Take first task group for further execution

[zetanumbers]

Continuing from #153768 (comment).

I thought that storing a first group of tasks for immediate execution instead of pushing and immediately poping it from rayon's local task queue in par_slice would avoid overwhelming work stealing potentially blocking the original thread. So I've implemented this change.

8 threads benchmarks:

Benchmark	baseline~~9	new~take-first-group~1
	Time	Time	%
🟣 hyper:check	0.1110s	0.1086s	💚 -2.13%
🟣 hyper:check:initial	0.1314s	0.1298s	💚 -1.23%
🟣 hyper:check:unchanged	0.0771s	0.0755s	💚 -2.14%
🟣 clap:check	0.3787s	0.3757s	-0.80%
🟣 clap:check:initial	0.4680s	0.4564s	💚 -2.48%
🟣 clap:check:unchanged	0.2337s	0.2301s	💚 -1.52%
🟣 syn:check	0.4321s	0.4265s	💚 -1.31%
🟣 syn:check:initial	0.5586s	0.5401s	💚 -3.31%
🟣 syn:check:unchanged	0.3434s	0.3429s	-0.14%
🟣 regex:check	0.2755s	0.2661s	💚 -3.40%
🟣 regex:check:initial	0.3350s	0.3347s	-0.11%
🟣 regex:check:unchanged	0.1851s	0.1832s	💚 -1.01%
Total	3.5296s	3.4695s	💚 -1.70%
Summary	1.0000s	0.9837s	💚 -1.63%

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[lqd]

I wonder what variance, and noise level, you're seeing on your benchmarking machine? BTZ, does this remove some small overhead a few times, or does it translate to good results on bigger benchmarks as well?

[jieyouxu]

@rustbot reroll

[zetanumbers]

I wonder what variance, and noise level, you're seeing on your benchmarking machine? BTZ, does this remove some small overhead a few times, or does it translate to good results on bigger benchmarks as well?

Here's baseline compiler running against itself:

Benchmark	baseline~~9	baseline~~9
	Time	Time	%
🟣 hyper:check	0.1143s	0.1157s	💔 1.25%
🟣 hyper:check:initial	0.1405s	0.1420s	💔 1.08%
🟣 hyper:check:unchanged	0.0805s	0.0797s	💚 -1.02%
🟣 clap:check	0.3914s	0.3909s	-0.14%
🟣 clap:check:initial	0.4765s	0.4823s	💔 1.21%
🟣 clap:check:unchanged	0.2345s	0.2329s	-0.68%
🟣 syn:check	0.4328s	0.4303s	-0.58%
🟣 syn:check:initial	0.5820s	0.5731s	💚 -1.53%
🟣 syn:check:unchanged	0.3695s	0.3696s	0.02%
🟣 regex:check	0.2668s	0.2720s	💔 1.95%
🟣 regex:check:initial	0.3224s	0.3289s	💔 2.02%
🟣 regex:check:unchanged	0.1901s	0.1846s	💚 -2.92%
Total	3.6015s	3.6021s	0.02%
Summary	1.0000s	1.0006s	0.06%

I have run these benchmarks on various changes before and never seen all greens like above.