Java CAS MESI 缓存一致性协议【最权威】

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MESI是缓存一致性协议，分别代表四种不同的状态。

Modified (M)

The cache line is present only in the current cache, and is dirty - it has been modified (M state) from the value in main memory. The cache is required to write the data back to main memory at some time in the future, before permitting any other read of the (no longer valid) main memory state. The write-back changes the line to the Shared state(S).

Exclusive (E)

The cache line is present only in the current cache, but is clean - it matches main memory. It may be changed to the Shared state at any time, in response to a read request. Alternatively, it may be changed to the Modified state when writing to it.

Shared (S)

Indicates that this cache line may be stored in other caches of the machine and is clean - it matches the main memory. The line may be discarded (changed to the Invalid state) at any time.

Invalid (I)

Indicates that this cache line is invalid (unused).

一下描述的3个CPU情况下，缓存一致性协议的工作流程

Step 1: As the cache is initially empty, so the main memory provides P1 with the block and it becomes exclusive state.

Step 2: As the block is already present in the cache and in an exclusive state so it directly modifies that without any bus instruction. The block is now in a modified state.

Step 3: In this step, a BusRd is posted on the bus and the snooper on P1 senses this. It then flushes the data and changes its state to shared. The block on P3 also changes its state to shared as it has received data from another cache. There is no main memory access here.

Step 4: Here a BusUpgr is posted on the bus and the snooper on P1 senses this and invalidates the block as it is going to be modified by another cache. P3 then changes its block state to modified.

Step 5: As the current state is invalid, thus it will post a BusRd on the bus. The snooper at P3 will sense this and so will flush the data out. The state of the both the blocks on P1 and P3 will become shared now. Notice that this is when even the main memory will be updated with the previously modified data.

Step 6: There is a hit in the cache and it is in the shared state so no bus request is made here.

Step 7: There is cache miss on P2 and a BusRd is posted. The snooper on P1 and P3 sense this and both will attempt a flush. Whichever gets access of the bus first will do that operation.

参考资料

https://en.wikipedia.org/wiki/MESI_protocol

Biased Locking in HotSpot [并发大神的博客]

https://blogs.oracle.com/dave/biased-locking-in-hotspot