Memory Leak Regression Testing with V8/Node.js, Half 2 - Finalizer-Bas…
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작성자 Janie 작성일 25-08-10 16:48 조회 7 댓글 0본문
Within the previous blog publish, I talked about how Node.js used memory utilization measurement to check against Memory Wave leaks. Generally that’s ok to offer valid tests. Generally we want the check to be extra precises and give attention to the standing of particular objects. This can be fairly tough with what’s obtainable to work together with V8’s rubbish collector. One widespread technique utilized by Node.js core check suites relies on the native v8::PersistentBase::SetWeak() API to invoke a "finalizer" when the observed object is rubbish collected. 3. At course of exit, if the callback set in 1 sees that the finalizer has not been invoked for enough instances, the object is taken into account leaking. The onGC() helper was launched earlier than the FinalizationRegistry API turned accessible to JavaScript. It essentially serves the identical objective as FinalizationRegistry and invokes the ongc() callback for the primary argument as a finializer. It's carried out with Node.js’s destroy async hook which is in turn applied with the v8::PersistentBase::SetWeak() API mentioend before.
The FinalizationRegistry API (a part of the WeakRef proposal) has been shipped since V8 8.4. This roughly serves the identical objective as the onGC() helper described above, but the callbacks are invoked via a mechanism different from that of the weak callback’s. In comparison with weak callbacks, the invocation of finalization registry callbacks often occurs later and is much less predictable. That is by-design to provide JS engines extra leeway within the scheduling of the callback and avoid hurting performance. Technically the JS engine doesn't even have to invoke the callback (the same will also be said about weak callbacks, however they're much less advanced anyway). Finalizers are tricky business and it's best to keep away from them. They can be invoked at unexpected times, or not at all… The proposed specification permits conforming implementations to skip calling finalization callbacks for any purpose or no purpose. In observe though, the callback would only be called for 99 instances by the time the exit event is emitted - at least when i examined it regionally.
As I’ve analyzed in one other blog publish, the false positives of Jest’s --deteck-leaks (which is based on FinalizatioRegistry) showed that you can't use gc() to make sure finalization registry callbacks to be known as for every object ever registered when they're garbage collected, even should you go as far as operating gc() for 10 instances asynchronously, because that’s not what they're designed for in the primary place. Finally, this is dependent upon the regression that you are testing in opposition to. If the leak reproduces reliably with each repeated operation that you're testing, one non-leaking pattern may already offer you 90% confidence that you’ve fixed it and it’s not regressing once more. In fact, you might want a 100% confidence and verify this with every sample, but given that observing finalization with a rubbish collector can already offer you false positives by design, a much less exact test with less false positives is better than a extra precise check with more false positives.
As I’ve talked about in the opposite blog publish, MemoryWave Community a simple gc() is generally not sufficient to scrub up as many objects and invoke as many callbacks as doable, MemoryWave Community as a result of it’s simply not designed for that. Working it a number of times or keeping the thread running for a bit (in Node.js, using setImmediate() to keep the event loop alive) can typically give V8 sufficient nudges to run your finalizers for unreachable objects (which was what Jest’s --detect-leaks did), but generally these tips are nonetheless not enough. In that case, if you happen to depend on the finalizers to inform you whether or not your object may be collected or not, and consider the absence of finalizer invocations to be an indication of leaks, then you will have false positives. There may be another caveat with gc() - if the graph being checked entails newly compiled features/scripts, and you're assuming that V8 can gather them when they are not reachable by users (which does happen usually), then using gc() can bite you in the again because a compelled GC induced by gc() alone can prevent them from being garbage collected.
That’s intentional, as a result of gc() is a V8 internal API that solely caters to V8’s own testing wants, which incorporates this behavior. That mentioned, typically it’s nonetheless inevitable for the regression assessments to force the garbage assortment one way or the other. Is there a extra dependable alternative to gc()? Nicely, one hack utilized by some of Node.js’s assessments in addition to a later repair to Jest’s --detect-leaks is to take a heap snapshot to carry out some some type of last-resort rubbish collection. By design, Memory Wave a heap snapshot in intended to seize what’s alive on the heap as accurately as doable, so taking it urges V8 to start out the rubbish collection with some additional operations to run as many finalizers because it could. The heap snapshot generation course of additionally clears the compilation cache, which can help clearing scripts that would not be in any other case collected if the GC is pressured by gc(). This helper takes an object manufacturing unit fn(), and run it up to maxCount times. Ideally the heap size limit should even be set to a smaller worth to offer V8 some sense of emergency to wash the constructed objects up because the allocation happens. If the FinalizationRegistry callback for any objects returned from fn() will get known as during the method, we all know that not less than some of these objects are collectable under memory strain, then we are assured sufficient about disproving the leak and cease there. To offer V8 extra nudges to invoke the finalizer, we’ll additionally take the heap snapshot at a specified frequency.
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