[jira] [Commented] (HBASE-6066) some low hanging read path improvement ideas

["Kannan Muthukkaruppan (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HBASE-6066?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13280755#comment-13280755 ] 

Kannan Muthukkaruppan commented on HBASE-6066:
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These are just some possible ideas. We probably need to dig deeper with a profiler to look for other improvements. In a quick and dirty hacky experiment I was able to get the single-threaded scan throughput from about 25MB/sec to 39MB/sec with some of these changes. But I am sure there is a lot of opportunity to tighten the code further.
                
> some low hanging read path improvement ideas 
> ---------------------------------------------
>
>                 Key: HBASE-6066
>                 URL: https://issues.apache.org/jira/browse/HBASE-6066
>             Project: HBase
>          Issue Type: Bug
>            Reporter: Kannan Muthukkaruppan
>
> I was running some single threaded scan performance tests for a table with small sized rows that is fully cached. Some observations...
> We seem to be doing several wasteful iterations over and/or building of temporary lists.
> 1) One such is the following code in HRegionServer.next():
> {code}
>    boolean moreRows = s.next(values, HRegion.METRIC_NEXTSIZE);
>    if (!values.isEmpty()) {
>      for (KeyValue kv : values) {              ------> #### wasteful in most cases
>        currentScanResultSize += kv.heapSize();
>    }
>    results.add(new Result(values));
> {code}
> By default the "maxScannerResultSize" is Long.MAX_VALUE. In those cases,
> we can avoid the unnecessary iteration to compute currentScanResultSize.
> 2) An example of a wasteful temporary array, is "results" in
> RegionScanner.next().
> {code}
>       results.clear();
>       boolean returnResult = nextInternal(limit, metric);
>       outResults.addAll(results);
> {code}
> results then gets copied over to outResults via an addAll(). Not sure why we can directly collect the results in outResults.
> 3) Another almost similar exmaple of a wasteful array is "results" in StoreScanner.next(), which eventually also copies its results into "outResults".
> 4) Reduce overhead of "size metric" maintained in StoreScanner.next().
> {code}
>   if (metric != null) {
>      HRegion.incrNumericMetric(this.metricNamePrefix + metric,
>                                copyKv.getLength());
>   }
>   results.add(copyKv);
> {code}
> A single call to next() might fetch a lot of KVs. We can first add up the size of those KVs in a local variable and then in a finally clause increment the metric one shot, rather than updating AtomicLongs for each KV.
> 5) RegionScanner.next() calls a helper RegionScanner.next() on the same object. Both are synchronized methods. Synchronized methods calling nested synchronized methods on the same object are probably adding some small overhead. The inner next() calls isFilterDone() which is a also a synchronized method. We should factor the code to avoid these nested synchronized methods.

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