Redis deletion policy and eviction policy

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Knowledge points of this article Expired data concept Data deletion policy Eviction algorithm

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Preface

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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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Expired data

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!

• -1 represents permanently valid data
• 775 This value is set on age, and the command used is setex age 1000 24, expressed as the remaining validity time
• -2 indicates expired data, or deleted data, or data that has no definition

Storage structure of expired data

• When we set a time-sensitive When the name is specified, redis stores a memory address 0x10101
• and then redis will open up another space to store the time-sensitive key
• But the storage method is the memory address corresponding to the key and the expiration timeThe redis deletion strategy we are talking about today is the deleted data.

Timed deletion

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.

Lazy deletion

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

Regular deletion

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

• If the key times out, delete the key directly
• The number of keys deleted in one round>ACTIVE_EXPIRE_CYCLE_SLOW_TIME_PERC*25% continues to cycle the process
• If the number of keys deleted in one round 25%, check the next expires[]

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 executionFeature 1: There is no high peak value in CPU usage, and the detection frequency can be customized The memory will be continuously cleaned

Eviction algorithmIn the above we talked about three deletion strategies , but these three strategies are only effective relative to the key with a validity period set.

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」

• Easy to detect Lost data (that is, we have set the validity period, but the data has not yet expired, which is expires[*] above)

  • volatile-lru: Select the data that is rarely used recently
  • volatile-lfu: Select the data that has been used the least recently
  • volatile-ttl: Select the data that will be eliminated
  • volatile-random: randomly select

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

  • allkeys-lru: Select data that has been used less recently
  • allkeys-lfu: Select the data with the least recent usage
  • allkeys-random: Randomly delete the entire database

• Give up data eviction

  • no-enviction: The data will not be deleted until the memory is used up, causing an oom error.

##Summary

The above is the redis pair Data deletion policy and eviction policy.

One thing to note here is that in the eviction policy, volatile data refers to the key with an expiration time set, not a permanent value.

The whole database refers to all values, with expiration time and permanent data set.

❝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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