Perf: Improve performance of multithreaded --check and undefined-field diagnostic

[tomlau10]

Recently I am integrating LuaLs into our project's CI and I am looking for ways to speed up the process. The following are 2 simple optimizations that I have found:

Work distribution in multi-threaded --check

I found this recently added multi-threaded --check in #2638. Upon some code reviewing I suspect that the hash function used to distribute work across multi threads is not that perfect, which limits its full potential. I simply changed it to Round Robin (after sorting the file list of course), and this yields another 10% performance gain in my project workspace (500+ lua files). Benchmarks as follow:

• CPU: Intel(R) Core(TM) i7-4790 CPU @ 3.60GHz

threads	ori-1	ori-2	ori-3	ori-avg		new-1	new-2	new-3	new-avg		diff
1	38.67	38.88	38.76	38.77		38.77	39.35	38.75	38.95667		0%
2	25.57	25.64	25.79	25.66667		22.93	22.89	22.86	22.89333		-11%
4	17.55	17.93	17.41	17.63		15.81	15.22	15.33	15.45333		-12%

I would like to mention @emmericp here as this function is originally implemented by him, to see if this small optimization can help shaving off a few more seconds in his project 😄 (great work btw 👍 )

Early break in undefined-field diagnostic

Upon some profiling, the undefined-field check is quite time consuming. By looking into the code I found the early break logic of it can be optimized. The #vm.getDefs(src) > 0 is used to check if there is any definition for a given src object, and there is no need to fetch the full definitions list. We can actually return as soon as the 1st definition is found. This yields a 25% performance gain in this single check within my workspace 🚀

• setting "diagnostics.severity": { "undefined-field": "Error!" } in .luarc.json then run with --checklevel=Error
  NOTE: The following is run in single thread to avoid confusion with the above threaded optimization.

	1	2	3	avg
ori	15.4	15.39	15.44	15.41
new	11.51	11.26	11.26	11.34333
diff				-26%

[C3pa]

Very nice tomlau10! Can we get some insight into how you profiled the LuaLS? I am primarily interested in how you found which functions to optimize.

[sumneko]

Great work!

[tomlau10]

Can we get some insight into how you profiled the LuaLS?

@C3pa Nothing advanced, I just profile each diagnostics one by one 😂 by setting up the .luarc.json in my workspace like this:

"diagnostics.severity": { "undefined-field": "Error!" }

And then I run LuaLS with --check --checklevel=Error, which will only check against those with severity Error or above.
(In this example setup is the undefined-field)

During my profiling, I also found that the invisible diagnostic is quite time consuming as well (on par with undefined-field check). But currently I have no idea what can be optimized. 😕

---

edit:

I am primarily interested in how you found which functions to optimize.

Oh~ I guess you are asking how I found that #vm.getDefs(src) > 0 can be optimized? Nothing special, just by some programmer instincts 😄

[C3pa]

Interesting. Thanks for your insight! I thought you might have used some kind of profiler setup.