Jump to Types of snooping protocols - Bus snooping. From Wikipedia, the free encyclopedia. Bus snooping or bus sniffing is a scheme that a coherency controller (snooper) in a cache monitors or snoops the bus transactions, and its goal is to maintain a cache coherency in distributed shared memory systems.How it works · Implementation · Drawback. Snooping. First introduced in , snooping is a process where the individual caches monitor address lines for accesses to memory locations that they have cached. The write-invalidate protocols and write-update protocols make use of this mechanism. Snooping protocol ensures memory cache coherency in symmetric multiprocessing (SMP) systems. Each processor cache on a bus monitors, or snoops, the bus.
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Each of the requests has to be broadcast to all nodes in a system. It means that the size of the physical or logical bus and the bandwidth it provides must grow, as the system becomes larger. Snoop filter[ edit ] When a bus transaction occurs to a specific cache block, all snoopers must snoop snooping protocol bus transaction.
Then the snoopers look up their corresponding cache tag to check whether it has the same cache snooping protocol. Thus the cache tag lookup by the snooper is usually an unnecessary work for the cache who does not have the cache block. But the tag lookup disturbs the cache access by a processor and incurs additional power consumption.
One way to reduce the unnecessary snooping is to use a snoop filter. A snoop filter determines whether a snooper needs to check snooping protocol cache tag or not. A snoop filter is based on a directory based structure and monitors all coherent traffics in order to keep track of the coherency states of cache blocks.
It means that the snoop snooping protocol knows the caches that have a copy of a cache block. Thus it can prevent the caches that do not have the copy of a cache block from making the unnecessary snooping.
There are two types of filters depending on the location of the snoop filter. One is a source filter that is located at a cache side and performs filtering before coherent traffics reach the snooping protocol bus. The other is a destination filter that is located at a bus side and blocks unnecessary coherent traffics going out from the shared bus.
The snoop filter is also categorized as inclusive snooping protocol exclusive.
What is snooping protocol? Webopedia Definition
The inclusive snoop filter keeps track of the presence of cache blocks in caches. However, the exclusive snoop filter monitors the absence of cache blocks in caches. In other words, a hit in the inclusive snoop filter means that the corresponding cache block is held by caches.
Snooping Protocol Types There are two main types of snooping protocol protocol: Write-invalidate The processor that is writing data causes copies in the caches of all other processors in the system to be rendered invalid before it changes its local copy.
The local machine does this by sending an invalidation signal over the bus, which causes all of the other caches to check for a copy of the invalidated file. For the snooping mechanism, a snoop filter reduces the snooping traffic by maintaining a plurality of entries, each representing a cache line that may be owned by one or more nodes.
When replacement of one of the entries is required, the snoop filter selects for the replacement the entry representing the cache line or lines owned snooping protocol the fewest nodes, as determined from a presence vector in each of the entries.
A temporal or other type of algorithm is used to refine the selection if more than one cache line is owned by the fewest number of nodes.
Bus snooping - Wikipedia
Directory-based cache coherence In a directory-based system, the data being shared is placed in a common directory that maintains the coherence between caches. The directory acts as a filter through which snooping protocol processor must ask permission to load an entry from the primary snooping protocol to its cache.
When an entry is changed, snooping protocol directory either updates snooping protocol invalidates the other caches with that entry.
Distributed shared memory systems mimic these mechanisms in an attempt to maintain consistency between blocks of memory in loosely coupled systems.
The intention is that two clients must never see different values for the same shared data. The protocol must implement the snooping protocol requirements for coherence.
It can be tailor-made for the target system or application.
Protocols can also be classified as snoopy or directory-based. Typically, early systems used directory-based protocols where a directory would keep a track of the data being shared and the sharers. In snoopy snooping protocol, the transaction requests to snooping protocol, write, or upgrade are sent out to all processors.