Hello,

I have the Twisted app that serves tons of short-lived TLS connections using TLSMemoryBIOFactory. I usually set loosened garbage collector thresholds in production environment for the sake of performance. But I've noticed that this app's RAM usage quickly grows up to unreasonable values. Digging into the issue using pdb and objgraph showed that protocol instances are still living long after they were closed.

I found two circular dependencies which are created for each TLS connection:

1. Between twisted.protocols.policies.ProtocolWrapper and its self.wrappedProtocol

2. Between twisted.protocols.tls.TLSMemoryBIOProtocol and its self._tlsConnection

Both of them cause protocol instance to not be deleted when the connection is closed. So all OpenSSL-related objects and all business-related data attached to that protocol instance are still living untill the next GC collection. This affects both RAM usage and performance (due to much more often GC collections)

I've tried to fix both circular dependencies:

replaced https://github.com/twisted/twisted/blob/trunk/src/twisted/protocols/policies.py#L75 by

self.wrappedProtocol.makeConnection(weakref.proxy(self))

and replaced https://github.com/twisted/twisted/blob/trunk/src/twisted/protocols/tls.py#L199 by:

self._tlsConnection = self.factory._createConnection(weakref.proxy(self))

Memory usage pattern changed drastically after this change.

I've created demo script that makes 10k TLS loopback connections with GC disabled and measures the number of objects are still living after the work is done and total resident RAM consumption:

https://gist.github.com/IlyaSkriblovsky/4dd3abfd5f67c64b13f1c673f56466f9

Output without the fix:

    N = 10000 , K = 100

    objects before 50136

    DummyServerProtocols still living 10000

    objects after 439919

    mem 778 mb

Output with the fix:

    N = 10000 , K = 100

    objects before 50133

    DummyServerProtocols still living 0

    objects after 159919

    mem 96 mb

So using weakrefs makes all protocol instances and instances of TLSMemoryBIOProtocol to be deleted right after a connection is closed. Less circular-dependent objects → less GC invocations → better performance. And I see much nicer RAM usage pattern in my app.

Is it possible to fix circular deps in some more clean way? Can this be solved at all while user's code is able to try to touch both sides of circular dep after connection is closed? Please advice

Thanks for consideration

Best regards,

    Ilya