Chat MySQL memory management, memory allocator, and operating systems

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When users encounter memory problems using any software (including MySQL), Our first reaction is a memory leak. As this article shows, that’s not always the case.

This article explains a bug about memory.

All Percona Support customers are eligible for bug fixes, but their options vary. For example, Advanced customers are offered a HotFix build prior to the public release of software with the patch. Premium customers do not even have to use Percona software: we may port our patches to upstream for them. But for Percona products all Support levels have the right to have a fix.

all percona supported customers All are eligible for bug fixes, but they also have different options. For example, VIP customers can get the hotfiix version before the software patch is officially released. Premium customers don't even need to use Percona's software. We can also push the patch upstream for them. But for Percona products, all support levels are entitled to bug fixes.

Even so, this does not mean we will fix every unexpected behavior, even if we accept that behavior to be a valid bug. One of the reasons for such a decision might be that while the behavior is clearly wrong for Percona products, this is still a feature request.

Even so, this does not mean that we will fix all unexpected situations, even if we accept this situation as a valid bug. One of the reasons for making such a decision may be that although this unexpected situation is clearly wrong, it is indeed a product requirement for the percona product itself

as a bug

A good recent example of such a case is PS-5312 – the bug is repeatable with upstream and reported at bugs.mysql.com/95065

A good recent example of such a case is PS-5312— —This bug can be reproduced upstream and documented at bugs.mysql.com/95065.

This reports a situation whereby access to InnoDB fulltext indexes leads to growth in memory usage. It starts when someone queries a fulltext index, grows until a maximum, and is not freed for quite a long time.

This report describes a situation that causes memory usage to increase when accessing InnoDB's full-text index. This situation occurs in some full-text index queries. The memory will continue to grow until it reaches the maximum value and will not be released for a long time.

Yura Sorokin from the Percona Engineering Team investigated if this is a memory leak and found that it is not.

Yura Sorokin from the Percona Engineering Team investigated if this is a memory leak and found that it is not. Leakage scope.

When InnoDB resolves a fulltext query, it creates a memory heap in the function fts_query_phrase_search This heap may grow up to 80MB. Additionally, it has a big number of blocks ( mem_block_t ) which are not always used continuously and this , in turn, leads to memory fragmentation.

When InnoDB parses a full-text query, it creates a memory heap in the fts_query_phrase_search function, which may grow to 80M. In addition, this process will also use a large number of non-contiguous blocks (mem_block_t), resulting in memory fragmentation.

In the function exit , the memory heap is freed. InnoDB does this for each of the allocated blocks. At the end of the function, it calls free() which belongs to one of the memory allocator libraries, such as malloc or jemalloc. From the mysqld point of view, everything is done correctly: there is no memory leak.

At function exit, these memory heaps will be released. InnoDB does this for every block it allocates. At the end of function execution, call a free() operation in a memory allocator library, such as malloc or jemalloc. From the perspective of MySQL itself, this is no problem and there is no memory leak.

However while free() should release memory when called, it is not required to return it back to the operating system. If the memory allocator decides that the same memory blocks will be required soon, it may still keep them for the mysqld process. This explains why you might see that mysqld still uses a lot of memory after the job is finished and all de-allocations are done.

However, the free() function should indeed be freed when it is called memory, but does not need to return it to the operating system. If the memory allocator finds that these memory blocks are needed immediately, they will be reserved for the mysqld process. This explains why mysqld still takes up a lot of memory after completing its work and releasing memory.

This in practice is not a big issue and should not cause any harm. But if you need the memory to be returned to the operating system quicker, you could try alternative memory allocators, such as jemalloc. The latter was proven to solve the issue with PS-5312.

This is not a big problem in actual production, and it should not cause any accidents. But if you need to return memory to the operating system faster, you can try a non-traditional memory allocator, like jemallolc. It is proven to solve the problem of PS-5312.

Another factor which improves memory management is the number of CPU cores: the more we used for the test, the faster the memory was returned to the operating system. This, probably, can be explained by the fact that if you have multiple CPUs, then the memory allocator can dedicate one of them just for releasing memory to the operating system.

Another factor that improves memory management is the number of cpu cores: in testing, the more cpu cores , the memory will be returned to the operating system faster. It's possible that you have multiple CPUs and one of them can be used exclusively as a memory allocator to free up memory for the operating system.

The very first implementation of InnoDB full text indexes introduced this flaw. As our engineer Yura Sorokin found:

• ##The very first 5.6 commit which introduces Full Text Search Functionality for InnoDB WL#5538: InnoDB Full-Text Search Support – https://dev.mysql.com/worklog/task/?id=5538

• Implement WL #5538 InnoDB Full-Text Search Support, merge – https://github.com/mysql/mysql-server/commit/b6169e2d944 – also has this problem.

As our engineers

Yura Sorokin is the same as what was found. The following two points illustrate that the early implementation of InnoDB full-text index introduced this flaw:

• 5.6 version of MySQL was the first to implement InnoDB WL full-text index Introduction to the function introduction: #5538: InnoDB full-text search support – https://dev.mysql.com/worklog/task/?id=5538

• Implementing WL #5538 InnoDB full-text search Support and Merger - https://github.com/mysql/mysql-server/commit/b6169e2d944 - The same problem also exists Problem

Fixing method

We have a few options to fix this:

1. Change implementation of InnoDB fulltext index

2. Use custom memory library like jemalloc

Both have their advantages and disadvantages.

We have two ways to fix this problem:

1. Modify the implementation of InnoDB full-text index

2. Use a custom memory library such as

jemalloc

Both methods have their own pros and cons.

Option 1 means we are introducing an incompatibility with upstream, which may lead to strange bugs in future versions. This also means a full rewrite of the InnoDB fulltext code which is always risky in GA versions, used by our customers .

Method 1 means we introduce the risk of incompatibility with the software upstream, which may lead to unknown bugs in new versions. It also means completely rewriting part of the InnoDB full-text index code, which is risky in the GA version used by users.

Option 2 means we may hit flaws in the jemalloc library which is designed for performance and not for the safest memory allocation.

Option 2 means we may hit some A bug in the jemalloc library that is designed for performance but is not the safest memory allocation.

So we have to choose between these two not ideal solutions.

Since option 1 may lead to a situation when Percona Server will be incompatible with upstream, we prefer option 2and look forward for the upstream fix of this bug.

So we have to choose one of these two imperfect methods.

In view of the fact that method one may cause the percona service to be incompatible with the upstream, we prefer to use method two to solve the problem and look forward to the upstream fixing this bug.

Conclusion

If you are seeing a high memory usage by the mysqld process, it is not always a symptom of a memory leak. You can use memory instrumentation in Performance Schema to find out how allocated memory is used. Try alternative memory libraries for better processing of allocations and freeing of memory. Search the user manual for LD_PRELOAD to find out how to set it up at these pages here and here.

If you find that the mysqld process takes up a lot of memory, it does not necessarily mean that there is a memory leak. We can use memory instrumentation in Performance Schema to understand how the process uses allocated memory. You can also try replacing the memory library to better handle memory allocation and deallocation. For how to configure LD_RELOAD, please refer to the corresponding pages of the MySQL user manual mysqld-safe and using-system.

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