Generational ZGC regression with H2 database

[Brian S O'Neill]

I created a simple test which stresses a fresh H2 database by inserting 
a bunch of random rows into a simple key-value table. The H2 cache size 
is set to 4GB, and the max Java heap size is 12GB.

I ran the test using JDK-19 ZGC for 30 minutes, and the average 
throughput was 23,075 inserts per second. With generational ZGC, the 
average throughput dropped to 20,284 inserts per second, and 11 
allocation stalls were observed. The longest one lasted 724 
milliseconds. Note that no allocation stalls were observed with 
non-generational ZGC.

Here's the code:

import java.sql.*;
import java.util.*;

public class H2Perf {
     public static void main(String[] args) throws Exception {
         String path = args[0];

         long numToInsert = 100_000_000L;
         long maxDurationMillis = 30L * 60 * 1000;

         Class.forName("org.h2.Driver");
         String conString = "jdbc:h2:" + path + ";CACHE_SIZE=4000000";

         Connection con = DriverManager.getConnection(conString, "sa", "");

         try (Statement st = con.createStatement()) {
             st.execute("create table test_tab (" +
                        "name varchar(30) primary key," +
                        "val varchar(30) )");
         }

         PreparedStatement ps = con.prepareStatement
             ("insert into test_tab (name, val) values (?,?)");

         var rnd = new Random(234234);
         int total = 0;
         long durationMillis = 0;
         long start = System.currentTimeMillis();

         do {
             long n = rnd.nextLong();
             ps.setString(1, "key-" + n);
             ps.setString(2, "value-" + n);
             ps.execute();

             total++;

             if (total % 100_000 == 0) {
                 durationMillis = (System.currentTimeMillis() - start);
                 float rate = (float) ((total / (double) durationMillis) 
* 1000.0);
                 System.out.println("inserted: " + total + " @ " + rate 
+ " per second");
             }
         } while (total < numToInsert && durationMillis < 
maxDurationMillis);
     }
}