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From "Jean-Daniel Cryans (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HBASE-2959) Scanning always starts at the beginning of a row
Date Tue, 07 Sep 2010 20:52:33 GMT

    [ https://issues.apache.org/jira/browse/HBASE-2959?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12906966#action_12906966

Jean-Daniel Cryans commented on HBASE-2959:

One other way this issue is screwing us is that seeking through all those cells will also
put more load on the block cache (if the row is contained in more than one block). I've been
monitoring our deployment closely and the load is definitely much higher on our machines,
and the fsReadLatency_avg_time is 10x on average. Also, I grepped through our logs how many
times we see evictions per day; our most loaded server had at max 70 evictions before the
upgrade, yesterday our busiest server did 2246 evictions and today it's already at 1503.

> 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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