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Chapman Flack edited this page Sep 6, 2018 · 13 revisions

Tuning PL/Java performance

As of 2018, there is a strong selection of Java runtimes that can be used to back PL/Java, including at least:

  • Oracle's Java (and Hotspot JVM)
  • OpenJDK (with Hotspot JVM)
  • OpenJDK (with Eclipse OpenJ9 JVM)

These JVMs offer a wide variety of configurable options affecting both memory footprint and time performance of applications using PL/Java. The options include initial and limit sizes for different memory regions, aggressiveness of just-in-time and ahead-of-time compilation, choice of garbage-collection algorithm, and various forms of shared-memory caching of precompiled classes.

The formal PL/Java documentation contains [a fairly extensive treatment of useful Hotspot settings][hstune], including a section on plausible minimum settings for memory footprint achievable with different class-sharing and garbage-collector settings. The documentation there of the comparable options and limits for OpenJ9 is more sparse at present.

This wiki page is intended as a clearinghouse for tuning tips and performance measurements for various PL/Java workloads and the available Java runtimes, that can be updated more actively between releases of the formal documentation.

Tip for quickly comparing runtime configurations

Once the PL/Java extension is installed in a database, in any newly-created session, the Java virtual machine is started on the first use of a PL/Java function. The JVM that is started, and how, are determined by the settings of pljava.* configuration variables in effect at that moment, most importantly:

  • pljava.libjvm_location selects which Java runtime will be used
  • pljava.vmoptions supplies the options to be passed to it

Therefore, all without exiting psql, a new Java runtime or combination of options can be tested by switching to a new connection with \c, setting those options differently, and again calling the PL/Java function of interest.

It can be convenient to include the settings on the psql \c line. For example, to time functionOfInterest() on two different Java runtimes:

\c "dbname=postgres options='-c pljava.libjvm_location=/path/to/oracle/.../libjvm.so'"
EXPLAIN ANALYZE SELECT functionOfInterest();
\c "dbname=postgres options='-c pljava.libjvm_location=/path/to/openj9/.../libjvm.so'"
EXPLAIN ANALYZE SELECT functionOfInterest();

For obvious reasons, the pljava.libjvm_location and pljava.vmoptions variables require privilege to set, so the connection needs to be made with superuser credentials.

Sample workload: Java XML manipulation

We will create a table containing a single XML document:

CREATE TABLE catalog_as_xml AS
SELECT schema_to_xml('pg_catalog', true, false, '') AS x;

In PostgreSQL 11beta3, the resulting document has the following size:

SELECT octet_length(xml_send(x)) AS uncompressed, pg_column_size(x) AS toasted
FROM catalog_as_xml;
uncompressed toasted
14049808 1130828

A test query will return the string value of every element whose string value is exactly six characters (a query that may be artificial and contrived, but can be expressed nearly identically in XML Query (the standard-mandated language for SQL XMLTABLE) and in the PostgreSQL native XMLTABLE syntax, which is limited to XPath 1.0).

The baseline will be the query expressed in XPath 1.0 using the PostgreSQL XMLTABLE function:

EXPLAIN ANALYZE SELECT
  xmltable.*
FROM
  catalog_as_xml,
  XMLTABLE('//*[string-length(.) = 6]'
	   PASSING x
	   COLUMNS s text PATH 'string(.)'
	  );

It will be compared to the equivalent query expressed in XQuery 1.0 and the "xmltable" function defined in the not-built-by-default org.postgresql.pljava.example.saxon.S9 example, relying on the Saxon-HE library:

EXPLAIN ANALYZE SELECT
  "xmltable".*
FROM
  catalog_as_xml,
  LATERAL (SELECT x AS ".") AS p,
  "xmltable"('//*[string-length(.) eq 6]',
	     PASSING => p,
	     COLUMNS => array[ 'string(.)' ]
	    )		  AS (  s text     );

The Java query will be run in both Oracle Java 8 (on the Hotspot JVM) and OpenJDK 8 (with the OpenJ9 JVM), with different choices of class-sharing options:

tag description
pg Baseline, PostgreSQL XMLTABLE
hs Hotspot, no sharing
hs-cds Hotspot, class data sharing (Java runtime classes only)
hs-appcds Hotspot, AppCDS (commercial feature), Java runtime, PL/Java, Saxon
j9 OpenJ9, no -Xquickstart, no sharing
j9q OpenJ9, -Xquickstart, no sharing
j9s OpenJ9, no -Xquickstart, sharing (Java runtime, PL/Java, Saxon)
j9qs OpenJ9, -Xquickstart, sharing (as above)

iteration | pg | hs | hs-cds | hs-appcds | j9 | j9q | j9s | j9qs ----------|-----:|---------:|------------:|-----:|------:|------:|-------: 1st |908.231|1969.658|1889.124|1544.612|3250.965|3095.733|2443.649|2644.991 2nd |879.483|747.658|777.894|719.146|1229.200|1855.513|1073.335|1932.083 4th |881.302|694.945|683.854|665.100|1011.018|1708.208|987.191|1912.010 8th |880.766|649.411|640.055|617.055|962.517|1660.867|952.857|1870.506 16th|880.622|679.538|675.594|655.664|967.805|1656.651|943.923|1941.888

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