Interviewer: do you understand redis' shared object pool?

I'm waiting anxiously in the interview room , Suddenly I heard footsteps outside the door , The door is then opened , The young man with clean clothes and beautiful face came in , A faint fragrance of men's perfume comes to us. .

interviewer ：“ Usually used in work Redis Do you ？”

I ：“ It's used a lot .” I'm happy in my heart ,Redis I'm familiar with it , What are the five data types 、 The two persistence methods are back to back .

interviewer ：“Redis Shared object pool, see ？”

“ This .. Not much in-depth understanding .” I hesitated , The palms of my hands are already sweating .

interviewer ：“ Go back and wait for the news .”

This sentence is clear , And then there's no then . Failure is the mother of success , I'm not discouraged , Decided to mend it right away .

Shared object pool

Creating a large number of duplicate integer types is bound to consume a lot of memory , So in Redis Internally maintained a from 0 To 9999 Integer object pool for , This is it. Shared object pool .

To verify and understand , We use object refcount Command to check the number of object references , The effect is as follows ：

127.0.0.1:6379> set one-more-num1 404OK127.0.0.1:6379> object refcount one-more-num1(integer) 2172.24.130.22:6379> set one-more-num2 404OK127.0.0.1:6379> object refcount one-more-num2(integer) 3

Set up one-more-num1 by 404 after , Directly use integer objects in the shared pool , So the number of references is 2（ Another reference is on the object pool ）; To set up one-more-num2 by 404 after , The number of references becomes 3.

But here's the thing ： When setting the maximum memory value （maxmemory） And enable LRU Relevant elimination strategies （ Such as ：volatile-lru、allkeys-lru） when , Shared object pools will be disabled .

Why is there no string object pool ？

A key operation in the shared object pool is to determine whether objects are equal .

Redis There are only integer type object pools in the , Because the time complexity of integer comparison algorithm is O(1), Only kept 10000 An integer to prevent excessive waste of the object pool .

Relatively speaking , The time complexity of string comparison algorithm is O(n), In particular, the comparison of long strings is more performance consuming .

and , There is a high probability that integer types will be reused , The probability that a string will be reused is much lower than , So in Redis Only integer type objects are shared in the pool .

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Interviewer, you'll see , You don't care about me today , Tomorrow I'll keep you up , Ha ha ha ...

reference ：
《Redis Design and implementation 》
《Redis Development and operations 》
《Redis Deep Adventure ： Core principles and application practice 》