6/28/2023 0 Comments Direct mapped vs set associativeMESI protocol Fig.4 - State transitions in MESI protocol.Įvery cache line is marked with one of the four following states (coded in two bits): Further, MOSI adds an "Owned" state to reduce the traffic caused by write-backs of blocks that are read by other caches. MESI addes an "Exclusive" state to reduce the traffic caused by writes of blocks that only in one cache (a silent write in MESI). achieves benefits of both MESI and MOSI.MOSI solution: adds O state (owner), indicating that the current core owns this block, and will service requests from other cores for the block.MSI observation: on M→S transitions, must write back line.MESI solution: adds E state (exclusive, clean), writes on such lines happen silently (don't tell other caches to invalidate the line), transition to M (exclusive, dirty).MSI observation: doing read-modify-write sequences on private data is common, and hence the traffic can be reduced for writes of block on only one cache.writes to the same location are serialized.writes eventually become visible to all processors.Cache coherence is to ensure that the changes in the values of shared operands are propagated throughout the system in a timely fashion. In a shared memory multiprocessor system, an operand can have multiple copies in main memory and in caches. Accordingly, cache and TLB accesses can begin simultaneously, and tag comparion is made after both accesses are completed. Page offset bits are not translated and thus can be presented to the cache immediately. We must flush the cache on a context switch to avoid "aliasing". If we translate before we go to the cache, we have a " physical cache" which works on physical addr.Ĭritical path = TLB access time + cache access timeĪlternatively, we could translate after the cache (only for cache misses), we have a " virtual cache". TLBs are small (maybe 64 entries), fully-associative caches for page table entries. Write-through is lower but cleaner (memory always consistent), write-back is faster but complicated when multi cores sharing memory, requiring cache coherency protocol. Evictions of a dirty cacheline cause a write to memory. Any writes to memory need to be the entire cacheline since no way to distinguish which word was dirty with only a single dirty bit. Evictions do not need to write to memory.Ī cache with a write-back policy (and write-allocate) reads an entire block (cacheline) from memory on a cache miss, may need to write dirty cacheline first. No-write-allocate: write it directly to memory without allocationĪ cache with a write-through policy (and write-allocate) read an entire block (cacheline) from memory on a cache miss and writes only the updated item to memory for a store.Write-allocate: allocate a cache line (put it in cache) for new data (and maybe write-through).Write-back: CPU writes only to cache cache writes to main memory when the dirty block is later evicted.Write-through: writes go to main memory and cache.No-write: writes invalidate the cache and go directly to memory.Cache write policies ↑top if data is already in the cache. when evicting a line: if D=0 (memory data is NOT stale), just set V=0 if D=1 (memory data is stale), write-back the data and then set D=0 and V=0.ģ.when writing a line in response to write hit, set D=1.when allocating line, set V=1, D=0 (clean) and fill in tag and data.
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