![]() ![]() As the connections got opened, the free memory reduced from approximately 5.26 G to 4.22 G. The free memory chart shows no significant difference between the run with and without DISCARD ALL. The following chart shows the connections count and the memory utilization on an RDS PostgreSQL instance. Postgres=> select state, query, count(1) from pg_stat_activity where usename='app_user' group by 1,2 order by 1,2 Leave the connections idle for 10 minutes.ĭuring the 10-minute wait, check the connection state as follows:.This basic test determines the memory impact of newly opened connection. Connections test #1: Connections with no activity For each test, a run with DISCARD ALL has been added to see if there is any change in the memory utilization. This statement is often used by connection poolers before reusing the connection for the next client. It discards all temporary tables, plans, and sequences, along with any session-specific configurations. The DISCARD ALL statement discards the session state. For storage, I used an IO1 EBS volume with 3000 IOPS. I used the RDS instance type db.m5.large for the test runs, which provides 2 vCPUs and 8GB memory. The tests are performed using Amazon RDS for PostgreSQL 11.6Īlthough this post shows the results for Amazon RDS for PostgreSQL 11.6 only, these tests were also performed with Aurora PostgreSQL 11.6, PostgreSQL on EC2, and Amazon RDS for PostgreSQL 12 to confirm that we see a similar resource utilization trend.The second test runs DISCARD ALL on the connection before leaving it idle.The connections are left idle for 10 minutes before closing the connections.Depending on the test case, some activity is performed on each connection before leaving it idle.Each test run opens 100 PostgreSQL connections.The tests include the following parameters: To determine the impact of idle PostgreSQL connections, I performed a few tests using a Python script that uses the Psycopg 2 for PostgreSQL connectivity. However, this is incorrect-they’re consuming server resources. A common explanation is that they’re just idle connections and not actually doing anything. It’s very common to see a huge number of connections against the database. Idle connections are one of the common causes of bad database performance. For more information about fork functionality, see the entry in the Linux manual. This method reduces some memory overhead when PostgreSQL forks a child process on receiving the new connection. At that point, a copy of the changed data gets created. This continues until the parent or the child process actually changes the data. As an optimization, Linux kernel uses the copy-on-write method, which means that initially the new process keeps pointing to the same physical memory that was available in the parent process. On Linux, when a new process is forked, a copy of the process gets created. This memory is used for maintaining the process state, data sorting, storing temporary tables data, and caching catalog data. The process memory contains process-specific data. This shared memory is used by all the PostgreSQL processes. ![]() The shared_buffers parameter configures the size of the shared memory. Shared memory is a chunk of memory used primarily for data page cache. PostgreSQL uses shared memory and process memory. The max_connections parameter controls the total number of connections that can be opened simultaneously. The following screenshot shows that a user app_user is connected to the database mydb from a remote host (10.0.0.123). A new child process is started for each new connection received by the database. The main process goes back to wait for the next connection, and the newly forked child process takes care of all activities related to this new client connection. When a new connection is received, the main process forks to create a child process to handle this new connection. This extension is available in the latest RDS PostgreSQL minor versions.Īfter initializing the maintenance child processes, the main PostgreSQL process starts waiting for new client connections. If you need to view the complete list of processes, consider using the pg_proctab extension to query system statistics. The process list in Enhanced Monitoring limits the total number of processes that are shown on the console. These child processes take care of activities such as logging, checkpointing, stats collection, and vacuuming. For more information, see Enhanced Monitoring. ![]() You can see this process tree in Amazon RDS and Aurora PostgreSQL by enabling enhanced monitoring and looking at the OS Process List page (see the following screenshot). \_ postgres: logical replication launcher usr/pgsql-11/bin/postmaster -D /var/lib/pgsql/11/data ![]()
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