Allowed memory size of 2147483648 bytes exhausted (tried to allocate 20480 bytes) in vendor/magento/framework/Serialize/Serializer/Json.php on line 37 when accessing a category page in your Magento 2.4.1 installation, don't panic.  This issue can be resolved with some investigation and adjustments.

Understanding the Issue

First, let's understand the error.

This error occurs when PHP reaches the memory limit defined in the PHP configuration (php.ini) while trying to allocate more memory for a process.

It's worth noting that increasing server memory or PHP memory limit is not always the solution, as the error can indicate a memory leak or inefficient memory usage in the codebase.

Investigating the Root Cause

To start troubleshooting, you should review the category page code, and specifically the logic related to product loading and rendering.

In Magento, displaying all products in a top-level category can potentially lead to high memory consumption if not handled optimally.

Querying and processing a large number of products can be resource-intensive, especially for complex product types like configurable products.

Memory Leak Detection

Given that the issue manifests after a specific duration on the production environment, it's crucial to investigate if there are memory leaks in the codebase.

Utilize PHP memory profiling tools or extensions like Xdebug to analyze memory usage patterns and identify any areas where memory is not being effectively released.

Configuration and Resource Optimization

Besides code review, consider optimizing server configurations.

Ensure that Nginx, PHP-FPM, and relevant server modules are properly configured and compatible with Magento 2.4.1.

Additionally, review the PHP-FPM configuration to guarantee that the increased memory limit has been applied effectively.

Be aware of any overrides in the .user.ini file and confirm that the memory limit modification is applied in the correct scope.

Code-Level Optimization

Optimizing the category page rendering and product loading mechanisms can significantly mitigate memory consumption.

Utilize Magento's caching mechanisms, pagination, and lazy loading techniques to distribute memory usage and enhance the overall performance.

Review any customizations or extensions that modify the category page behavior and ensure they align with best practices for memory-efficient operations.

Lastly, regular Magento 2 updates and patches often include memory and performance optimizations.

Ensuring that your Magento installation is up-to-date with the latest version can address known memory-related issues and enhance stability.

By methodically addressing code-level, configuration, and resource optimization issues, you can effectively resolve the PHP Fatal error related to memory exhaustion in Magento 2.4.1 and ensure a stable and efficient storefront for your customers.