches are updated through bus snooping. This method broadcasts a write data to all caches throughout a bus. It incurs larger bus traffic than write-invalidate protocol. That is why this method is uncommon. Dragon and firefly protocols belong to this category.
Official source
About this result
This page is automatically generated and may contain information that is not correct, complete, up-to-date, or relevant to your search query. The same applies to every other page on this website. Please make sure to verify the information with EPFL's official sources.