On Sun, Sep 23, 2018 at 7:54 PM 'Kenton Varda' via v8-users < v8-users@googlegroups.com> wrote:
> Hi v8-users, > > I'm trying to understand how to correctly instrument my wrappers for > multi-threaded garbage collection, and it seems I'm doing something wrong. > Currently, my code seems to work only in --predictable mode > (single-threaded), but I'd like to take advantage of GC in a background > thread. > > When in multi-threaded mode, I observe that occasionally, when V8 traces > into a particular wrapper, the weak handles held by that wrapper have > already been collected. The tracing is happening, but V8 seemingly > prematurely collects some handles before I get a chance to mark them. > > fyi: The cross-component tracing used for EmbedderHeapTracer always goes through the main thread, i.e., none of its methods are called concurrently. > Here are the things I've done: > > Per-object: > > - When I create a new wrapper object, I give it two internal fields > set to the native object pointer and a tracing callback function. > - The tracing callback, when run, invokes RegisterExternalReference() > on each persistent handle reachable through the native object. > - Once the internal pointers are set, I invoke SetWeak() (with no > arguments) on all handles reachable through the native object. > > Globally: > > - I call AddGCPrologueCallback() to register a callback for > v8::kGCTypeScavenge. > - In the scavenge callback, I call VisitWeakHandles() with a visitor > that calls MarkActive() on every handle. > > Looks like Isolate::VisitWeakHandles <https://cs.chromium.org/chromium/src/v8/src/api.cc?q=Isolate::VisitWeakHandles&sq=package:chromium&g=0&l=8852> only iterates over weak handles with a non-zero class id. Any chance that this one is still 0? > > - I call SetEmbedderHeapTracer() to register my own heap tracer > implementation. > - In the heap tracer's RegisterV8References() method, I invoke each > tracing callback using the pair of internal pointers (which calls > RegisterExternalReference() on reachable weak handles, as described above). > - Currently, I don't do anything in AdvanceTracing() -- I complete all > tracing before RegisterV8References() returns. (It appears this is what > Blink does in its unified_heap_controller.cc, so I followed the example.) > However, if I instead only save a copy of the pointer list in > RegisterV8References() and then do the tracing later in AdvanceTracing(), > then I observe problems with prematurely-collected handles more often. I'd > guess this is because the underlying problem is a race condition, in which > another thread is collecting those handles before I get a chance to trace > them. > > The symptom described here hints to the Scavenger collecting objects. From the above description it looks like you wanted to preserve those objects on Scavenge. Some ideas: - Maybe class id is 0 (see above)? - Maybe the handle are not SetWeak <https://cs.chromium.org/chromium/src/v8/include/v8.h?type=cs&q=v8::PersistentBase::SetWeak&sq=package:chromium&g=0&l=514> immediately but only after some time where a Scavenge could've happened? > > - I don't do anything in any of the other EmbedderHeapTracer callbacks. > > Anything I'm missing here? > > I haven't been able to find any documentation on how to use these > interfaces. Please let me know if there are docs I missed. > > There's no additional documentation and you already found the existing implementations for wrapper tracing <https://cs.chromium.org/chromium/src/third_party/blink/renderer/platform/bindings/script_wrappable_marking_visitor.h?sq=package:chromium&dr&q=blink::ScriptWrappableMarking&g=0&l=32> and unified heap garbage collections <https://cs.chromium.org/chromium/src/third_party/blink/renderer/platform/heap/unified_heap_controller.h?sq=package:chromium&dr&q=blink::UnifiedHeap&g=0&l=31> . The Scavenger quirks are definitely something we should document though. Cheers, -Michael -- -- v8-users mailing list v8-users@googlegroups.com http://groups.google.com/group/v8-users --- You received this message because you are subscribed to the Google Groups "v8-users" group. To unsubscribe from this group and stop receiving emails from it, send an email to v8-users+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
