Question

Difference between write and invalidate protocol

Answer 1

Hye.!!!

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"The performance differences between write update and write invalidate protocols arise from three characteristics: 1 Multiple writes to the same word with no intervening reads require multiple write broadcasts in an update protocol, but only one initial invalidation in a write invalidate protocol. 2 With multiword cache blocks, each word written in a cache block requires a write broadcast in an update protocol, although only the first write to any word in the block needs to generate an invalidate in an invalidation protocol. An invalidation protocol works on cache blocks, while an update protocol must work on individual words (or bytes, when bytes are written). It is possible to try to merge writes in a write broadcast scheme. 3 The delay between writing a word in one processor and reading the written value in another processor is usually less in a write update scheme, since the written data are immediately updated in the reader’s cache.

#riShu:-)

Answer 2

Bus snoop or bus sniffing is a program that tracks and/or snoops bus transactions by a coherence controller(snooper) in a cache and aims at preserving cache coherence in distributed memory systems. A snoopy cache is a cache containing a coherence controller (snooper).

Explanation:

• There are two types of snooping protocol (i) Write invalidate and (ii) Write-update
• Write Invalidation: The processor which 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. This can be done by giving the local machine an invalidation signal over the bus to search for a copy of an invalidated file by all other caches. Once the cache copies are invalidate , the data can be restored on your local machine until another processor needs them.
• Write Update: When a processor writes on a shared cache block, all the shared copies of the other caches are updated through bus snooping. This way all caches receive written data in the bus. It takes up more bus traffic than a protocol that is write invalidate. It is therefore uncommon to use this method. Protocols for dragons and fireflies are of that type