Michael McCandless created LUCENE-10572:
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Summary: Can we optimize BytesRefHash
Key: LUCENE-10572
URL: https://issues.apache.org/jira/browse/LUCENE-10572
Project: Lucene - Core
Issue Type: Improvement
Reporter: Michael McCandless
I was poking around in our nightly benchmarks
([https://home.apache.org/~mikemccand/lucenebench]) and noticed in the JFR
profiling that the hottest method is this:
{noformat}
PERCENT CPU SAMPLES STACK
9.28% 53848 org.apache.lucene.util.BytesRefHash#equals()
at org.apache.lucene.util.BytesRefHash#findHash()
at org.apache.lucene.util.BytesRefHash#add()
at org.apache.lucene.index.TermsHashPerField#add()
at
org.apache.lucene.index.IndexingChain$PerField#invert()
at
org.apache.lucene.index.IndexingChain#processField()
at
org.apache.lucene.index.IndexingChain#processDocument()
at
org.apache.lucene.index.DocumentsWriterPerThread#updateDocuments() {noformat}
This is kinda crazy – comparing if the term to be inserted into the inverted
index hash equals the term already added to {{BytesRefHash}} is the hottest
method during nightly benchmarks.
Discussing offline with [~rcmuir] and [~jpountz] they noticed a few
questionable things about our current implementation:
* Why are we using a 1 or 2 byte {{vInt}} to encode the length of the inserted
term into the hash? Let's just use two bytes always, since IW limits term
length to 32 K (< 64K that an unsigned short can cover)
* Why are we doing byte swapping in this deep hotspot using {{VarHandles}}
(BitUtil.VH_BE_SHORT.get)
* Is it possible our growth strategy for {{BytesRefHash}} (on rehash) is not
aggressive enough? Or the initial sizing of the hash is too small?
* Maybe {{MurmurHash}} is not great (causing too many conflicts, and too many
{{equals}} calls as a result?) – {{Fnv}} and {{xxhash}} are possible "upgrades"?
* If we stick with {{{}MurmurHash{}}}, why are we using the 32 bit version
({{{}murmurhash3_x86_32{}}})?
* Are we using the JVM's intrinsics to compare multiple bytes in a single SIMD
instruction ([~rcmuir] is quite sure we are indeed)?
* [~jpountz] suggested maybe the hash insert is simply memory bound
* {{TermsHashPerField.writeByte}} is also depressingly slow (~5% of total CPU
cost)
I pulled these observations from a recent (5/6/22) profiler output:
[https://home.apache.org/~mikemccand/lucenebench/2022.05.06.06.33.00.html]
Maybe we can improve our performance on this crazy hotspot?
Or maybe this is a "healthy" hotspot and we should leave it be!
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