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https://issues.apache.org/jira/browse/HBASE-20628?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16487819#comment-16487819
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stack commented on HBASE-20628:
-------------------------------
Here is a comment from Anoop on a back-channel (hope you don't mind me quoting
you Anoop) that might help: "In 1.4 we can see that we will create a snapshot
for flush and that will use CollectionBackedScanner as scanner. The
MemStoreScanner is used for normal user scan over memstore. In 2.0 all will use
SegmentScanner. Seems the SegmentScanner is making the cost. Specially impl
like reseek(). We will get calls to reseek() so many times within a flush op...
> SegmentScanner does over-comparing when one flushing
> ----------------------------------------------------
>
> Key: HBASE-20628
> URL: https://issues.apache.org/jira/browse/HBASE-20628
> Project: HBase
> Issue Type: Sub-task
> Components: Performance
> Reporter: stack
> Assignee: stack
> Priority: Critical
> Fix For: 2.0.1
>
> Attachments: HBASE-20628.branch-2.0.001.patch,
> HBASE-20628.branch-2.0.001.patch, hits-20628.png
>
>
> 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 perf 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
> one) Scanners and the scan is via the generic SegmentScanner which is all
> about a bunch of stuff we don't need when doing a flush so it seems to do
> more work than is necessary. 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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