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https://issues.apache.org/jira/browse/SOLR-5894?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Toke Eskildsen updated SOLR-5894:
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Attachment: SOLR-5894_test.zip
sparse_50510000docs_20140328-152807.png
SOLR-5894.patch
Patch with several bugfixes and updated test scripts.
The chart sparse_50510000docs_20140328-152807.png shows results form a test run
on 50M documents / 150M unique tags. The black line is standard Solr, the red
is a sparse counter with 100% overhead, the cyan is an attempt of compromising
with 8% sparse overhead.
The performance hit for passing the 8% cut-off is huge. One way to avoid it
could be to make a numhits-based guess of the expected number of unique tags,
when choosing facet implementation. The consequence of a wrong guess would
"just" be poorer performance, with the worst-case being the max of the black
and the cyan line for any given number of unique tags in the search result.
> Speed up high-cardinality facets with sparse counters
> -----------------------------------------------------
>
> Key: SOLR-5894
> URL: https://issues.apache.org/jira/browse/SOLR-5894
> Project: Solr
> Issue Type: Improvement
> Components: SearchComponents - other
> Affects Versions: 4.6.1, 4.7
> Reporter: Toke Eskildsen
> Priority: Minor
> Fix For: 4.6.1
>
> Attachments: SOLR-5894.patch, SOLR-5894.patch, SOLR-5894.patch,
> SOLR-5894.patch, SOLR-5894_test.zip, SOLR-5894_test.zip,
> author_7M_tags_1852_logged_queries_warmed.png,
> sparse_50510000docs_20140328-152807.png
>
>
> Field based faceting in Solr has two phases: Collecting counts for tags in
> facets and extracting the requested tags.
> The execution time for the collecting phase is approximately linear to the
> number of hits and the number of references from hits to tags. This phase is
> not the focus here.
> The extraction time scales with the number of unique tags in the search
> result, but is also heavily influenced by the total number of unique tags in
> the facet as every counter, 0 or not, is visited by the extractor (at least
> for count order). For fields with millions of unique tag values this means
> 10s of milliseconds added to the minimum response time (see
> https://sbdevel.wordpress.com/2014/03/18/sparse-facet-counting-on-a-real-index/
> for a test on a corpus with 7M unique values in the facet).
> The extractor needs to visit every counter due to the current counter
> structure being a plain int-array of size #unique_tags. Switching to a sparse
> structure, where only the tag counters > 0 are visited, makes the extraction
> time linear to the number of unique tags in the result set.
> Unfortunately the number of unique tags in the result set is unknown at
> collect time, so it is not possible to reliably select sparse counting vs.
> full counting up front. Luckily there exists solutions for sparse sets that
> has the property of switching to non-sparse-mode without a switch-penalty,
> when the sparse-threshold is exceeded (see
> http://programmingpraxis.com/2012/03/09/sparse-sets/ for an example). This
> JIRA aims to implement this functionality in Solr (a proof of concept patch
> will be provided shortly).
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