Skip to content
New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Add small bias towards bit set encoding. #14155

Merged
merged 1 commit into from
Jan 23, 2025
Merged

Add small bias towards bit set encoding. #14155

Changes from all commits
Commits
Show all changes
1 commit
Select commit
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
17 changes: 9 additions & 8 deletions lucene/core/src/java/org/apache/lucene/codecs/lucene101/Lucene101PostingsWriter.java
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -424,15 +424,17 @@ private void flushDocBlock(boolean finishTerm) throws IOException {
long numSkipBytes = level0Output.size();
// Now we need to decide whether to encode block deltas as packed integers (FOR) or unary
// codes (bit set). FOR makes #nextDoc() a bit faster while the bit set approach makes
// #advance() sometimes faster and #intoBitSet() much faster. Since the trade-off is not
// obvious, we make the decision purely based on storage efficiency, using the approach that
// requires fewer bits to encode the block.
// #advance() usually faster and #intoBitSet() much faster. In the end, we make the decision
// based on storage requirements, picking the bit set approach whenever it's more
// storage-efficient than the next number of bits per value (which effectively slightly biases
// towards the bit set approach).
int bitsPerValue = forDeltaUtil.bitsRequired(docDeltaBuffer);
int sum = Math.toIntExact(Arrays.stream(docDeltaBuffer).sum());
int numBitSetLongs = FixedBitSet.bits2words(sum);
int numBitsNextBitsPerValue = Math.min(Integer.SIZE, bitsPerValue + 1) * BLOCK_SIZE;
if (sum == BLOCK_SIZE) {
level0Output.writeByte((byte) 0);
} else if (version < VERSION_DENSE_BLOCKS_AS_BITSETS || bitsPerValue * BLOCK_SIZE < sum) {
} else if (version < VERSION_DENSE_BLOCKS_AS_BITSETS || numBitsNextBitsPerValue <= sum) {
level0Output.writeByte((byte) bitsPerValue);
forDeltaUtil.encodeDeltas(bitsPerValue, docDeltaBuffer, level0Output);
} else {
Expand All @@ -444,10 +446,9 @@ private void flushDocBlock(boolean finishTerm) throws IOException {
s += i;
spareBitSet.set(s);
}
// Since we use the bit set encoding when it's more storage efficient than storing deltas,
// we know that each doc ID uses less than 32 bits, the maximum number of bits required to
// store a delta between consecutive doc IDs. So in the end, the bit set cannot have more
// than BLOCK_SIZE * Integer.SIZE / Long.SIZE = 64 longs, which fits on a byte.
// We never use the bit set encoding when it requires more than Integer.SIZE=32 bits per
// value. So the bit set cannot have more than BLOCK_SIZE * Integer.SIZE / Long.SIZE = 64
// longs, which fits on a byte.
assert numBitSetLongs <= BLOCK_SIZE / 2;
level0Output.writeByte((byte) -numBitSetLongs);
for (int i = 0; i < numBitSetLongs; ++i) {
Expand Down
Loading