How to deal with Redis memory fragmentation?

Redis memory fragmentation refers to the existence of small free areas in the allocated memory that cannot be reassigned. Coping strategies include: Restart Redis: completely clear the memory, but interrupt service. Optimize data structures: Use a structure that is more suitable for Redis to reduce the number of memory allocations and releases. Adjust configuration parameters: Use the policy to eliminate the least recently used key-value pairs. Use persistence mechanism: Back up data regularly and restart Redis to clean up fragments. Monitor memory usage: Discover problems in a timely manner and take measures.

Redis memory fragments, this thing has troubled many people, and so do I. Simply put, some places are used in the memory allocated by Redis, and some are empty, but these empty areas are too small to be allocated to new data structures, wasting space. It's like your refrigerator, stuffed with all kinds of leftover pieces. Although the refrigerator has a lot of space, there is no place to put a big watermelon in it.

In this article, let’s talk about how to deal with this annoying memory fragment. After reading it, you will understand the causes of Redis memory fragmentation and several coping strategies to avoid falling into the pits I have stepped on.

First, you have to figure out how Redis manages memory. It uses jemalloc, a very efficient memory allocator. jemalloc will divide memory into different blocks with different sizes to meet the needs of different data structures. But no matter how awesome jemalloc is, it cannot avoid the creation of fragments. Frequent memory allocation and release, especially frequent operations of small objects, are most likely to cause fragmentation. Think about it, you keep stuffing snacks into the refrigerator, take out some, and then stuff them in, and finally the refrigerator is full of fragments.

So, how to solve it? The most direct way is to restart Redis. It's like completely emptying the refrigerator and reorganizing it. But this is certainly not the best solution, because restarting will interrupt services and cause the risk of data loss.

Another idea is to optimize your data structure and usage. If your data structure is relatively small, frequent additions, deletions, modifications and checks will lead to a large number of small fragments. At this time, you can consider using some data structures that are more suitable for Redis, such as Hash instead of List, to reduce the number of memory allocation and release times. It's like you switch to some more reasonable storage boxes to organize the pieces in the refrigerator.

Another method is to adjust the configuration parameters of Redis. maxmemory-policy parameter controls Redis's policy when there is insufficient memory. You can choose different strategies, such as allkeys-lru , to let Redis eliminate some of the least recently used key-value pairs. This is equivalent to cleaning the refrigerator regularly, throwing away some of the near-expiration food, and making room. But it should be noted that choosing the right strategy requires decisions based on your actual application scenario. Choosing the wrong one may lead to the loss of important data.

Going further, you can consider using Redis's persistence mechanism, such as RDB or AOF. Back up data regularly, then restart Redis to clean up memory fragmentation. It's like taking out everything in the refrigerator regularly, wiping the refrigerator clean, and putting the important things back. While this can solve the problem, the backup and recovery process can be time and resources.

Finally, I would like to remind you that monitoring Redis's memory usage is very important. You can use Redis's own monitoring commands or some third-party monitoring tools to promptly discover memory fragmentation problems and take corresponding measures. It's like you often check the refrigerator to see if it's too messy and clean it up in time.

In short, there is no one-time solution to deal with Redis memory fragmentation, and you need to choose the appropriate strategy based on the actual situation. Remember that prevention is better than treatment. Only by having good programming habits and reasonable configuration parameters can we fundamentally reduce the generation of memory fragmentation. Don't forget that the code should be written elegantly and the annotations should be written clearly, so that it will be easier for you to find solutions when facing problems. My code style is more concise and efficient. Here is a simple example showing how to use Lua scripts to perform batch deletion operations to reduce fragmentation:

 <code class="lua">local keys = redis.call('KEYS', 'myprefix:*') for i, key in ipairs(keys) do redis.call('DEL', key) end</code>

This is just a simple example, and more complex logic may be required in practical applications. Remember, choosing the right tools and methods can better meet challenges.

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