On 3/29/09 12:43 AM, Earwin Burrfoot wrote:
There are three cases when we can override readNNN methods and provide
implementations with zero or minimum method invocations -
RAMDirectory, MMapDirectory and BufferedIndexInput for
FSDirectory/CompoundFileReader. Anybody tried this?
A while ago I tried overriding the read* methods in BufferedIndexInput
like this:
public int readVInt() throws IOException {
if (5 <= (bufferLength-bufferPosition)) {
return readVIntFast();
}
return super.readVInt();
}
private int readVIntFast() throws IOException {
byte b = buffer[bufferPosition++];
int i = b & 0x7F;
for (int shift = 6; (b & 0x80) != 0; shift += 7) {
b = buffer[bufferPosition++];
i |= (b & 0x7F) << shift;
}
return i;
}
Notice that I don't rely on ArrayIndexOutOfBoundsException, instead I do
one range check in readVInt() and then call the readVIntFast() method,
which accesses the buffer array directly to avoid multiple range checks.
Surprisingly I did not see any performance improvement. In my test I
wrote a huge file (several GBs) to disk with VInts, making sure they
occupied more than just a single byte each. Reading the file with and
without this "optimization" in BufferedIndexInput made almost no
difference. Only when I ran it in a profiler I saw a big difference,
because with his change there are less method calls, hence less
invocation count overhead.
I'm still surprised there was no performance improvement at all. Maybe
something was wrong with my test and I should try it again...
-Michael
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