GitHub user darekwojciechowski added a comment to the discussion:
sqlalchemy.exc.TimeoutError: QueuePool limit of size 8 overflow 8 reached,
connection timed out
That error is about Superset's metadata DB pool, and it's a per-worker pool.
Each gunicorn worker has its own pool of pool_size + max_overflow = 8 + 8 = 16
connections. When a big dashboard refreshes, one worker fires many chart
queries at once (each needs a metadata connection for logging/results), blows
past 16 concurrent, and the 17th waits pool_timeout=90s then throws.
Raising the pool "didn't help much" because you also have a hard ceiling: total
connections = workers × (pool_size + max_overflow). With gevent and 10 workers
you must keep that under Postgres max_connections=200, minus Celery's own
usage. 10 × 16 = 160 today; you can't just crank pool_size to 50 (10 × 100 =
1000 >> 200).
Two ways forward:
1. Right-size within the ceiling. Fewer, fatter workers give each one a bigger
pool without exceeding Postgres. For example 5 workers:
SQLALCHEMY_ENGINE_OPTIONS = {
"pool_size": 15,
"max_overflow": 15,
"pool_pre_ping": True,
"pool_recycle": 1800,
"pool_timeout": 30,
}
5 × 30 = 150 connections, but each worker now tolerates 30 concurrent chart
queries instead of 16. Also bump Postgres max_connections (e.g. 300) if you
have RAM.
2. The scalable fix: put PgBouncer in front of the metadata Postgres in
transaction pooling mode. Then hundreds of gevent greenlets share a small set
of real Postgres connections, and pool exhaustion basically goes away. This is
the standard answer for gevent workers + Postgres connection explosion.
For a quick unblock use option 1; for a real load pattern, do option 2.
GitHub link:
https://github.com/apache/superset/discussions/39993#discussioncomment-17633420
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