[Twisted-Python] Circular references in TLSMemoryBIOProtocol

Ilya Skriblovsky ilyaskriblovsky at gmail.com
Wed Jan 17 14:09:21 MST 2018


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
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