lec25 - ² Also maintains the “status” of a block...

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LECTURE - 25
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Topic for Today's Lecture ± Cache coherence protocols ± ± Scribe for today?
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Cache Coherence ± In what kind of multi-processors do we need cache coherence? ± What are the kinds of data which are cached? ² Shared (read) data – repl ication ² Private data – migr ation
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Notions of Coherence and Consistency ± Coherence: ² Program order preservation within a processor ² Write by P1, Read by P2, if “sufficiently separated”, should get the value of write ² Write serialization: same order of writes seen by all processors ± Specifying when a read should get the value of a write: memory consistency model
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Styles of Coherence Protocols ± Directory-based: central directory maintains the “s tatus” o f each block ± Snooping-based: ² In a centralized shared memory machine ² Each processor snoops on the common bus
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Unformatted text preview: ² Also maintains the “status” of a block locally (no central directory) ² Snooping helps maintain coherence Styles of Snooping Protocols ± Write-invalidate: processor makes sure that it has the only copy of a block before writing ² Invalidates other copies by sending an invalidate command on the bus ± Write-update or write-broadcast: processor updates all copies of a block when it writes ² Send the written data on the common bus Write-invalidate vs. Write-update Write-invalidate Consecutive writes to a location does not cause repeated traffic on bus Writes to consecutive locations does not cause extra traffic on bus Write-update Writes appear for readers with lesser latency...
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This note was uploaded on 07/14/2011 for the course CS 422 taught by Professor Hogakoi during the Spring '10 term at IIT Kanpur.

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lec25 - ² Also maintains the “status” of a block...

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