Opened 14 months ago
Last modified 12 months ago
#34865 assigned Bug
DatabaseWrapper are not GC and connections are not closed
Reported by: | Fábio Domingues | Owned by: | Priyank Panchal |
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Component: | Database layer (models, ORM) | Version: | 4.2 |
Severity: | Normal | Keywords: | |
Cc: | Carlton Gibson, Andrew Godwin, Andreas Pelme, David Wobrock, Florian Apolloner | Triage Stage: | Accepted |
Has patch: | yes | Needs documentation: | no |
Needs tests: | yes | Patch needs improvement: | yes |
Easy pickings: | no | UI/UX: | no |
Description
I have found possible memory leak when investigating why database connections where not closed after the thread/coroutine ended (https://code.djangoproject.com/ticket/33497 and https://github.com/django/django/pull/17275), the DatabaseWrapper
ends up in a circular reference with DatabaseFeatures
/DatabaseCreation
/DatabaseClient
/... and the GC could not cleanup the objects.
I tryed use WeakRef
from the "child" objects to the wrapper or deleting the __dict__
and the wrapper was collected and the connections was closed.
Attachments (2)
Change History (14)
by , 14 months ago
Attachment: | databasewrapper.png added |
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follow-up: 2 comment:1 by , 14 months ago
Resolution: | → duplicate |
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Status: | new → closed |
comment:2 by , 14 months ago
Replying to Mariusz Felisiak:
Fabio, thanks for the report. Is there a reason to open a new ticket and don't add your findings in the #33497? As far as I'm aware, this is a duplicate.
Fixing this memory leak will not solve the problem of the persistent connections not been reuse, only the fact that they will not pill up (and starve the DB) if inaccessible.
But I'm new here, still learning. Should I move the findings to #33497?
comment:3 by , 14 months ago
Cc: | added |
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Resolution: | duplicate |
Status: | closed → new |
Triage Stage: | Unreviewed → Accepted |
We can keep this separately.
Tentatively accepted, I'd like to check out the patch proposal.
comment:4 by , 14 months ago
...I'd like to check out the patch proposal.
Yes, please.
Any chance you could give a minimal example showing the behaviour here, to look at?
Various folks have reported leaky behaviour on Channels/Daphne, but we've not been able to pin it down.
… ends up in a circular reference ... and the GC could not cleanup the objects.
Just out of interest, could you try throwing in a gc.collect()
— that should get it — unless we never let the DatabaseWrapper
go, which is why it would be nice to look at the example — but WeakRef
is hopefully right in this case.
comment:5 by , 14 months ago
Tested using Uvicorn and Daphne, the database engine should not matter, I tested with PostgreSQL, CONN_MAX_AGE
could be any value and the effect is the same using async or sync views.
get_refs = lambda: [o for o in gc.get_objects() if isinstance(o, BaseDatabaseWrapper)] plot_refs = lambda: objgraph.show_backrefs(get_refs(), filename="sample-graph.png") def sync_view(request): MyModel.objects.count() # gc.collect() # plot_refs() return HttpResponse(f"Number of BaseDatabaseWrapper instances: {len(get_refs())}") async def async_view(request): await MyModel.objects.acount() # gc.collect() # plot_refs() return HttpResponse(f"Number of BaseDatabaseWrapper instances: {len(get_refs())}")
On each refresh you should see the number of instances increase. If you plot the references you would see many diagrams like the one in the attachment. Throwing gc.collect()
actually get it, but it take some milliseconds, even if the underlying database connection is already closed.
comment:6 by , 14 months ago
One more detail. Using objgraph.show_growth
without doing gc.collect
give us this on each new request:
list 1990 +10 dict 6798 +3 deque 20 +2 lock 43 +2 OrderedDict 11 +2 partial 24 +2 ReferenceType 3848 +1 DatabaseWrapper 6 +1 DatabaseClient 6 +1 DatabaseCreation 6 +1 DatabaseFeatures 6 +1 DatabaseIntrospection 6 +1 DatabaseOperations 6 +1 BaseDatabaseValidation 6 +1 DatabaseErrorWrapper 5 +1 PGconn 5 +1 Connection 5 +1 PrepareManager 5 +1 AdaptersMap 7 +1 TypesRegistry 7 +1
with the patch the output of show_growth
is empty.
comment:7 by , 13 months ago
Cc: | added |
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comment:8 by , 13 months ago
Owner: | changed from | to
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Status: | new → assigned |
comment:9 by , 13 months ago
Throwing gc.collect() actually get it, but it take some milliseconds, even if the underlying database connection is already closed.
with "actually get it" do you mean that it cleans it up properly? If yes there does not seem to be real leak here since garbage collection usually runs every now and then (or do I miss something). It might still be beneficial to improve the performance here but that might require a test or two :D
comment:10 by , 13 months ago
Has patch: | set |
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comment:11 by , 13 months ago
Needs tests: | set |
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Patch needs improvement: | set |
Marking as needs improvement per Florian's comment.
comment:12 by , 12 months ago
Cc: | added |
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Fabio, thanks for the report. Is there a reason to open a new ticket and don't add your findings in the #33497? As far as I'm aware, this is a duplicate.