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[jira] [Created] (HBASE-20628) SegmentScanner does over-comparing


stack created HBASE-20628:
-----------------------------

             Summary: SegmentScanner does over-comparing
                 Key: HBASE-20628
                 URL: https://issues.apache.org/jira/browse/HBASE-20628
             Project: HBase
          Issue Type: Sub-task
          Components: Performance
            Reporter: stack
            Assignee: stack
             Fix For: 2.0.1


Flushing memstore is taking too long. It looks like we are doing a bunch of comparing out of a new facility in hbase2, the Segment scanner at flush time.

Below is a patch from [~anoop.hbase]. I had a similar more hacky version. Both undo the extra comparing we were seeing in per tests.

[~anastas] and [~eshcar]. Need your help please.

As I read it, we are trying to flush the memstore snapshot (default, no IMC case). There is only ever going to be one Segment involved (even if IMC is enabled); the snapshot Segment. But the getScanners is returning a list of Scanners and the scan is via the generic SegmentScanner which is all about Scanning over multiple Segments of a pipeline and so seems to do more work than is necessary in this case at least. It also supports scanning backwards which is not needed when trying to flush memstore.

Do you see a problem doing a version of Anoops patch (whether IMC or not)? It makes a big difference in general throughput when the below patch is in place. Thanks.


{code}
diff --git a/hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/MemStoreSnapshot.java b/hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/MemStoreSnapshot.java
index cbd60e5da3..c3dd972254 100644
--- a/hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/MemStoreSnapshot.java
+++ b/hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/MemStoreSnapshot.java
@@ -40,7 +40,8 @@ public class MemStoreSnapshot implements Closeable {
     this.cellsCount = snapshot.getCellsCount();
     this.memStoreSize = snapshot.getMemStoreSize();
     this.timeRangeTracker = snapshot.getTimeRangeTracker();
-    this.scanners = snapshot.getScanners(Long.MAX_VALUE, Long.MAX_VALUE);
+    //this.scanners = snapshot.getScanners(Long.MAX_VALUE, Long.MAX_VALUE);
+    this.scanners = snapshot.getScannersForSnapshot();
     this.tagsPresent = snapshot.isTagsPresent();
   }

diff --git a/hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/Segment.java b/hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/Segment.java
index 70074bf3b4..279c4e50c8 100644
--- a/hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/Segment.java
+++ b/hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/Segment.java
@@ -33,6 +33,7 @@ import org.apache.hadoop.hbase.KeyValueUtil;
 import org.apache.hadoop.hbase.io.TimeRange;
 import org.apache.hadoop.hbase.util.Bytes;
 import org.apache.hadoop.hbase.util.ClassSize;
+import org.apache.hadoop.hbase.util.CollectionBackedScanner;
 import org.apache.yetus.audience.InterfaceAudience;
 import org.slf4j.Logger;
 import org.apache.hbase.thirdparty.com.google.common.annotations.VisibleForTesting;
@@ -130,6 +131,10 @@ public abstract class Segment {
     return Collections.singletonList(new SegmentScanner(this, readPoint, order));
   }

+  public List<KeyValueScanner> getScannersForSnapshot() {
+    return Collections.singletonList(new CollectionBackedScanner(this.cellSet.get(), comparator));
+  }
+
   /**
    * @return whether the segment has any cells
    */
{code}





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