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From "Sameer Paranjpye (JIRA)" <j...@apache.org>
Subject [jira] Created: (HADOOP-87) SequenceFile performance degrades substantially compression is on and large values are encountered
Date Thu, 16 Mar 2006 22:51:02 GMT
SequenceFile performance degrades substantially compression is on and large values are encountered
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         Key: HADOOP-87
         URL: http://issues.apache.org/jira/browse/HADOOP-87
     Project: Hadoop
        Type: Improvement
  Components: io  
    Versions: 0.1    
    Reporter: Sameer Paranjpye
     Fix For: 0.1


The code snippet in quesiton is:

     if (deflateValues) {
        deflateIn.reset();
        val.write(deflateIn);
        deflater.reset();
        deflater.setInput(deflateIn.getData(), 0, deflateIn.getLength());
        deflater.finish();
        while (!deflater.finished()) {
          int count = deflater.deflate(deflateOut);
          buffer.write(deflateOut, 0, count);
        }
      } else {
  
A couple of issues with this code:

1. The value is serialized to the 'deflateIn' buffer which is an instance of 'DataOutputBuffer',
this grows as large as needed to store the serialized value and stays as large as the largest
serialized value encountered. If, for instance a stream has a single 8MB value followed by
several 8KB values the size of the buffer stays at 8MB. The problem is that the *entire* 8MB
buffer is always copied over the JNI boundary regardless of the size of the value. We've observed
this over several runs where compression performance degrades by a couple of orders of magnitude
when a very large value is encountered. Shrinking the buffer fixes the problem.

2. Data is copied lots of times. First the value is serialized into 'deflateIn'. Second, the
value is copied over the JNI boundary in *every* iteration of the while loop. Third, the compressed
data is copied piecemeal into 'deflateOut'. Finally, it is appended to 'buffer'.


Proposed fix:

1. Don't let big buffers persist. Allow 'deflateIn' to grow to a *persistent* maximum reasonable
size, say 64KB. If a larger value is encountered, grow the buffer in order to process the
value, then shrink it back to the maximum size. To do this, we add a 'reset' method which
takes a buffer size.

2. Don't use a loop to deflate. The maximum size of the output can be determined by 'maxOutputSize
= inputSize * 1.01 + 12'. This is the maximum output size that zlib will produce. We allocate
a large enough output buffer and compress everything in 1 pass. The output buffer, of course,
needs to shrink as well.




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