The LWLock:buffer_content event occurs when a session is waiting to read or write a data
page in memory while another session has that page locked for writing. In RDS for PostgreSQL 13 and higher,
this wait event is called BufferContent.
Supported engine versions
This wait event information is supported for all versions of RDS for PostgreSQL.
Context
To read or manipulate data, PostgreSQL accesses it through shared memory buffers. To read from the buffer, a process gets a lightweight lock (LWLock) on the buffer content in shared mode. To write to the buffer, it gets that lock in exclusive mode. Shared locks allow other processes to concurrently acquire shared locks on that content. Exclusive locks prevent other processes from getting any type of lock on it.
The LWLock:buffer_content (BufferContent) event
indicates that multiple processes are attempting to get a lock on contents of a specific
buffer.
Likely causes of increased waits
When the LWLock:buffer_content (BufferContent) event appears more than normal,
possibly indicating a performance problem, typical causes include the following:
- Increased concurrent updates to the same data
-
There might be an increase in the number of concurrent sessions with queries that update the same buffer content. This contention can be more pronounced on tables with a lot of indexes.
- Workload data is not in memory
-
When data that the active workload is processing is not in memory, these wait events can increase. This effect is because processes holding locks can keep them longer while they perform disk I/O operations.
- Excessive use of foreign key constraints
-
Foreign key constraints can increase the amount of time a process holds onto a buffer content lock. This effect is because read operations require a shared buffer content lock on the referenced key while that key is being updated.
Actions
We recommend different actions depending on the causes of your wait event. You might identify
LWLock:buffer_content (BufferContent) events by using Amazon RDS Performance
Insights or by querying the view pg_stat_activity.
Topics
Improve in-memory efficiency
To increase the chance that active workload data is in memory, partition tables or scale up your instance class. For information about DB instance classes, see DB instance classes.
Reduce usage of foreign key constraints
Investigate workloads experiencing high numbers of LWLock:buffer_content (BufferContent)
wait events for usage of foreign key constraints. Remove unnecessary foreign key constraints.
Remove unused indexes
For workloads experiencing high numbers of LWLock:buffer_content
(BufferContent) wait events, identify unused indexes and remove them.
Increase the cache size when using sequences
If your tables uses sequences, increase the cache size to remove contention on sequence pages and index pages. Each sequence is a single page in shared memory. The pre-defined cache is per connection. This might not be enough to handle the workload when many concurrent sessions are getting a sequence value.
