[YARN-5686] DefaultContainerExecutor random working dir algorigthm skews results

• Type: Bug
• Status: Resolved
• Priority: Minor
• Resolution: Fixed
• Fix Version/s: 2.9.0, 3.0.0-alpha2
• Hadoop Flags: Reviewed
• Assignee: Vrushali C
• Reporter: Miklos Szegedi
• Created: 28/Sep/16 23:46
• Updated: 27/Oct/16 09:03
• Resolved: 27/Oct/16 08:34

Description

long randomPosition = RandomUtils.nextLong() % totalAvailable;
    ...
    while (randomPosition > availableOnDisk[dir]) {
      randomPosition -= availableOnDisk[dir++];
    }

The code above selects a disk based on the random number weighted by the free space on each disk respectively. For example, if I have two disks with 100 bytes each, totalAvailable is 200. The value of randomPosition will be 0..199. 0..99 should select the first disk, 100..199 should select the second disk inclusively. Random number 100 should select the second disk to be fair but this is not the case right now.

We need to use

while (randomPosition >= availableOnDisk[dir])

instead of

while (randomPosition > availableOnDisk[dir])