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From "Benoit Sigoure (JIRA)" <j...@apache.org>
Subject [jira] Created: (HBASE-2959) Scanning always starts at the beginning of a row
Date Fri, 03 Sep 2010 18:00:35 GMT
Scanning always starts at the beginning of a row

                 Key: HBASE-2959
                 URL: https://issues.apache.org/jira/browse/HBASE-2959
             Project: HBase
          Issue Type: Bug
          Components: regionserver
    Affects Versions: 0.89.20100621, 0.20.6, 0.20.5, 0.20.4
            Reporter: Benoit Sigoure
            Priority: Blocker

In HBASE-2248, the code in {{HRegion#get}} was changed like so:
-  private void get(final Store store, final Get get,
-    final NavigableSet<byte []> qualifiers, List<KeyValue> result)
-  throws IOException {
-    store.get(get, qualifiers, result);
+  /*
+   * Do a get based on the get parameter.
+   */
+  private List<KeyValue> get(final Get get) throws IOException {
+    Scan scan = new Scan(get);
+    List<KeyValue> results = new ArrayList<KeyValue>();
+    InternalScanner scanner = null;
+    try {
+      scanner = getScanner(scan);
+      scanner.next(results);
+    } finally {
+      if (scanner != null)
+        scanner.close();
+    }
+    return results;
So instead of doing a {{get}} straight on the {{Store}}, we now open a scanner.  The problem
is that we eventually end up in {{ScanQueryMatcher}} where the constructor does: {{    this.startKey
= KeyValue.createFirstOnRow(scan.getStartRow());}}.  This entails that if we have a very wide
row (thousands of columns), the scanner will need to go through thousands of {{KeyValue}}s
before finding the right entry, because it always starts from the beginning of the row, whereas
before it was much more straightforward.

This problem was under the radar for a while because the overhead isn't too unreasonable,
but later on, {{incrementColumnValue}} was changed to do a {{get}} under the hood.  At StumbleUpon
we do thousands of ICV per second, so thousand of times per second we're scanning some really
wide rows.  When a row is contented, this results in all the IPC threads being stuck on acquiring
a row lock, while one thread is doing the ICV (albeit slowly due to the excessive scanning).
 When all IPC threads are stuck, the region server is unable to serve more requests.

As a nice side effect, fixing this bug will make {{get}} and {{incrementColumnValue}} faster,
as well as the first call to {{next}} on a scanner.

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