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https://issues.apache.org/jira/browse/HBASE-20628?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16493107#comment-16493107
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Anoop Sam John commented on HBASE-20628:
----------------------------------------
We have 10 cols in each row. And the versions (max versions) for CF is default
1. So in the SQM logic, we get INCLUDE_AND_SEEK_NEXT_COL and in that flow we
will try reseek to the last cell in this column. There is no extra versions
there but the reseek call will happen. We need the versions tracking in flush
because the flush itself might remove some unwanted versions.
> 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
> Priority: Critical
> Fix For: 2.0.1
>
> Attachments: HBASE-20628.branch-2.0.001 (1).patch,
> HBASE-20628.branch-2.0.001.patch, HBASE-20628.branch-2.0.001.patch,
> HBASE-20628.branch-2.0.002.patch, Screen Shot 2018-05-25 at 9.38.00 AM.png,
> 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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