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buzzword [2015/04/10 16:37] kevincha |
buzzword [2015/04/10 16:38] kevincha [Lecture 28 (4/8 Wed.)] |
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===== Lecture 28 (4/8 Wed.) ===== | ===== Lecture 28 (4/8 Wed.) ===== | ||
+ | * Ordering of instructions | ||
+ | * Maintaining memory consistency when there are multiple threads and shared memory | ||
+ | * Need to ensure the semantic is not changed | ||
+ | * Making sure the shared data is properly locked when used | ||
+ | * Support mutual exclusion | ||
+ | * Ordering depends on when each processor is executed | ||
+ | * Debugging is also difficult (non-deterministic behavior) | ||
+ | * Dekker's algorithm | ||
+ | * Inconsistency -- the two processors did NOT see the same order of | ||
+ | operations to memory | ||
+ | * Sequential consistency | ||
+ | * Multiple correct global orders | ||
+ | * Two issues: | ||
+ | * Too conservative/strict | ||
+ | * Performance limiting | ||
+ | * Weak consistency: global ordering when sync | ||
+ | * programmer hints where the synchronizations are | ||
+ | * Memory fence | ||
+ | * More burden on the programmers | ||
+ | * Cache coherence | ||
+ | * Can be done in the software level or hardware level | ||
+ | * Snoop-based coherence | ||
+ | * A simple protocol with two states by broadcasting reads/writes on a bus | ||
+ | * Maintaining coherence | ||
+ | * Needs to provide 1) write propagation and 2) write serialization | ||
+ | * Update vs. Invalidate | ||
+ | * Two cache coherence methods | ||
+ | * Snoopy bus | ||
+ | * Bus based, single point of serialization | ||
+ | * More efficient with small number of processors | ||
+ | * Processors snoop other caches read/write requests to keep the cache block coherent | ||
+ | * Directory | ||
+ | * Single point of serialization per block | ||
+ | * Directory coordinates the coherency | ||
+ | * More scalable | ||
+ | * The directory keeps track of where the copies of each block resides | ||
+ | * Supplies data on a read | ||
+ | * Invalidates the block on a write | ||
+ | * Has an exclusive state |