The stub replaces the C++ version almost entirely, Isolate::RunMicrotasksInternal() invokes the stub if there are any pending tasks. The stub then consumes all pending tasks.
Without a proper entry stub or entry/exit code inlined into the function, the root list register will be uninitialized and cause access violations. And callee-saved regs would be corrupted, possibly introducing bugs on return to C++ after the stub call. Using a full JSFunction would allow the standard JSEntryStub to be used, avoiding those problems. Other alternatives include adding a different variant of JSEntryStub which calls this specific stub, or inlining entry/exit code in the stub. The RunMicrotasks JSFunction could be a movable root, it would just need to be accessible without an entered Context. > On Sep 1, 2017, at 1:28 AM, Benedikt Meurer <[email protected]> wrote: > > I don't think strong-rooting a JSFunction is a good idea. That might solve > one problem, but will likely create N+1 different problems in other places. > > I've been discussing this with +Jaroslav Sevcik and we probably don't > understand the underlying problem. So let's try to get that first: > You're porting the microtask queue pumping to the CSA now, and you need to > call into Blink C++ functions and JSFunctions from that. But there's also > still the C++ implementation of RunMicrotaskQueue still. Is that correct? > > -- Benedikt > >> On Fri, Sep 1, 2017 at 7:03 AM Caitlin Potter <[email protected]> wrote: >> I'm unclear on what you mean regarding code duplication. >> >> It's about ensuring A) fixed registers (e.g. the root array register) are >> initialized properly to avoid access violations when heap constants are >> used, and to make sure callee-saved regs are actually saved and restored. >> >> If I can strong-root a full JSFunction and just use the ordinary >> JSEntryStub, as Adam suggested, it may be a non-issue. >> >>> On Aug 31, 2017, at 11:00 PM, Benedikt Meurer <[email protected]> wrote: >>> >>> I like the idea of being able to run microtasks from CSA land. Not sure >>> about this JSEntryStub businesses tho, it all sounds dangerous to me. Is >>> this just to avoid some code duplication between CSA and C++? If so, then >>> I'd strongly recommend against it and just duplicate that logic for now. If >>> not then I have probably misunderstood the problem. >>> >>> -- Benedikt >>> >>> >>>> On Fr., 1. Sep. 2017, 03:35 Adam Klein <[email protected]> wrote: >>>>> On Thu, Aug 31, 2017 at 5:52 PM, Adam Klein <[email protected]> wrote: >>>>> Hi Caitlin, >>>>> >>>>> Jakob and I just spent some time digging into this, comments inline >>>>> (though we don't have answers to everything). >>>>> >>>>>> On Thu, Aug 31, 2017 at 9:01 AM, <[email protected]> wrote: >>>>>> Hi, >>>>>> >>>>>> Recently I've been trying out some things to get more out of Promises >>>>>> and async functions. >>>>>> >>>>>> Different people in/around the Node.js project have been writing various >>>>>> benchmarks which show cases where `await` >>>>>> seems to slow things down significantly. One simple example is >>>>>> https://github.com/tc39/proposal-async-iteration/issues/112#issuecomment-324885954. >>>>>> While it's invalid to compare >>>>>> the simple synchronous loop to the one with `await`, it does highlight >>>>>> that in situations like that, the v8 implementation >>>>>> can seem to be very slow, when really it should be more similar to the >>>>>> sync. loop (~20 times slower seems like a steeper >>>>>> price to pay than is necessary). >>>>>> >>>>>> I drafted an informal document to come up with some ideas for speeding >>>>>> up Await in v8. In general, the solutions were >>>>>> split into 2 categories: >>>>>> >>>>>> 1) reduce heap use and GC overhead (allocate fewer objects for Await). >>>>>> 2) avoid JS->C and C->JS transitions where possible (mainly accomplished >>>>>> by translating >>>>>> Isolate::RunMicrotasksInternal() and Isolate::EnqueueMicrotask() >>>>>> into code stubs). This generally makes JS-defined >>>>>> microtasks (for Promises and Await) much faster, but may cause >>>>>> DOM-defined microtasks to slow down a bit (unclear >>>>>> at this time). I expect Promises and Await to be used more >>>>>> frequently in tight loops, and certainly DOM microtasks don't >>>>>> affect Node.js at all, so this may be something worth going after. >>>>>> >>>>>> The first approach did not make much of a dent in any benchmarks. More >>>>>> useful profiles of actual applications did not >>>>>> show `await` to be a bottleneck at all. Reducing overall memory use >>>>>> seems like a good thing in general, however. >>>>>> >>>>>> The second approach yielded a significant improvement (~60% over 10 >>>>>> runs) for the simple benchmark (in a very >>>>>> simple prototype implementation with some limitations discussed below). >>>>>> >>>>>> So there are some constraints WRT implementing RunMicrotasks in JIT'd >>>>>> code. Particularly, it needs to be possible to >>>>>> perform RunMicrotasks() when no context has been entered. I've tried a >>>>>> few things to work around this: >>>>>> >>>>>> Initially, I had wrote the stub with JS linkage, and used the typical >>>>>> JSEntryStub to invoke it. This is partly >>>>>> wasteful, and partly problematic. There need not be a separate >>>>>> JSFunction for RunMicrotasks in each >>>>>> context. More importantly, the function ought not to be associated with >>>>>> a context at all, given the >>>>>> constraint that it must be possible to invoke it without a context >>>>>> having been entered. >>>>> >>>>> From looking at the JSEntryStub codepath for JSFunctions, it appears to >>>>> us that for a function marked as native, strict, or both (which seems >>>>> appropriate in this case) there shouldn't be any need for a Context. So >>>>> it seems like you could unblock your prototype by creating a single >>>>> JSFunction (as a strong root on the Heap) which wraps the builtin, and >>>>> call through that from the C++ API. If you already tried something like >>>>> this and ran into trouble it'd be interesting to hear what went wrong. >>>>> >>>>>> A second approach involved creating new TF operators to initialize the >>>>>> roots register (the main >>>>>> manifestation of problems when not using the JSEntryStub was that the >>>>>> roots register was not initialized, >>>>>> leading to access violations when using heap constants). I didn't spend >>>>>> much time with this, because I >>>>>> felt that it was more important to make sure callee-saved registers were >>>>>> restored properly, even though >>>>>> there wasn't much going on in the sole caller of the function. I >>>>>> thought it might be interesting to produce >>>>>> more general operators which would handle entry and exit for stubs which >>>>>> need to be invoked from C, >>>>>> but it seemed like a lot of work and I haven't gotten around to doing >>>>>> this yet. >>>>>> >>>>>> Finally, I tried adding a new variant to JSEntryStub, which call the >>>>>> RunMicrotasks stub rather than the various entry >>>>>> trampolines. At this moment, it's mostly in working order, but it's >>>>>> possible there are still problems with >>>>>> StackFrameIteration and exception handling. >>>>> >>>>> These approaches seem too involved just for this one case, I'd prefer the >>>>> JSFunction approach above if it works. >>>>> >>>>>> Another limitation is, previously SaveContexts (which seem to matter to >>>>>> the debugger and API in some way, though I >>>>>> haven't really looked at why yet) were not set up when calling >>>>>> API-defined microtask callbacks. In my prototype, I >>>>>> always set up the SaveContext before entering the RunMicrotasks stub. >>>>>> It's yet unclear if this breaks anything, or if it >>>>>> would be possible (or even a good idea) to mimic the old behaviour in >>>>>> the stub rather than always pushing the SaveContext. >>>>>> This is a subtle difference, but as noted it could have some bad effects. >>>>> >>>>> Still digging into this. It appears I may have inadvertently removed the >>>>> regression test for this code in >>>>> https://codereview.chromium.org/1909433003 when I removed support for >>>>> Object.observe, but