Advanced Filter Caching in Solr

10 02 2012


Note: my blog has moved. Please see : Advanced Filter Caching in Solr



Ranges over Functions in Solr 1.4

6 07 2009

Solr 1.4 contains a new feature that allows range queries or range filters over arbitrary functions.  It’s implemented as a standard Solr QParser plugin, and thus easily available for use any place that accepts the standard Solr Query Syntax by specifying the frange query type.  Here’s an example of a filter specifying the lower and upper bounds for a function:

fq={!frange l=0 u=2.2}log(sum(user_ranking,editor_ranking))

The other interesting use for frange is to trade off memory for speed when doing range queries on any type of single-valued field.  For example, one can use frange on a string field provided that there is only one value per field, and that numeric functions are avoided.

For example, here is a filter that only allows authors between martin and rowling, specified using a standard range query:
fq=author_last_name:[martin TO rowling]

And the same filter using a function range query (frange):
fq={!frange l=martin u=rowling}author_last_name

This can lead to significant performance improvements for range queries with many terms between the endpoints, at the cost of memory to hold the un-inverted form of the field in memory (i.e. a FieldCache entry – same as would be used for sorting). If the field in question is already being used for sorting or other function queries, there won’t be any additional memory overhead.

The following chart shows the results of a test of frange queries vs standard range queries on a string field with 200,000 unique values. For example, frange was 14 times faster when executing a range query / range filter that covered 20% of the terms in the field. For narrower ranges that matched less than 5% of the values, the traditional range query performed better.

Percent of terms covered Fastest implementation Speedup (how many times faster)
100% frange 43.32
20% frange 14.25
10% frange 8.07
5% frange 1.337
1% normal range query 3.59

Of course, Solr 1.4 also contains the new TrieRange functionality that will generally have the best time/space profile for range queries over numeric fields.

Solr scalability improvements

1 12 2008

With CPU cores constantly increasing, there has been some major work done in Lucene/Solr to increase the scalability under multi-threaded load.

Read-only IndexReaders

One bottleneck was synchronization around the checking of deleted docs in a Lucene IndexReader.  Since another thread could delete a document at any time, the IndexReader.isDeleted() call was synchronized.  It’s a very quick call, simply checking if a bit is set in a BitVector, but the problem was that it can be called millions of times in the process of satisfying a single query. The Read-only IndexReader feature allowed for the removal of this synchronization by prohibiting deletion.

Use of NIO to read index files

The standard method for Lucene to read index files is via Java’s RandomAccessFile.  Reading a part of the file involves two calls, a seek() to position the file pointer followed by a read() to get the data.  For multiple threads to share the same RandomAccessFile instance, this obviously involves synchronization to avoid one thread changing the file pointer before another thread gets to read at the file position it set.   If the data to be read isn’t in the operating system cache, it’s even worse news… the synchronization causes all other reads to block while the data is retrieved from disk, even if some of those reads could have been quickly satisified.

The preferred solution would be to have a method on RandomAccessFile that accepted an offset to read from.  This could easily be implemented by the JVM via a pread() system call.  But since Sun has not provided this functionality, we need to use something else.  NIO’s FileChannel does have the type of method we are looking for: dst, long position)

Solr now uses the non-synchronizing NIO method of reading index files (via Lucene’s NIOFSDirectory) by default if you are on a non-Windows platform.  Windows systems default to the older method since it turns out to be faster than the new method – the reason being a long standing “bug” in Java that still synchronizes internally even when using

Non blocking caches

Solr’s standard LRU cache implementation use a synchronized LinkedHashMap.  A single cache could be checked hundreds or thousands of times during the course of a single request that involves faceting.  A non-blocking ConcurrentLRUCache was developed as an alternative implementation, and is now the default for Solr’s filter cache.  One user indicated that this has doubled their query throughput under ideal circumstances.

Where to find this scalability goodness?

Solr 1.3 has read-only IndexReaders, but for the other scalability improvements, including the improved faceting, you’ll have to grab a nightly Solr build.

Solr Faceted Search Performance Improvements

25 11 2008

See facet performance benchmarks on my new blog for the latest performance benchmarks.

Distributed Search for Solr

27 02 2008

A new chapter in Solr scalability has been opened with the addition of distributed search!

Distributed Search splits an index into multiple shards, and queries across all the shards, combining the results and presenting a single merged response that looks like it came from a single server.

Solr’s current implementation uses SolrJ (the solr java client) to talk to other Solr servers via HTTP, in two main phases. The first phase collects matching document ids and scores, as well as doing any requested faceting. The second phase retrieves the stored fields for selected documents, does highlighting, and may include additional faceting requests to nail down exact facet counts.