> On Sep 1, 2017, at 2:36 AM, Benedikt Meurer <[email protected]> wrote: > > I think we're trying to solve an actual problem, plus a problem that we might > not have. Let's just keep the C++ version of RunMicrotaskQueue for now and > work on the real issue only. This code doesn't change often and there's > otherwise no advantage but a lot of potential breakage when messing with > strong-rooted JSFunctions, JSEntryStub and friends.
Which is the "actual" problem, and which is the one we might not have? > -- Benedikt > >> On Fri, Sep 1, 2017 at 8:28 AM Caitlin Potter <[email protected]> wrote: >> >>> 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. 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