Serial Processing for Large Documents
Saxon 8.5 introduces a new optimization, specifically designed for processing of large
documents, where the need to allocate sufficient memory to hold the entire source tree is
traditionally a problem.
This currently works only in XSLT, and is supported only in Saxon-SA (though the stylesheet
does not need to be schema-aware). It involves no new language constructs, but needs to be enabled
by means of the extension attribute
saxon:read-once="yes". It is invoked by
a stereotypical coding pattern that the optimizer recognizes and treats specially.
The typical code is as follows:
It's not necessary for such a stylesheet to have a principal source document, the transformation
can be invoked instead using the
-it main option from the command line, or its equivalent
in the Java API.
The important factors here are:
xsl:copy-of instruction must be used, with a select expression that starts
with a call on the
doc() function. The significance of this is
that because the instruction is making a copy of the nodes in the external document, there is no requirement
that the nodes returned by multiple attempts to access the same document should have the same identity.
Saxon therefore doesn't need to include the source document in its in-memory document pool.
(The "copy" is purely notional of course, since the original source tree corresponding to the
customers.xml document is never materialized.)
The path expression introduced by the call on
doc must conform
to the rules for path expressions appearing in identity constraints in XML Schema. This means there must be
no predicates; the first step (but only the first) can be introduced with "//"; the last step can optionally
use the attribute axis; all other steps must be simple Axis Steps using the child axis. These restrictions allow
Saxon to use the same code for serial XPath processing that is already used for validating identity constraints
against a schema (which is one reason the facility is available only in Saxon-SA).
It is not absolutely essential that the
xsl:copy-of instruction is used within a function,
but this is the easiest way of ensuring that the instruction is executed in pull mode, which is a prerequisite
for this optimization to be activated.
The optimization is enabled only if the
saxon:read-once attribute is present and is
set to "yes", and if the stylesheet is processed using Saxon-SA.
The optimization should not be enabled if the source document is read more than once in the course
of the transformation. There are two reasons for this: firstly, performance will be better in this case if the
document is read into memory; and secondly, when this optimization is used, there is no guarantee that the
document() function will be stable, that is, that it will return the same results when called
repeatedly with the same URI.
The implementation of this facility uses multithreading. One thread (which operates as a push pipeline)
is used to read the source document and filter out the nodes selected by the path expression. The nodes are then
handed over to the main processing thread, which iterates over the selected nodes using an XPath pull pipeline.
Because multithreading is used, this facility is not used when tracing is enabled. It should also be disabled
when using a debugger (there is a method in the Configuration object to achieve this.)
Note that a tree is built for each selected node, and its subtree. The saving in memory comes when these nodes
are processed one at a time, because each subtree can then be discarded as soon as it has been processed. There
is no benefit if the stylesheet needs to perform non-serial processing, such as sorting. There is also no benefit
if the path expression selects a node that contains most or all of the source document, for example its outermost
Saxon can handle expressions that select nested nodes, for example
//section where one section
contains another. However, the need to deliver nodes in document order makes the pipeline somewhat turbulent
in such cases, increasing memory usage.
Serial processing in this way is not actually faster than conventional processing (in fact,
it may only run at half the speed). Its big advantage is that it saves memory, thus making it possible to
process documents that would otherwise be too large for XSLT to handle. There may also be environments
where the multithreading enables greater use of the processor capacity available.
To run without this optimization,
either change the
xsl:copy-of instruction to
saxon:read-once to "no".