HDFS unit test failure in AArch64. TestDirectoryScanner.testThrottling: Throttle is too permissive

[Guangming Zhang]

Hadoop Version:  3.1.0-SNAPSHOT
Environment: ARMv8 AArch64, Ubuntu16.04
Description:  Hi,  When I ran the HDFS unit test and got a failure in 
TestDirectoryScanner.java test case :
TestDirectoryScanner.testThrottling:624 Throttle is too permissive
detail:
                Running 
org.apache.hadoop.hdfs.server.datanode.TestDirectoryScanner
                Tests run: 7, Failures: 1, Errors: 0, Skipped: 0, Time elapsed: 
227.046 sec <<< FAILURE! - in 
org.apache.hadoop.hdfs.server.datanode.TestDirectoryScanner
                
testThrottling(org.apache.hadoop.hdfs.server.datanode.TestDirectoryScanner)  
Time elapsed: 198.014 sec  <<< FAILURE!
                java.lang.AssertionError: Throttle is too permissive
                                                at 
org.junit.Assert.fail(Assert.java:88)
                                                at 
org.junit.Assert.assertTrue(Assert.java:41)
                                                at 
org.apache.hadoop.hdfs.server.datanode.TestDirectoryScanner.testThrottling(TestDirectoryScanner.java:624)

And below is the failure part of source code TestDirectoryScanner.java:
                      ...........
      while ((retries > 0) && ((ratio < 7f) || (ratio > 10f))) {
        scanner = new DirectoryScanner(dataNode, fds, conf);
        ratio = runThrottleTest(blocks);
        retries -= 1;
      }

      // Waiting should be about 9x running.
      LOG.info("RATIO: " + ratio);
      assertTrue("Throttle is too restrictive", ratio <= 10f);
      assertTrue("Throttle is too permissive", ratio >= 7f);
    ............
    private float runThrottleTest(int blocks) throws IOException {
      scanner.setRetainDiffs(true);
      scan(blocks, 0, 0, 0, 0, 0);
      scanner.shutdown();
      assertFalse(scanner.getRunStatus());
      return (float)scanner.timeWaitingMs.get() / scanner.timeRunningMs.get();
    }
  .............

The ratio in my test is 6.0578866, which is smaller than 7f in the code. So the 
code thrown out an assertTrue failure.
My questions are:
1. Why the ratio was set between 7f and 10f, is it a empirical value?
       2. The ratio is smaller than 7f in AArch64 platform, is this value 
within normal range?

Could anyone help? Thanks a lot.

Best Regards
Guangming Zhang
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