MAPREDUCE-6059. Speed up history server startup time (Siqi Li via aw)

(cherry picked from commit fd57ab2002f97dcc83d455a5e0c770c8efde77a4)

Conflicts:

	hadoop-mapreduce-project/CHANGES.txt

hadoop-mapreduce-project/CHANGES.txt

@@ -48,6 +48,8 @@ Release 2.7.0 - UNRELEASED
     from key and value at the end of the iteration to save memory. 
     (Zhihai Xu via kasha)
 
+    MAPREDUCE-6059. Speed up history server startup time (Siqi Li via aw)
+
   BUG FIXES
 
     MAPREDUCE-6210. Use getApplicationAttemptId() instead of getApplicationId()

hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-hs/src/main/java/org/apache/hadoop/mapreduce/v2/hs/HistoryFileManager.java

@@ -263,6 +263,10 @@ public class HistoryFileManager extends AbstractService {
     public HistoryFileInfo get(JobId jobId) {
       return cache.get(jobId);
     }
+
+    public boolean isFull() {
+      return cache.size() >= maxSize;
+    }
   }
 
   /**
@@ -668,6 +672,10 @@ public class HistoryFileManager extends AbstractService {
     for (FileStatus fs : timestampedDirList) {
       // TODO Could verify the correct format for these directories.
       addDirectoryToSerialNumberIndex(fs.getPath());
+    }
+    for (int i= timestampedDirList.size() - 1;
+        i >= 0 && !jobListCache.isFull(); i--) {
+      FileStatus fs = timestampedDirList.get(i); 
       addDirectoryToJobListCache(fs.getPath());
     }
   }