#34865: DatabaseWrapper are not GC and connections are not closed
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Reporter: Fábio Domingues | Owner: Priyank
| Panchal
Type: Bug | Status: assigned
Component: Database layer | Version: 4.2
(models, ORM) |
Severity: Normal | Resolution:
Keywords: | Triage Stage: Accepted
Has patch: 1 | Needs documentation: 0
Needs tests: 1 | Patch needs improvement: 1
Easy pickings: 0 | UI/UX: 0
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Comment (by Patryk Zawadzki):
Florian, it does clean up ''eventually'' thanks to the garbage collection,
but garbage collection is a last resort method of freeing unused memory.
There are two important problems with relyin on GC:
1. It happens very rarely (the default thresholds are `(700, 10 10)`,
which means the deepest collection cycle—generation 2—only runs every
70,000 dereferenced objects. That's very rare; in a simple Django
application, that can mean a hundred requests.
2. It also locks the global interpreter lock and effectively freezes the
entire Python process (and all its threads) until it goes through all
uncollectable objects. The more objects it finds (not just the ones with
reference cycles but also everything else they hold a reference to), the
longer the freeze. In extreme cases, it can randomly add seconds to your
response times.
Aggregation of garbage leads to memory fragmentation as unused memory
cannot be reused until the garbage collection thresholds are met, which
leads to the overall memory use slowly creeping up.
What makes it worse is that in this case, it holds a reference to a
`psycopg` connection object, holding all of the psycopg internals hostage
and preventing proper finalization of the connection (and, I assume, in
Django 5.x also preventing the connection from being reclaimed by the
connection pool? I did not verify that).
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Ticket URL: <https://code.djangoproject.com/ticket/34865#comment:13>
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