What is caching and what is his role

The CPU can access data very quickly, and can access and process one billion instructions and data per second (term: CPU master.

frequency 1G), and the memory is much slower, and the fast memory can reach tens of megabytes is good, which shows the speed difference between the two.

What a large cache is to solve the speed difference between CPU speed and memory speed.

The most frequently accessed data and instructions in memory are copied into the cache in the CPU, so that the CPU can come to the elephant less often.

The CPU only needs to go to the cache to fetch the data, and the speed of the cache is comparable.

The memory is much faster.

Cache, which can be divided into first-level cache and second-level cache (see **detailed explanation)Cache, which can be divided into first-level cache and second-level cache (see **detailed explanation)Hard disk cache (see **detailed explanation).

Caching (see **Details).

Caching is meant to solve the speed difference between CPU speed and memory speed. Nonsense.

The most frequently accessed data and instructions in memory are copied to the cache in the CPU, so that the CPU can not often go to the slow memory like a "snail" to retrieve data, the CPU only needs to go to the cache, and the cache speed is much faster than the memory.

In particular, it is important to note that:

1. Because the cache is only a copy of a small part of the data in the memory, when the CPU goes to the cache to find the data, it will also be unable to find the situation, at this time, the CPU will still go to the memory to find the data, so that the speed of the system will slow down, but the CPU will copy the data to the cache, so that the next time do not go to the memory to retrieve;

2. Because with the change of time, the most frequently accessed data is not static, that is to say, the data that is not frequent just now needs to be frequently accessed at this time, and the data that is still the most frequent just now is not frequent, so the data in the cache should be replaced frequently according to a certain algorithm, so as to ensure that the data in the cache is the most frequently accessed.

What the caching does:

1. Pre-reading.

When the hard disk is controlled by the CPU instruction to start reading data, the control chip on the hard disk will control the head to read the data in the next or several clusters of the cluster being read into the cache (because the data storage on the hard disk is relatively continuous, so the read hit rate is high), when it is necessary to read the data in the next or several clusters.

The hard disk does not need to read the data again, and can directly transfer the data in the cache to the memory, because the cache rate is much higher than the read and write rate of the head, so it can achieve the purpose of significantly improving the performance.

2. Write. When the hard disk receives the instruction to write data, it will not immediately write the data to the platter, but will temporarily store it in the cache, and then send a signal to the system that the data has been written, then the system will think that the data has been written and continue to perform the following work, while the hard disk will write the data in the cache to the platter when it is idle (when it is not reading or writing).

3. Temporary storage.

For example, the cache inside the hard disk (a type of scratchpad) will store some data that is read more frequently in the cache, and when it is read again, it can be directly transferred from the cache.