GenShen: Regression in LRU cache benchmark

From Christine Flood, see https://github.com/openjdk/jdk/pull/14185#issuecomment-1579278537 :

I wrote an LRU program back in 2017 which allocates trees and stores them in an array in a round robin fashion, freeing the last allocated. At the time this was written it's purpose was to show how generational GCs can hit the wall and start performing very badly. I ran this on a clean openjdk build, a genshen build in generational mode and a genshen build in non-generational mode. These results are repeatable for me.

I would like to understand where the degradation is coming from before moving forward with this patch since it appears to penalize those who wish to just run traditional Shenandoah.

Clean
cflood@fedora java_programs]$ ~/genshen/cleanjdk/build/linux-x86_64-server-release/images/jdk/bin/java -XX:+UnlockExperimentalVMOptions -XX:+UseShenandoahGC LRU 1000 1000
Took 341892ms to allocate 1000000 trees in a cache of 1000

Genshen generational (we expect this to be bad)
[cflood@fedora java_programs]$ ~/genshen/jdk/build/linux-x86_64-server-release/images/jdk/bin/java -XX:+UnlockExperimentalVMOptions -XX:+UseShenandoahGC -XX:ShenandoahGCMode=generational LRU 1000 1000
Took 442012ms to allocate 1000000 trees in a cache of 1000

Genshen non-generational (shows what I feel is a significant degradation from the clean build)
[cflood@fedora java_programs]$ ~/genshen/jdk/build/linux-x86_64-server-release/images/jdk/bin/java -XX:+UnlockExperimentalVMOptions -XX:+UseShenandoahGC LRU 1000 1000
Took 395679ms to allocate 1000000 trees in a cache of 1000

I think that generational Shenandoah can be a big win for some applications, but I want to fully understand the cost for all applications.

I can't attach a .java file so here it is inline in the post.

class TreeNode {
  public TreeNode left, right;
  public int val;
}

public class LRU {
static int cache_size;
static int reps;
static int tree_height=16;

private static TreeNode[] trees;

private static int getIndex(int i) {return i % cache_size;}
private static TreeNode makeTree(int h) {
    if (h == 0) { return null;}
    else {
    TreeNode res = new TreeNode();
    res.left = makeTree(h - 1);
    res.right = makeTree(h - 1);
    res.val = h;
    return res;
}
}

public static void main(String[] args) {
    if (args.length != 2) {
        System.err.println("LRU requires args: cache_size reps");
        return;
    }
    cache_size = Integer.parseInt(args[0]);
    reps = Integer.parseInt(args[1]) * cache_size;
    trees = new TreeNode[cache_size];

    long start = System.currentTimeMillis();
    for (int i = 0; i < reps; i++)
        trees[getIndex(i)] = makeTree(tree_height);
    long end = System.currentTimeMillis();
    long ms = end - start;

    System.out.println("Took " + ms + "ms to allocate " + reps + " trees in a cache of " + cache_size);
}
}