mikemccand commented on PR #12624:
URL: https://github.com/apache/lucene/pull/12624#issuecomment-1828548480
Hmm I'm running `Test2BFSTs` on this patch and noticed it seems to take very
much longer during the `TEST: now verify` step where it confirms the built FST
accepts all the inputs it just compiled into it.
I `jstack`'d it and found this:
```
java.lang.Thread.State: RUNNABLE
at
java.nio.ByteBuffer.position(java.base@17.0.9/ByteBuffer.java:1516)
at java.nio.ByteBuffer.position(java.base@17.0.9/ByteBuffer.java:267)
at java.nio.Buffer.<init>(java.base@17.0.9/Buffer.java:246)
at java.nio.ByteBuffer.<init>(java.base@17.0.9/ByteBuffer.java:288)
at
java.nio.HeapByteBuffer.<init>(java.base@17.0.9/HeapByteBuffer.java:95)
at
java.nio.HeapByteBufferR.<init>(java.base@17.0.9/HeapByteBufferR.java:102)
at
java.nio.HeapByteBufferR.duplicate(java.base@17.0.9/HeapByteBufferR.java:135)
at
java.nio.HeapByteBufferR.asReadOnlyBuffer(java.base@17.0.9/HeapByteBufferR.java:148)
at
org.apache.lucene.store.ByteBuffersDataInput.<init>(ByteBuffersDataInput.java:60)
at
org.apache.lucene.store.ByteBuffersDataOutput.toDataInput(ByteBuffersDataOutput.java:279)
at
org.apache.lucene.util.fst.ReadWriteDataOutput.getReverseBytesReader(ReadWriteDataOutput.java:52)
at org.apache.lucene.util.fst.FST.getBytesReader(FST.java:1181)
at org.apache.lucene.util.fst.Util.get(Util.java:50)
at org.apache.lucene.util.fst.Test2BFST.test(Test2BFST.java:113)
```
It looks like getting the reversed reader has maybe become quite a bit more
expensive than `BytesStore` was?
Note that `Test2BFSTs` is quite silly -- it uses `Util.get` for every
lookup, instead of pulling and reusing a `ReverseBytesReader` like most "real"
usages of FST (e.g. terms dict) will do. So perhaps this performance change
doesn't really matter in practice? Though I wonder if all consumers of FST are
re-using their reversed readers?
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