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Posted to issues@lucene.apache.org by "Michael McCandless (Jira)" <ji...@apache.org> on 2022/05/13 19:14:00 UTC
[jira] [Updated] (LUCENE-10572) Can we optimize BytesRefHash?
[ https://issues.apache.org/jira/browse/LUCENE-10572?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]
Michael McCandless updated LUCENE-10572:
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Summary: Can we optimize BytesRefHash? (was: Can we optimize BytesRefHash)
> 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
> Priority: Major
>
> 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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