Cache trashing in PostgreSQL 15 occurs when there is high contention for the shared buffer cache. This can happen when there are many concurrent queries accessing the same data, or when there is a lack of available memory for the cache. When the cache becomes full, the oldest or least frequently used pages are evicted to make room for new pages. This process, called replacement, can lead to cache trashing if the evicted pages are immediately requested again, causing the cache to constantly swap pages in and out. This results in a decrease in performance as it takes longer to retrieve the data from memory, and the CPU becomes bogged down with cache management tasks.
To avoid cache trashing, you can increase the shared_buffers configuration setting to increase the amount of memory available for the cache, or you can use the pg_prewarm extension to pre-load frequently used pages into the cache. Additionally, you can monitor the hit rate of the cache using the pg_stat_bgwriter and pg_stat_database views, and make adjustments to your queries and indexes accordingly.
Crash trashing can negatively impact the performance, scalability, and reliability of a PostgreSQL database in several ways:
Performance: When crash trashing occurs, the background writer process uses a significant amount of CPU and I/O resources to clean dirty pages. This can cause a significant increase in the overall system load and slow down the performance of other queries running on the system.
Scalability: Crash trashing can also lead to increased memory pressure, which can cause PostgreSQL to start swapping to disk. This can cause a significant decrease in the performance of the system and make it difficult to scale.
Reliability: Crash trashing can also cause the database to become unresponsive or crash, which can lead to data loss or corruption. Additionally, if the background writer process is not able to clean dirty pages fast enough, it can lead to a buildup of dirty pages and eventual system failure.
It is important to monitor the background writer process and address any issues related to crash trashing as soon as they arise to maintain the performance, scalability, and reliability of a PostgreSQL database.
Monitoring crash trashing happening to PostgreSQL in real-time
You can download a Python script for monitoring real-time crash trapping to PostgreSQL 15 from our GitHub repository here: https://github.com/shiviyer/Troubleshooting-PostgreSQL-Performance/blob/main/psql-cache-trash.py