the regression test should be able to be adapted for >>>>> Promises (see https://codereview.chromium.org/332923003 for the test). >>>>> I'm going to try restoring the test and playing around with this code in >>>>> the current C++ version to see if I can get a better handle on it. But >>>>> from an initial reading, it really shouldn't make a difference for the >>>>> C++ callback case anyway. >>>> >>>> Looking more deeply at this (and at >>>> https://bugs.chromium.org/p/chromium/issues/detail?id=385349, the bug for >>>> that regression test), I'm not convinced this is actually necessary >>>> anymore. +yangguo in case he has a less fuzzy memory than me (this was all >>>> >3 years ago). >>>> >>>>>> Finally, a somewhat strange behaviour of the stub is that it enters >>>>>> contexts by itself when it needs to, inlining >>>>>> HandleScopeImplementer::EnterMicrotaskContext and >>>>>> LeaveMicrotaskContext(), and overwriting Isolate::context(). >>>>>> I believe this is done in a valid way in the prototype, but it's not >>>>>> something that comes up in other stubs, so there isn't really >>>>>> any other code to model it on. >>>>> >>>>> This seems fine to me, it's rather special behavior even in its C++ form. >>>>> >>>>>> --- >>>>>> >>>>>> I was wondering if anyone thought reducing the C++->JS->C++ overhead in >>>>>> RunMicrotasks for that 60% boost in certain >>>>>> very simple and unrepresentative-of-real-code benchmarks might be worth >>>>>> doing properly and upstreaming? While it's >>>>>> unclear what the impact would be on real-world code, it seems like a >>>>>> reasonable expectation that you'd see some kind of >>>>>> significant benefit (though perhaps not on the order of 60% as in the >>>>>> very simple benchmark mentioned above). >>>>>> >>>>>> If (in the opinion of the v8 team) it might be worth my time to try to >>>>>> upstream this, I'd love some feedback on the approaches >>>>>> taken to address the problems listed above, and get an idea of what sort >>>>>> of approach you'd all be happiest with. >>>>> >>>>> If we can pin down the answers to all the stuff above to our >>>>> satisfaction, then yes, my inclination is that this is a worthwhile thing >>>>> to do: the code may be a bit verbose (what with having to deal with the >>>>> different sorts of things stored in the queue), but it's at least >>>>> relatively straightforward. >>>>> >>>>> - Adam >>>> >>>> -- >>>> -- >>>> v8-dev mailing list >>>> [email protected] >>>> http://groups.google.com/group/v8-dev >>>> --- >>>> You received this message because you are subscribed to the Google Groups >>>> "v8-dev" group. >>>> To unsubscribe from this group and stop receiving emails from it, send an >>>> email to [email protected]. >>>> For more options, visit https://groups.google.com/d/optout. >>> >>> -- >>> -- >>> v8-dev mailing list >>> [email protected] >>> http://groups.google.com/group/v8-dev >>> --- >>> You received this message because you are subscribed to the Google Groups >>> "v8-dev" group. >>> To unsubscribe from this group and stop receiving emails from it, send an >>> email to [email protected]. >>> For more options, visit https://groups.google.com/d/optout. >> >> -- >> -- >> v8-dev mailing list >> [email protected] >> http://groups.google.com/group/v8-dev >> --- >> You received this message because you are subscribed to the Google Groups >> "v8-dev" group. >> To unsubscribe from this group and stop receiving emails from it, send an >> email to [email protected]. >> For more options, visit https://groups.google.com/d/optout. > > -- > -- > v8-dev mailing list > [email protected] > http://groups.google.com/group/v8-dev > --- > You received this message because you are subscribed to the Google Groups > "v8-dev" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected]. > For more options, visit https://groups.google.com/d/optout. -- -- v8-dev mailing list [email protected] http://groups.google.com/group/v8-dev --- You received this message because you are subscribed to the Google Groups "v8-dev" group. 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