benchmark: disable concurrent recompilation+sweeping?

[bnoordhuis]

I've noticed that many benchmarks have lots of variance unless you pass --noconcurrent_sweeping --noconcurrent_recompilation which forces V8 to run the mark-sweep GC phase and the optimizing compiler on the main thread.

Does it make sense to run with those flags by default? Considerations:

1. It makes the benchmarks more reliable.
2. It's not representative of how node is normally run.
3. Micro-benchmarks aren't representative of normal use either.
4. Is the variance indicative of problems with V8's thread pool?

(I'd say the answer to 4. is no, variable latency is practically a given with thread pools, but I haven't extensively tested what impact different values for --v8-pool-size=... have.)

cc @nodejs/benchmarking

[joyeecheung]

I am a bit worried that this could make the microbenchmarks even more micro and results in optimizations landing when they can cause slowdowns in real-word usage, or prevent optimizations from landing even they benefit real-world usage..

[kunalspathak]

Having the flag will also make it difficult for other VMs to get comparable results with node-v8 which was one of the thing that #9615 addressed.

[hashseed]

V8 has a --predictable flag to reduce performance flakiness at the cost of running somewhat slower, by turning off a set of concurrency features.

[Trott]

This issue has been inactive for sufficiently long that it seems like perhaps it should be closed. Feel free to re-open (or leave a comment requesting that it be re-opened) if you disagree. I'm just tidying up and not acting on a super-strong opinion or anything like that.