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Re: [Jython-dev] PyType publication with better concurrency
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From: Jeff A. <ja...@fa...> - 2017-09-24 09:19:59
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As I suspected, the critical distinction is between the *initiating* class loader and the *defining* class loader. The defining class loader here turns out to be an instance of sun.misc.Launcher$AppClassLoader, the grandparent of our URLClassLoader that gets consulted before it tries the JAR URL we gave. org.python.tests.Callbacker appears in jython-dev.jar, so it is found there first. This is the same loader that holds PyObject, etc., so essentially permanent and keeps our test class alive. When this is all fixed by making the parent explicitly None in make_jar_classloader, everything is GC'd as we'd hope. (Phew!) https://bitbucket.org/tournesol/jython-fgtype/commits/4fd2baafb7499929f876ac20a607511668b3e222 Jeff On 18/09/2017 00:45, Jim Baker wrote: > Jeff, > > I looked at the new test, and it's interesting to see the ClassLoader > gets collected, but not the corresponding PyJavaType. To me this > implies the Class was necessarily collected; but it's not observable > directly in Python code. While you are busy on other things, I will > try playing with this changeset some more to see what is going on > here. Maybe it ends just being a simple question of setting up a ref > queue on Class GC and calling ClassValue#remove. > > It's rather awesome about the simplification of fromClass, and > removing the separate implementation of fromClassSkippingInners. > > - Jim > > On Sun, Sep 17, 2017 at 2:41 AM, Jeff Allen <ja...@fa... > <mailto:ja...@fa...>> wrote: > > Jim: > > I figured out needsInners (I think!) and how I could make > fromClass and fromClassSkippingInners into one method using the > state of the registry to steer the action to be almost the same -- > and reach the same conclusion -- with a net reduction in code. > > https://bitbucket.org/tournesol/jython-fgtype/commits/f781dc1f52c9b3733841b375831f8a3c0d27a93e > <https://bitbucket.org/tournesol/jython-fgtype/commits/f781dc1f52c9b3733841b375831f8a3c0d27a93e> > > I have added a test to prove the PyType (actually PyJavaType) is > GC'd when we cease to reference it, using make_jar_classloader, > but it is presently failing. Applied retrospectively to successive > change sets it begins to fail exactly where I introduce ClassValue. :( > > Maybe ClassValue doesn't quite work as I thought. Or I'm still not > using it as John Rose intended. It seems wrong we should have to > layer weak references on top or have some kind of removal queue. > > I tried visualvm to see where it might be referenced, but I'm not > sure how to interpret what it's telling me. However, this week, > and maybe longer, other things have priority. Ideas welcome for > when I'm back to this. > > For exposed PyObjects, I'm pretty sure classToBuilder always was > keeping a strong reference that would keep them alive now. (But > not for the type under test.) I've removed this effect. But maybe > we don't mind that? > https://bitbucket.org/tournesol/jython-fgtype/commits/f781dc1f52c9b3733841b375831f8a3c0d27a93e#Lsrc/org/python/core/PyType.javaT1208 > <https://bitbucket.org/tournesol/jython-fgtype/commits/f781dc1f52c9b3733841b375831f8a3c0d27a93e#Lsrc/org/python/core/PyType.javaT1208> > > Jeff > > > On 12/09/2017 01:37, Jim Baker wrote: >> [I'm been having some problem posting to the jython-dev mailing >> list, let's see if this response gets through!] >> >> On Mon, Sep 11, 2017 at 1:47 PM, Jeff Allen <ja...@fa... >> <mailto:ja...@fa...>> wrote: >> >> Jim: >> >> Thanks for the pointers. I agree with you on #2 that what >> I've got *should* behave correctly with class loaders, but is >> there a test you can suggest (or one I've missed) that >> conclusively demonstrates it? >> >> >> So I would start with test_support.make_jar_classloader, just >> because it's easier to test this behavior in Python. >> test_classpathimporter seems to use this support functionality >> nicely. >> >> If we want to add testing of our expectations around ClassValue's >> lifetime, we can also add some sort of weakref scheme, in >> conjunction with a multi-round GC, to verify that the PyType gets >> collected as expected. Of course we should expect ClassValue to >> behave as expected, but this would ensure we are not holding >> anything, now or in the future implementation, in the Jython >> runtime that would violate this contract. >> >> >> I'll press in on the question of inners. I tried just calling >> fromClass, but it fails when the inner extends the outer >> (test_import_jy:: test_selfreferential_classes fails). You >> have to initialise the outer, then visit the inners. In a >> way, it's the PyJavaType counterpart of the deferred >> initialisation of exposed types that we control by >> identifying bootstrap types. But here it has to be self-guiding. >> >> I have an idea that if I can make fromClass and >> fromClassSkippingInners the same thing, then that can become >> the implementation classToValue.computeValue, and I won't >> need the NotReady exception or findExistingType. >> >> >> So Java classes can form a general type graph (directed, with >> cycles). It seems to me that the NotReady acts as some sort of >> coloring in the graph exploration