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means that the snoop filter 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 three types of filters depending on the location of the snoop filters. One is a source filter that is located at a cache side and performs filtering before coherence traffic reaches the shared bus. Another is a destination filter that is located at receiver caches and prevents unnecessary cache-tag look-ups at the receiver core, but this type of filtering fails to prevent the initial coherence message from the source. Lastly, in-network filters prune coherence traffic dynamically inside the shared bus. The snoop filter is also categorized as inclusive and exclusive. 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. On the other hand, a hit in the exclusive snoop filter means that no cache has the requested cache block.
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63:. 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
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The caching logic monitors the bus and detects if any cached memory is requested. If the cache is dirty and shared and there is a request on the bus for that memory, a dirty snooping element will supply the data to the requester. At that point either the requester can take on responsibility for the
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to check whether it has the same cache block. In most cases, the caches do not have the cache block since a well optimized parallel program doesn’t share much data among threads. Thus the cache tag lookup by the snooper is usually an unnecessary work for the cache who does not have the cache block.
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One way to reduce the unnecessary snooping is to use a snoop filter. A snoop filter determines whether a snooper needs to check its cache tag or not. A snoop filter is a directory-based structure and monitors all coherent traffic in order to keep track of the coherency states of cache blocks. It
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On a read miss to the local cache, the read request is broadcast on the bus. All cache controllers monitor the bus. If one has cached that address and it is in the state "dirty", it changes the state to "valid" and sends the copy to requesting node. The "valid" state means that the cache line is
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Each cache line is in one of the following states: "dirty" (has been updated by local processor), "valid", "invalid" or "shared". A cache line contains a value, and it can be read or written. Writing on a cache line changes the value. Each value is either in main memory (which is very slow to
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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.
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When invalidating an address marked as dirty (i.e. one cache would have a dirty address and the other cache is writing) then the cache will ignore that request. The new cache will be marked as dirty, valid and exclusive and that cache will now take responsibility for the address.
285:. Frequent snooping on a cache causes a race with an access from a processor, thus it can increase cache access time and power consumption. 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
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current. On a local write miss (an attempt to write that value is made, but it's not in the cache), bus snooping ensures that any copies in other caches are set to "invalid". "Invalid" means that a copy used to exist in the cache, but it is no longer current.
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When specific data are 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
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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.
50:. 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.
269:. The data being shared is placed in a common directory that maintains the coherence between caches in a directory-based system. Bus snooping is normally faster if there is enough
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data (marking the data as dirty), or memory can grab a copy (the memory is said to have "snarfed" the data) and the two elements go to the shared state.
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When a bus transaction occurs to a specific cache block, all snoopers must snoop the bus transaction. Then the snoopers look up their corresponding
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Tag | ID | V | D | S --------------------- 1111 | 00 | 1 | 1 | 0 0000 | 01 | 0 | 0 | 0 0000 | 10 | 1 | 0 | 1 0000 | 11 | 0 | 0 | 0
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Tag | ID | V | D | S --------------------- 1111 | 00 | 1 | 0 | 0 0000 | 01 | 0 | 0 | 0 0000 | 10 | 1 | 0 | 1 0000 | 11 | 0 | 0 | 0
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access), or in one or more local caches (which is fast). When a block is first loaded into the cache, it is marked as "valid".
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of the cache block. It also involves a change of cache block state depending on the cache coherence protocol.
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it provides must grow, as the system becomes larger. Since the bus snooping does not scale well, larger
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Proceedings of the 16th annual international symposium on
Computer architecture - ISCA '89
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376:"Evaluating the performance of four snooping cache coherency protocols"
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Design of a Snoop Filter for Snoop-Based Cache
Coherency Protocols
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The advantage of using bus snooping is that it is faster than
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After a write of address 1111 00, it would change into this:
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is a scheme by which a coherency controller (snooper) in a
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Ravishankar, Chinya; Goodman, James (February 28, 1983).
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For example, an initial state might look like this:
199:One of the possible implementations is as follows:
483:Agarwal, N.; Peh, L.; Jha, N. K. (December 2009).
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542:. Norwegian University of Science and Technology.
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337:Cache Implementation for Multiple Microprocessors
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440:. Morgan Kaufmann Publishers. pp. 469–471.
436:Patterson, David A.; Hennessy, John L. (1990).
413:Computer Architecture: A Quantitative Approach
411:Hennessy, John L; Patterson, David A. (2011).
361:Fundamentals of Parallel Computer Architecture
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438:Computer Architecture A Quantitative Approach
281:The disadvantage of bus snooping is limited
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