L09_c_multiproc2

L09_c_multiproc2 - CS211 - Graduate Computer Architecture...

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CS211 - Fernandez - 1 CS211 - Graduate Computer Architecture Multiprocessor 2: Snooping Protocol, Directory Protocol, Synchronization, Consistency Reference Textbook: “Computer Architecture - A Quantitative Approach” by Patterson and Hennesy - Third edition The Bulk of These Notes are From The Textbook’s Resource Web Site and UCB’s CS152 and CS252 Adapted by Rolando A. Fernandez
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CS211 - Fernandez - 2 Review: Multiprocessor Basic issues and terminology Communication: share memory, message passing Parallel Application: Commercial workload: OLTP, DSS, Web index search Multiprogramming and OS Scientific/Technical Amdahl’s Law: Speedup <= 1 / Sequential_Frac Cache Coherence: serialization App Scaling compu - tation Scaling communi cation comp - to-comm FFT n log n/p n/p log n LU n/p n 1/2 /p 1/2 n 1/2 /p 1/2 Barnes n log n/p n 1/2 log n /p 1/2 n /p 1/2 Ocean 1/2 /p 1/2 n /p 1/2
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CS211 - Fernandez - 3 Potential HW Coherence Solutions Snooping Solution (Snoopy Bus): Send all requests for data to all processors Processors snoop to see if they have a copy and respond accordingly Requires broadcast, since caching information is at processors Works well with bus (natural broadcast medium) Dominates for small scale machines (most of the market) Directory-Based Schemes (discuss later) Keep track of what is being shared in 1 centralized place (logically) Distributed memory => distributed directory for scalability (avoids bottlenecks) Send point-to-point requests to processors via network Scales better than Snooping Actually existed BEFORE Snooping-based schemes
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CS211 - Fernandez - 4 Bus Snooping Topology Memory: centralized with uniform access time (“uma”) and bus interconnect Examples: Sun Enterprise 5000 , SGI Challenge, Intel System Pro
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CS211 - Fernandez - 5 Basic Snoopy Protocols (Review slides 5 to 21) Write Invalidate Protocol: Multiple readers, single writer Write to shared data: an invalidate is sent to all caches which snoop and invalidate any copies Read Miss: » Write-through: memory is always up-to-date » Write-back: snoop in caches to find most recent copy Write Broadcast Protocol (typically write through): Write to shared data: broadcast on bus, processors snoop, and update any copies Read miss: memory is always up-to-date Write serialization : bus serializes requests! Bus is single point of arbitration
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CS211 - Fernandez - 6 Basic Snoopy Protocols Write Invalidate versus Broadcast: Invalidate requires one transaction per write-run Invalidate uses spatial locality: one transaction per block Broadcast has lower latency between write and read
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CS211 - Fernandez - 7 An Example Snoopy Protocol Invalidation protocol, write-back cache Each block of memory is in one state: Clean in all caches and up-to-date in memory ( Shared ) OR Dirty in exactly one cache ( Exclusive ) OR Not in any caches Each cache block is in one state (track these): Shared : block can be read OR Exclusive
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L09_c_multiproc2 - CS211 - Graduate Computer Architecture...

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