that fromClass is doing. But as >> I said, I find this code for inner classes rather subtle! Good >> luck in your coding here, it would be really nice to get some >> further simplification. >> >> >> Jeff >> >> On 10/09/2017 01:12, Jim Baker wrote: >> >> Thanks for working on this change set, it's really good >> stuff. In particular, that's a very nice handoff you do >> in the PyType.Registry class, specifically moving >> computed types from classToNewType to classToType; and >> managing the NotReady state. Perhaps it needs some sort >> of state diagram to illustrate, but as someone who has >> been in that code, it makes perfect sense. >> >> With respect to GC, the following should be the case: >> >> * As long as the ClassLoader for the mapped Java class >> instance C is >> around, the corresponding PyType instance T will >> live. This >> follows from the definition of ClassValue as you >> point out in the >> code comments. (Good!) >> * As long as Python code holds a hard ref to T, the >> corresponding C >> will live. (Good!) (So we would see this in from >> `some_java_pkg >> import baz`; and it follows from the underlying_class >> field >> holding a hard ref from T to C.) >> * As long as Python code holds a hard ref to an >> instance of C, the >> corresponding T will also live. (Good!) >> >> Otherwise, this mapping will allow for GC of T, and >> subsequently of C, per the definition of ClassValue. I >> was also looking at Groovy's usage of a similar mapping, >> and corresponding bug, >> https://bugs.openjdk.java.net/browse/JDK-8136353 >> <https://bugs.openjdk.java.net/browse/JDK-8136353>, and >> it looks like the problem reported there doesn't hold. >> This is because there are no strong refs outside of >> PyType itself. >> >> So everything looks good for getting this change in. >> >> On Sat, Sep 9, 2017 at 2:12 PM, Jeff Allen >> <ja...@fa... <mailto:ja...@fa...> >> <mailto:ja...@fa... <mailto:ja...@fa...>>> >> wrote: >> >> Jim, all: >> >> By a series of small steps I've come quite a long way >> in this >> aspect of PyType. It might be worth a look. >> https://bitbucket.org/tournesol/jython-fgtype/commits/3fc5d8ff48bf5d20cb57169cd3f43a3cd90df0cd >> <https://bitbucket.org/tournesol/jython-fgtype/commits/3fc5d8ff48bf5d20cb57169cd3f43a3cd90df0cd> >> >> <https://bitbucket.org/tournesol/jython-fgtype/commits/3fc5d8ff48bf5d20cb57169cd3f43a3cd90df0cd >> <https://bitbucket.org/tournesol/jython-fgtype/commits/3fc5d8ff48bf5d20cb57169cd3f43a3cd90df0cd>> >> >> In this last commit I've entirely dispensed with any >> long-term >> reference to the created PyType in the type registry: >> only the >> ClassValue holds one. The explicit management of >> reachability goes >> away and we have just the behaviour given us by >> ClassValue. The >> only down-side I see is that a PyType created for a >> Java class you >> handle briefly from Python will not be collectable >> until the class is. >> >> The old test for weakness, which I've been running in >> parallel >> with the new, requires PyType.getClassToType(), and >> can no longer >> be supported. (You can't enumerate the ClassValue.) I >> believe the >> replacement tests the same thing -- they used to pass >> and fail >> together. >> >> >> Right, it makes sense to remove this test. This is >> because we don't want to support the necessary mapping, >> even though it could be readily supported with an >> appropriate concurrent weak mapping - the only semantics >> it would be supporting is for this very test. >> >> >> I've moved some actions around, and removed some of >> the checks. We >> seemingly expected the desired PyType to be created >> at any moment >> by a side effect. Reason and regression testing tell >> me that >> doesn't happen in the new arrangement. I think there >> is slightly >> less code to execute, but masses more comment and >> debug -- it's my >> way of figuring it out. It's all very educational. >> >> I still don't understand: >> >> 1. Whether we find the downside (PyType not GC'd >> until the class is) >> important. >> >> >> So we do know that Jython will continue to work if we >> remove a mapping entry, since it happens all the time >> currently in some code like json, re, and socket, where >> classes are not directly imported :) We could always do >> some sort of LRU and then call ClassValue#remove, but I >> don't think it makes sense. There are many other other >> optimizations in the runtime first. >> >> 2. Whether I have broken something that the >> regression tests don't >> cover (like use with isolating class loaders). >> >> >> These are different Java classes if in different class >> loaders. For Python usage of these Java classes, it's all >> the same, thanks to duck typing, but we need to maintain >> a different mapping entry. Again #1 demonstrates we have >> been handling this correctly in the current codebase. >> >> 3. What needsInners is really up to. I see *what* it >> does, but not >> *why* well enough to consider alternatives. It >> would simplify >> computeValue if there were no >> fromClassSkippingInners: could >> needsInners be a registry global? And why do we >> skip processing >> them, when we do? >> >> >> That codepath is exceedingly subtle, at least to me. But >> it would be an interesting experiment to see if it could >> be made global, vs this potentially expanding context >> that is passed through. >> >> - Jim >> >> >> > > |
