Bug 240984 – [JSC] Promise/async is about 6x slower than v8

• Status: NEW
• Component: JavaScriptCore

[Jarred Sumner]

Created attachment 459797 [details]
safari

Running https://github.com/v8/promise-performance-tests in jsc vs v8 reports the following on macOS arm64

with JSC:

❯ fd . --exec bun '{}'
Time(doxbee-async-es2017-native): 55.3 ms.
Time(doxbee-promises-es2015-native): 60.5 ms.
Time(doxbee-async-es2017-babel): 73.4 ms.
Time(parallel-promises-es2015-native): 255.8 ms.
Time(parallel-async-es2017-babel): 383.9 ms.
Time(parallel-async-es2017-native): 414.5 ms.
Time(fibonacci-async-es2017-babel): 785.8 ms.
Time(fibonacci-async-es2017-native): 1923.3 ms.

with V8:

❯ fd . --exec node '{}'
Time(doxbee-async-es2017-native): 20.9 ms.
Time(doxbee-promises-es2015-native): 26.6 ms.
Time(doxbee-async-es2017-babel): 42 ms.
Time(parallel-promises-es2015-native): 55.6 ms.
Time(parallel-async-es2017-native): 63.1 ms.
Time(parallel-async-es2017-babel): 82.7 ms.
Time(fibonacci-async-es2017-native): 110.9 ms.
Time(fibonacci-async-es2017-babel): 222.6 ms.

the bluebird ones were skipped because bun doesn't support node's async_hooks

In another microbench, we can run the following in jsc's shell with Mitata (https://github.com/evanwashere/mitata)

import {run, bench} from './mitata/src/cli.mjs';

bench("sync", () => {});
bench("async", async () => {});

await run();


That reports:

❯ node a.mjs
cpu: Apple M1 Max
runtime: node v18.2.0 (arm64-darwin)

benchmark      time (avg)             (min … max)       p75       p99      p995
------------------------------------------------- -----------------------------
sync       320.19 ps/iter  (304.1 ps … 258.77 ns)  316.7 ps  358.3 ps  370.8 ps
async       57.01 ns/iter  (52.72 ns … 122.63 ns)  59.06 ns 107.38 ns 115.63 ns


❯ jsc a.mjs -m
cpu: unknown
runtime: unknown (unknown)

benchmark      time (avg)             (min … max)       p75       p99      p995
------------------------------------------------- -----------------------------
sync       322.29 ps/iter     (300 ps … 15.65 ns)  316.7 ps  387.5 ps  416.6 ps
async      369.04 ns/iter (357.71 ns … 519.04 ns) 365.94 ns  476.6 ns 519.04 ns


Running this in Safari (attached screenshot) shows async to be about 565 ns/iter, something like 10x slower compared to V8/node


Running the https://github.com/v8/promise-performance-tests/blob/master/src/parallel-async-es2017-native.js with Instruments, shows this: 
https://user-images.githubusercontent.com/709451/170589528-852d693f-94c3-40ab-a460-f697225e150b.png

We can see that a lot of time is spent in bookkeeping – preparing to call microtasks, creating JSC::JSMicrotask, destroying JSC::JSMicrotask, writing to handleset, etc

[Jarred Sumner]

If we look at V8's implementation

We can see that they don't use a generic JSC::JSMicrotask-like type with a JS implementation. Instead, they have specific types for each job.

https://github.com/v8/v8/blob/f32335fea75b7bde495e0800d7f7349253f81a7c/src/builtins/builtins-microtask-queue-gen.cc#L118


They also implement an optimization for resolving promises that skips allocating a temporary promise.
 
https://github.com/v8/v8/blob/f32335fea75b7bde495e0800d7f7349253f81a7c/src/builtins/promise-jobs.tq#L15

[Jarred Sumner]

These patches had some impact:
- https://github.com/Jarred-Sumner/WebKit/commit/2640da9d0023f6e70ef0a1f7331f2eeb8b93717b
- https://github.com/Jarred-Sumner/WebKit/commit/e049be38d79ec80be0a4e6687f9d99f6e97ac208
- https://github.com/Jarred-Sumner/WebKit/commit/d8fdbb963f79baa30848fdb77914486055a6bc57
- https://github.com/Jarred-Sumner/WebKit/commit/e63c5d1faf1ba7c00f45f90acef0a0f30141ed7d

There was also a JSC::Strong in Bun's code that was not good.

async function calls are about 59% faster in bun than default jsc shell now

[Radar WebKit Bug Importer]

<rdar://problem/94314760>

[Yusuke Suzuki]

(In reply to Jarred Sumner from comment #1)
> 
> They also implement an optimization for resolving promises that skips
> allocating a temporary promise.
>  
> https://github.com/v8/v8/blob/f32335fea75b7bde495e0800d7f7349253f81a7c/src/
> builtins/promise-jobs.tq#L15

Note that JSC is doing the same thing internally :)

[Brent Fulgham]

*** Bug 243472 has been marked as a duplicate of this bug. ***