Mike Sokolov created LUCENE-8781:
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Summary: Explore FST direct array arc encoding
Key: LUCENE-8781
URL: https://issues.apache.org/jira/browse/LUCENE-8781
Project: Lucene - Core
Issue Type: Improvement
Reporter: Mike Sokolov
This issue is for exploring an alternate FST encoding of Arcs as full-sized
arrays so Arcs are addressed directly by label, avoiding binary search that we
use today for arrays of Arcs.
h3. Testing
ant test passes. I added some unit tests that were helpful in uncovering bugs
while
implementing which are more difficult to chase down when uncovered by the
randomized testing we already do. They don't really test anything new; they're
just more focused.
I'm not sure why, but ant precommit failed for me with:
{noformat}
...lucene-solr/solr/common-build.xml:536: Check for forbidden API calls failed
while scanning class
'org.apache.solr.metrics.reporters.SolrGangliaReporterTest'
(SolrGangliaReporterTest.java): java.lang.ClassNotFoundException:
info.ganglia.gmetric4j.gmetric.GMetric (while looking up details about
referenced class 'info.ganglia.gmetric4j.gmetric.GMetric')
{noformat}
I also got Test2BFST running (it was originally timing out due to excessive
calls to ramBytesUsage(), which seems to have gotten slow), and it passed; that
change isn't include here.
h4. Micro-benchmark
I timed lookups in FST via FSTEnum.seekExact in a unit test under various
conditions.
h5. English words
A test of looking up existing words in a dictionary of ~170000 English words
shows improvements; the numbers listed are % change in FST size, time to look
up (FSTEnum.seekExact) words that are in the dict, and time to look up random
strings that are not in the dict. The comparison is against the current
codebase with the optimization disabled. A separate comparison of showed no
significant change of the baseline (no opto applied) vs the current master FST
impl with no code changes applied.
load=2 || load=4 || load=16
+4, -6, -7 | +18, -11, -8 | +22, -11.5, -7
The "load factor" used for those measurements controls when direct array arc
encoding is used;
namely when the number of outgoing arcs was > load * (max label - min label).
h5. sequential and random terms
The same test, with terms being a sequence of integers as strings shows a
larger improvement, around 20% (load=4). This is presumably the best case for
this delta, where every Arc is encoded as a direct lookup.
When random lowercase ASCII strings are used, a smaller improvement of around
4% is seen.
h4. luceneutil
Testing w/luceneutil (wikimediumall) we see improvements mostly in the PKLookup
case. Other results seem noisy, with perhaps a small improvment in some of the
queries.
{noformat}
Task QPS base StdDev QPS opto StdDev
Pct diff
OrHighHigh 6.93 (3.0%) 6.89 (3.1%)
-0.5% ( -6% - 5%)
OrHighMed 45.15 (3.9%) 44.92 (3.5%)
-0.5% ( -7% - 7%)
Wildcard 8.72 (4.7%) 8.69 (4.6%)
-0.4% ( -9% - 9%)
AndHighLow 274.11 (2.6%) 273.58 (3.1%)
-0.2% ( -5% - 5%)
OrHighLow 241.41 (1.9%) 241.11 (3.5%)
-0.1% ( -5% - 5%)
AndHighMed 52.23 (4.1%) 52.41 (5.3%)
0.3% ( -8% - 10%)
MedTerm 1026.24 (3.1%) 1030.52 (4.3%)
0.4% ( -6% - 8%)
HighTerm 1111.10 (3.4%) 1116.70 (4.0%)
0.5% ( -6% - 8%)
HighTermDayOfYearSort 14.59 (8.2%) 14.73 (9.3%)
1.0% ( -15% - 20%)
AndHighHigh 13.45 (6.2%) 13.61 (4.4%)
1.2% ( -8% - 12%)
HighTermMonthSort 63.09 (12.5%) 64.13 (10.9%)
1.6% ( -19% - 28%)
LowTerm 1338.94 (3.3%) 1383.90 (5.5%)
3.4% ( -5% - 12%)
PKLookup 120.45 (2.5%) 130.91 (3.5%)
8.7% ( 2% - 15%)
{noformat}
h4.FST perf tests
I ran LookupBenchmarkTest to see the impact on the suggesters which make heavy
use of FSTs. Some show little or no improvement, but in some cases there are
substantial gains.
chart TK
h3. API change / implementation notes
The only change in the public FST API is that the Builder constructor now takes
an additional
boolean ("useDirectArcAddressing") controlling whether or not this new
optimization is
applied. However, FST's internal details are not really hidden, so in practice
any change to its
encoding can have ripple effects in other classes.
This is because the FST decoding is repeated in several places in the code
base, sometimes with
subtle variations: eg FST, FSTEnum, and fst.Util have very similar, but not
shared, traversal code,
and there are a few other places this same or similar decoding algorithm
appears as well (eg
blocktreeords codec). I do think it would be useful to make this code more DRY,
and to hide the
traversal implementation within a single class (or at least in the package),
but it seemed better to
do that separately. I also think there are possibilities for improving this
encoding further, maybe
by encoding Arcs in contiguous blocks with gaps in between, but before
attempting anything like
that, I think it would good to do that refactor to make the code in its current
form easier to
understand and work with.
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