❝Knowledge points of this article Expired data concept Data deletion policy Eviction algorithm
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❝Kaka compiled a roadmap to create an interview guide, and prepared to write articles according to such a roadmap. Later, I found that I was adding knowledge points that were not supplemented. I also look forward to your partners joining in to help add some information. See you in the comments section!
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Let’s first look at the three key values, namely sex, name, and age.
The instructions for setting these three values are set name kaka setex age 100 24 setex sex 10 1
In redis we can use ttl to get a certain key Status, let’s use ttl to get the status of name, age, and sex respectively.
You can see three values, which are -1 775 -2
So what is the information given by these three values!
setex age 1000 24, expressed as the remaining validity time
The redis deletion strategy we are talking about today is the deleted data. Timed deletion is to write a timer and then use it as the key After the time expires, the timer task immediately deletes the expired key
Advantages: It is conceivable that the key will be deleted when it expires. It is definitely the most memory-friendly and saves memory
Disadvantages: The single-threaded feature of redis is that all commands are executed in a certain order. Deleting the key value when it expires will put more pressure on the CPU, which will directly affect the redis server response time and IO
Scheduled deletion means using time in exchange for space
After the scheduled deletion is completed, the data corresponding to the key value will be deleted, and the expired memory area will also be deleted directly. 
Looking at this picture, when the key value expires, it will not be deleted directly. When will it be deleted? Continue reading
When we use lazy deletion, the data will not be automatically deleted when it expires. Then its deletion method is to make a judgment the next time the key value is obtained. , determine whether the key has expired, and delete it if it expires.
That is to say, when get name is executed again, a function will be executed expirelfNeeded() This function is to determine whether the key has expired. Return nil when expired, and then delete it from the memory
Advantages: It will reduce a certain amount of CPU performance, and will only be deleted when it must be deleted
Disadvantages: That is definitely the case The memory pressure is high, such as some hot news. After the hot spot is over, almost no one accesses it. If no one accesses the key, it will always exist, which will occupy a certain amount of memory space for a long time.
In other words, this method It is to exchange space for time
In the above we mentioned two deletion methods, one is One is scheduled deletion, and the other is lazy deletion. One is to trade space for time. One is to trade time for space. Both options are relatively extreme approaches. Then let's take a look at the implementation plan for regular deletion.
Let’s first take a look at the storage space of redis. There are 16 by default. There is a configuration parameter database in redis.conf that is controlled by this parameter. Each database has its own expiration partition, which stores data address and data expiration time.
Implementation method
When redis starts, it will read the hz value under the server, which defaults to 10. This value can be viewed directly on the terminal by using info server
and then the server will be executed hz times per second for serverCron() polling
Continue to use databasesCron to access the information of the 16 redis libraries one by one.
When accessing, activeExpireCycel will be executed to detect each expires[*] one by one. The time of each execution is 250ms/server hz parameter
When detecting each expirs[*] one by one, ACTIVE_EXPIRE_CYCLE_SLOW_TIME_PERC will be randomly selected for detection
Now the problem comes, our 250ms/hz time has been executed, but the 16 databases of expires have not been cycled. How to do it! Which library will I cycle next time? In fact, this value exists, it is the value of current_db. This value will be recorded in activeExpireCycel next time you enter expires[*] for execution
Feature 1: There is no high peak value in CPU usage, and the detection frequency can be customized The memory will be continuously cleaned
Now that our redis uses insufficient memory, we will use the eviction policy to ensure the normal use of redis.
Redis will call freeMemorylfNeeded() before each command is executed to check whether the memory is sufficient. When it is insufficient, some keys will be cleared. This data clearing strategy is called an eviction strategy.
The parameter of the maximum usable memory of redis is: maxmemory. The default is 0. It refers to the proportion of physical memory occupied. Generally, setting it to 50% is enough.
Select the number of keys to be deleted each time: maxmemory-samples
Deletion policy: maxmemory-policy
「Let’s focus on the deletion strategy」
Look at a picture next
9s It is the current time
The longest key that is 9 seconds away is age
Use The key with the least frequency is gender
If you follow volatile-lru, age will be deleted
If you follow volatile-lfu, gender will be deleted
Detect all data
Give up data eviction
❝Persistence in learning, persistence in blogging, and persistence in sharing are the beliefs that Kaka has always upheld since his career. I hope that Kaka’s articles in the huge Internet can bring you a little Please help. See you in the next issue. ❞
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