title: Spark - Shuffle Read Blocked Time
body: <p>Lately I've been tuning the performance of some large, shuffle heavy jobs. Looking at the spark UI, I noticed an option called "Shuffle Read Blocked Time" under the additional metrics section.</p>
  
  <p>This "Shuffle Read Blocked Time" seems to account for upwards of 50% of the task duration for a large swath of tasks.</p>
  
  <p>While I can intuit some possibilities for what this means, I can't find any documentation that explains what it actually represents. Needless to say, I also haven't been able to find any resources on mitigation strategies.</p>
  
  <p>Can anyone provide some insight into how I might reduce Shuffle Read Blocked Time?</p>