> On Sep 1, 2017, at 1:57 AM, Yang Guo <[email protected]> wrote: > > I don't like strong-rooting a JSFunction either. You will get some issues > with the serializer when creating the snapshot.
Fair enough, but then we do need a way to enter the stub cleanly. > I also feel like I don't understand the problem. We run the MicrotaskQueue > when we are about to return to the embedder with re-entrancy level of 0, so > we already need to leave JS and enter JS once. With the micro-benchmark, only > one microtask gets queued per microtask run, so you don't benefit from > staying in JS. Of course you could run the MicrotaskQueue before leaving JS, > but that doesn't seem like what you are trying to do. Even though only one task is enqueued at a time, RunMicrotasks() never finishes until the entire benchmark is run (since each resume enqueues another task). Many tasks are run in a single JS entry, whereas in v8 ToT you enter/exit JS thousands of times. > Cheers, > > Yang > >> On Fri, Sep 1, 2017 at 7:29 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. > > -- > > > > > • Yang Guo > • Google Germany GmbH > • Erika-Mann-Str. 33 > • 80636 Munich > • [email protected] > > Geschäftsführer: Paul Manicle, Halimah DeLaine Prado > Registergericht und -nummer: Hamburg, HRB 86891 Sitz der Gesellschaft: Hamburg > Diese E-Mail ist vertraulich. Wenn Sie nicht der richtige Adressat sind, > leiten Sie diese bitte nicht weiter, informieren Sie den Absender und löschen > Sie die E-Mail und alle Anhänge. Vielen Dank. This e-mail is confidential. If > you are not the right addressee please do not forward it, please inform the > sender, and please erase this e-mail including any attachments. Thanks. > -- > -- > 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.
