To: pgsql-general@lists.postgresql.org
From: Square Bob <square_bob@yahoo.com>
Subject: amazon aroura config - seriously overcommited defaults? (May be Off
 Topic)
Message-ID: <a053cc2d-bf74-cd33-09e1-024b7e87b494@yahoo.com>
Date: Sat, 8 Dec 2018 12:00:33 -0700
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All;


My apologies if this is off topic.


Our company is moving to Aurora, In the past I would take care not to 
allow postgresql to over-commit memory beyond the actual memory on the 
server, which meant I would add the buffer pool + (work_mem * 
max_connections) + (maintenance_work_mem * autovacuum threads)


However as I look at the aroura defaults they are all off the charts, 
for example, based on the calculations in the config (amazon doesn't 
make it easy, some settings are in pages, some are in kb, some are who 
knows what) I see the following settings as default in our aroura config:


The instance size is db.r4.xlarge


this instance size is listed as having 30.5GB of ram


Here's the default settings:


shared_buffers: {DBInstanceClassMemory/10922}

which equates to 24GB


work_mem:   64000 (kb)

which equates to 65.5MB


maintenance_work_mem: GREATEST({DBInstanceClassMemory/63963136*1024},65536)

which equates to 4.2GB


max_connections: LEAST({DBInstanceClassMemory/9531392},5000)

which equates to 3,380


According to my math (If I got it right)  in a worst case scenario,

if we maxed out max_connections, work_mem and maintenance_work_mem limits

the db would request 247GB of memory


Additionally amazon has set effective_cache_size =
{DBInstanceClassMemory/10922}

which equates to about 2.9MB (which given the other outlandish setting 
may be the only appropriate setting in the system)


What the hell is amazon doing here? Am I missing the boat on tuning 
postgresql memory? Is amazon simply counting on the bet that users will 
never fully utilize an instance?


Thanks in advance