[HBASE-9488] Improve performance for small scan

• Type: Improvement
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 0.98.0, 0.96.0, 0.94.13
• Component/s: Client, Performance, Scanners
• Hadoop Flags: Reviewed
• Release Note: Better perfomance for small scan (e.g. scan range is within one data block(64KB)) through setting 'small' attribute as true in Scan Object
• Assignee: Chunhui Shen
• Reporter: Chunhui Shen
• Created: 10/Sep/13 03:06
• Updated: 08/Nov/13 00:03
• Resolved: 18/Sep/13 02:07

Description

review board:
https://reviews.apache.org/r/14059/

Performance Improvement
Test shows about 1.5~3X improvement for small scan where limit<=50 under cache hit ratio=100%.
See more performance test result from the picture attachment

Usage:
Scan scan = new Scan(startRow,stopRow);
scan.setSmall(true);
ResultScanner scanner = table.getScanner(scan);

Set the new 'small' attribute as true for scan object, others are the same

Now, one scan operation would call 3 RPC at least:
openScanner();
next();
closeScanner();

I think we could reduce the RPC call to one for small scan to get better performance

Also using pread is better than seek+read for small scan (For this point, see more on HBASE-7266)

Implements such a small scan as the patch, and take the performance test as following:

a.Environment：
patched on 0.94 version
one regionserver;
one client with 50 concurrent threads;
KV size:50/100;
100% LRU cache hit ratio;
Random start row of scan

b.Results:
See the picture attachment