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Date: Fri, 3 Sep 2010 14:55:33 -0400 (EDT)
From: "ryan rawson (JIRA)" <jira@apache.org>
To: issues@hbase.apache.org
Subject: [jira] Commented: (HBASE-2959) Scanning always starts at the
 beginning of a row
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    [ https://issues.apache.org/jira/browse/HBASE-2959?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12906016#action_12906016 ] 

ryan rawson commented on HBASE-2959:
------------------------------------

We start at the beginning of the row because there might be delete family
entries there. If we didn't pick those up we'd not properly mask deletes

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.
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.
{{incrementColumnValue}} faster, as well as the first call to {{next}} on a
scanner.


> 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: 0.20.4, 0.20.5, 0.20.6, 0.89.20100621
>            Reporter: Benoit Sigoure
>            Priority: Blocker
>
> In HBASE-2248, the code in {{HRegion#get}} was changed like so:
> {code}
> -  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;
>    }
> {code}
> 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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