Tuning transaction_prealloc_size variable

Basic Details

The transaction_prealloc_size variable in MySQL specifies the amount of memory preallocated for each transaction's statement execution and row locking information.

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transaction_prealloc_size – Usage

A transaction in MySQL is a sequence of SQL statements that are treated as a single logical unit of work. MySQL uses various memory buffers and temporary storage mechanisms to manage the data manipulation and querying operations that occur during a transaction.

The transaction_prealloc_size setting is specifically related to the memory allocation strategy for these operations. Preallocating memory for transactions helps to avoid the overhead of repeatedly allocating and deallocating memory during transaction processing, which can significantly degrade performance.

Specifically, transaction_prealloc_size determines the size of the memory buffer allocated in advance for transactions. This buffer is used to store various transaction-related data, such as statement execution information and row-locking details. The primary goal of preallocating memory is to reduce the overhead associated with dynamic memory allocations during transaction execution.

Default Value

The default value of transaction_prealloc_size is 4KB (4096 bytes). This default setting is designed to balance memory usage and performance across a wide range of transaction sizes and complexities.

Note: If a transaction requires more memory than initially allocated (transaction_prealloc_size), the memory pool is increased by a certain amount specified by another parameter, 'transaction_alloc_block_size'.

transaction_prealloc_size – Configuration

The transaction_prealloc_size variable can be configured using the command line or set at startup using the configuration file:
Command Line Configuration:
mysqld> set global transaction_prealloc_size = XX

Replace XX with value to suit your database needs. To verify that the variable has been changed:

mysqld> show global variables like 'transaction_prealloc_size'
Configuration File:
transaction_prealloc_size variable = XX

Replace XX with value to suit your database needs. Must restart MySQL server to see changes applied.

transaction_prealloc_size – Considerations

Configuring transaction_prealloc_size requires a careful balance to optimize database performance while avoiding excessive disk I/O. Here are some considerations and best practices:

Memory Constraints

Increasing transaction_prealloc_size can lead to improved transaction performance due to reduced memory allocation overhead. However, setting this value too high can also result in excessive memory consumption, especially under heavy transaction loads, as each transaction will consume more memory upfront. It's essential to strike a balance based on observed workload patterns and performance metrics.

Workload Characteristics

Administrators may consider adjusting this variable in response to specific workload characteristics. For example, a higher value could be beneficial for environments with long-running transactions or transactions that involve a significant amount of data manipulation, as it can reduce the need for additional memory allocations during transaction execution.

Other Variables

When considering adjustments to transaction_prealloc_size, it's also important to account for other related MySQL variables and settings that influence memory usage and transaction performance, such as innodb_buffer_pool_size and transaction_alloc_block_size. The interaction between these variables can significantly impact the efficiency of memory utilization and the performance of transaction processing.

Leverage Automation

Consider using tools like Releem for automated optimization, which continuously evaluates your server's performance and makes adjustments to settings, including transaction_prealloc_size, on its own. Releem simplifies the process of applying these changes, minimizing manual effort while aiming to improve resource management, decrease latency, and boost query efficiency.
Releem can automatically detect MySQL performance degradation and optimizes MySQL configuration files