Consider the following test:

    /*
     * @test
     * @run testng/othervm -Ddataproviderthreadcount=4 ParallelTestNg
     */
    public class ParallelTestNg {

        @DataProvider(parallel = true)
        public static Iterator<Object[]> integers() {
            List<Object[]> args = new LinkedList<>();
            for (int i = 0; i < 10; i++) {
                args.add(new Object[]{i});
            }
            return args.iterator();
        }

        @Test(dataProvider = "integers")
        public void test(Integer arg) throws InterruptedException {
            TimeUnit.MILLISECONDS.sleep(100);
            fail(String.valueOf(arg));
        }
    }

"dataproviderthreadcount=4" passed to TestNG in conjunction with "parallel =
true" element passed to @DataProvider annotation instructs TestNG to dispatch
invocations of "test" to a thread pool of size 4.

This was designed to speed up executions of tests with data supplied from data
providers.

However, when this test is run (jtreg -verbose:all ... ), it reports different
numbers of tests failed each time:

    TEST RESULT: Failed. Execution failed: `main' threw exception: java.lang.Exception: failures: 6
    
or
    
    TEST RESULT: Failed. Execution failed: `main' threw exception: java.lang.Exception: failures: 9
    
etc. In the same output TestNG consistently reports the same correct number:

    ===============================================
    ParallelTestNg.java
    Total tests run: 10, Failures: 10, Skips: 0
    ===============================================