RFR: 8325403: Add SystemGC JMH benchmarks [v2]

[Stefan Johansson]

On Tue, 13 Feb 2024 11:39:53 GMT, Thomas Schatzl <tschatzl at openjdk.org> wrote:

>> Stefan Johansson has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Restructure and rename
>
> test/micro/org/openjdk/bench/vm/gc/system_gc/AllDead.java line 23:
> 
>> 21:  * questions.
>> 22:  */
>> 23: package org.openjdk.bench.vm.gc.system_gc;
> 
> I do not like a package name with an underscore in it, but I can live with that.

I kind of agree, maybe `org.openjdk.bench.vm.gc.systemgc` would have been better, but I found this and at least `_` is allowed:
https://docs.oracle.com/javase/tutorial/java/package/namingpkgs.html

> test/micro/org/openjdk/bench/vm/gc/system_gc/AllDead.java line 39:
> 
>> 37: 
>> 38: @BenchmarkMode(Mode.SingleShotTime)
>> 39: @Fork(value=25, jvmArgsAppend={"-Xmx5g", "-Xms5g", "-Xmn3g"})
> 
> Please use `-XX:+AlwaysPreTouch` by default. Otherwise some of the tests degrade to testing the memory paging.
> 
> E.g. `gc+cpu` logging gives
> 
> `[0,837s][info][gc,cpu   ] GC(0) User=0,31s Sys=0,15s Real=0,45s`
> 
> when running with serial gc, i.e. 1/3rd of time is spent outside paging in memory.

As discussed I'm not against it, I usually run with `-XX:+AlwaysPreTouch` and bound to a single NUMA node when doing my own benchmarking. So adding this as a default is ok with me.

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PR Review Comment: https://git.openjdk.org/jdk/pull/17784#discussion_r1487731047
PR Review Comment: https://git.openjdk.org/jdk/pull/17784#discussion_r1487730258