[LUCENE-4682] Reduce wasted bytes in FST due to array arcs

• Type: Improvement
• Status: Open
• Priority: Minor
• Resolution: Unresolved
• Component/s: core/FSTs
• Lucene Fields: New
• Assignee: Unassigned
• Reporter: Michael McCandless
• Created: 12/Jan/13 14:32
• Updated: 28/Aug/22 13:36

Description

When a node is close to the root, or it has many outgoing arcs, the FST writes the arcs as an array (each arc gets N bytes), so we can e.g. bin search on lookup.

The problem is N is set to the max(numBytesPerArc), so if you have an outlier arc e.g. with a big output, you can waste many bytes for all the other arcs that didn't need so many bytes.

I generated Kuromoji's FST and found it has 271187 wasted bytes vs total size 1535612 = ~18% wasted.

It would be nice to reduce this.

One thing we could do without packing is: in addNode, if we detect that number of wasted bytes is above some threshold, then don't do the expansion.

Another thing, if we are packing: we could record stats in the first pass about which nodes wasted the most, and then in the second pass (paack) we could set the threshold based on the top X% nodes that waste ...

Another idea is maybe to deref large outputs, so that the numBytesPerArc is more uniform ...

Issue Links

• is related to: LUCENE-8084 FSTs can be very space-inefficient on array-expanded nodes