Server GC mode for fsc.exe

[latkin]

I did some testing today, trying out server GC mode on fsc.exe. Results look promising:

Compiler	Target	Trials	Workstation GC	Server GC	% Improvement
proto debug fsc	Debug FSharp.Core.dll	10	17.4 s	15.1 s	13.2%
proto debug fsc	Release FSharp.Compiler.dll	3	64.2 s	54.4 s	15.3%
Release NGENed fsc	Test area fsharp\core\portable	2	75.3 s	70.5 s	6.4 %
Release NGENed fsc	Test tool FSharp.Compiler.Hosted	10	0.903 s	0.705 s	22.0 %

FWIW the Roslyn command-line compilers use server GC.

Potential downsides:

• On single-core machines, server GC might actually be slower
• With server mode, GCs are less-frequent but bigger. Result is that process working set gets much larger in between GCs.
• Server mode gives higher throughput, but worse responsiveness (collections are more intrusive). That's ok for command-line compiler, but we would not want to change it for IDE language service compiler backend.

Note also that we already have GCLatencyMode.Batch enabled now in fsc, which gave us a nice perf boost.

I'll send a PR next week adding this, unless anyone objects.

[dsyme]

Well, that looks great :)

[dsyme]

@vzarytovskii @KevinRansom I'm reopening as I'd like to revalidate whether using gcServer is a perf win or not. On bootstrapping the compiler it gives a noticeable slowdown.

[dsyme]

I was quite surprised to find out that we use server GC for fsc.exe on .NET Framework which has an ​fsc.exe.config​ with GC Server enabled as well as a few other settings.

You can see this applies in the product here:

​C:\Program Files\Microsoft Visual Studio\2022\Enterprise\Common7\IDE\CommonExtensions\Microsoft\FSharp\Tools\fsc.exe.config​

This surprised me. My experiments on compiling-the-compiler show a 30% slowdown with this setting. I think this should never have been re-enabled, despite what's reported above. Either way we need to revalidate this setting.

When we build our .NET 6/7 compiler in artifacts is lays down a ​fsc.dll.config​ with ​​ as well as a couple of other settings. However I don't know if .NET 6/7 even takes any notice of these config files at all

[dsyme]

Closing in favour of #13739