IoBuffer.normalizeCapacity improvement

Key: DIRMINA751
URL: https://issues.apache.org/jira/browse/DIRMINA751
Project: MINA
Issue Type: Improvement
Components: Core
Affects Versions: 2.0.0RC1
Environment: N/A
Reporter: Bogdan Pistol
Priority: Minor
Fix For: 2.0.0RC1
The technique of computing the minimum power of 2 that is bigger than the requestedCapacity
in the org.apache.mina.core.buffer.IoBuffer.normalizeCapacity() is not optimal.
The current computation is as follows:
int newCapacity = 1;
while ( newCapacity < requestedCapacity ) {
newCapacity <<= 1;
if ( newCapacity < 0 ) {
return Integer.MAX_VALUE;
}
}
The time complexity of this is O(n), where n is the number of bits of the requestedCapacity
integer, that is log2(requestedCapacity)  maximum 31.
This creates an unnecessary overhead in some high IoBuffer allocations scenarios that are
calling IoBuffer.normalizeCapacity() a lot when creating IoBuffers. I observed this when benchmarking
a MINA server with hprof.
There is a better solution to this problem than to iterate the bits from 0 to log2(requestedCapacity).
The alternative is to use a binary search technique that has optimal time complexity of O(5).
Because requestedCapacity is an integer and has a maximum of 2^5 (32) bits we can binary search
in the set of bits and determine in O(5) comparisons the minimum power of 2 that is bigger
than the requestedCapacity.

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