Multithreaded programming, CPU is if you troubleshoot multi-threaded memory access problems

CPU modification of memory variables is to read the memory data into the CPU cache, and then by the CPU to do the operation, after the completion of the operation to continue to write to memory

In a single-core CPU, this is completely fine, but in multicore CPUs, each CPU core has its own independent cache

When you access the same memory address at the same time, the memory value is copied to the cache of multiple CPUs

At this point, if you modify the value in the cache, the data will be inconsistent, when written to memory, the data in memory will not reach the expected value

In order to solve this problem, the early CPU, the use of bus lock method, a CPU to memory operation, the bus lock, until the operation is completed and then unlock

The bus contains a lot of device control, lock will greatly reduce the speed of resource processing, so Intel proposed the MESI protocol

Intel has been introducing the MESI protocol since the Pentium, and many CPUs are now using variants of the Protocol

In Mesi, a cache is called row, a copy of the same memory address in different CPU caches , and their Row is the same

There are 4 states of row, and they are

State	Describe
M (Modified)	This line of data is valid, the data is modified, and the data in memory is inconsistent, the data only exists in this cache.
E (Exclusive)	This line of data is valid, the data is consistent with the data in memory, and the data exists only in this cache.
S (Shared)	This line of data is valid, the data is consistent with the data in memory, and the data exists in many caches.
I (Invalid)	This row of data is invalid.

Exclusive Monopoly

M Modify State

Each CPU listens to its own cache row while listening to the other CPU's cache row

Set to E (Exclusive) state when only one CPU has a memory copy

Set to the S (Shared) state when the second CPU reads a memory copy

Set to I (Invalid) state when other CPUs modify the memory copy

When the current CPU modifies the memory copy, it is set to the M (Modified) state

Each CPU Cache row has a status of its own

The migration process between the Mesi states is as follows:

Current status	Event	Behavior	Next state
I (Invalid)	Local Read	If the other cache does not have this data, this cache takes data from memory, the cache line status becomes E; If the other cache has this data, and the status is M, the data will be updated to memory, the cache and then fetch data from memory, the cache line status of 2 cache is changed to S; If the other cache has this data and the status is S or E, this cache takes data from memory and the cache line status of these caches becomes s	e/s
	Local Write	Fetch data from memory, modify it in cache, change status to M; If the other cache has this data and the status is M, the data should be updated to memory first; If the other cache has this data, the cache line status of the other cache becomes I	M
	Remote Read	Since it's invalid, the operation of the other nuclei has nothing to do with it.	I
	Remote Write	Since it's invalid, the operation of the other nuclei has nothing to do with it.	I
E (Exclusive)	Local Read	Fetch data from cache, state unchanged	E
	Local Write	Modify the data in the cache, the status becomes m	M
	Remote Read	Data and other cores are shared, the state becomes S	S
	Remote Write	The data is modified, this cache line can no longer be used, the status becomes I	I
S (Shared)	Local Read	Fetch data from cache, state unchanged	S
	Local Write	Modify the data in the cache, the status becomes M, Other kernel-shared cache line status becomes I	M
	Remote Read	The state does not change	S
	Remote Write	The data is modified, this cache line can no longer be used, the status becomes I	I
M (Modified)	Local Read	Fetch data from cache, state unchanged	M
	Local Write	Modify the data in the cache with the same status	M
	Remote Read	This line of data is written into memory so that other nuclear energy is used to the latest data, the state becomes S	S
	Remote Write	This line of data is written into memory so that other nuclear energy is used to the latest data, as other cores modify the line of data, The state becomes I	I

Multithreaded programming, CPU is if you troubleshoot multi-threaded memory access problems