How to cite this paper
Parallel Bit Stream Technology as a Foundation for XML Parsing Performance
International Symposium on Processing XML Efficiently: Overcoming Limits on Space,
Time, or Bandwidth
August 10, 2009
While particular XML applications may benefit from special-purpose hardware such as XML
chips [Leventhal and Lemoine 2009] or appliances [Salz, Achilles and Maze 2009], the bulk
of the world's XML processing workload will continue to be handled by XML software stacks
on commodity processors. Exploiting the SIMD capabilities of such processors such as the
SSE instructions of x86 chips, parallel bit stream technology offers the potential of
dramatic improvement over byte-at-a-time processing for a variety of XML processing tasks.
Character set issues such as Unicode validation and transcoding [Cameron 2007], normalization of line breaks and white space and XML character validation can be
handled fully in parallel using this representation. Lexical item streams, such as the bit
stream marking the positions of opening angle brackets, can also be formed in parallel.
Bit-scan instructions of commodity processors may then be used on lexical item streams to
implement rapid single-instruction scanning across variable-length multi-byte text blocks
as in the Parabix XML parser [Cameron, Herdy and Lin 2008]. Overall, these techniques may be
combined to yield end-to-end performance that may be 1.5X to 15X faster than alternatives
[Herdy, Burggraf and Cameron 2008].
Continued research in parallel bit stream techniques as well as more conventional
application of SIMD techniques in XML processing offers further prospects for improvement
of core XML components as well as for tackling performance-critical tasks further up the
stack. A newly prototyped technique for parallel tag parsing using bitstream addition is
expected to improve parsing performance even beyond that achieved using sequential bit
scans. Several techniques for improved symbol table performance are being investigated,
including parallel hash value calculation and length-based sorting using the cheap length
determination afforded by bit scans. To deliver the benefits of parallel bit stream
technology to the Java world, we are developing Array Set Model (ASM) representations of
XML Infoset and other XML information models for efficient transmission across the JNI
Amplifying these software advances, continuing hardware advances in commodity processors
increase the relative advantage of parallel bit stream techniques over traditional
byte-at-a-time processors. For example, the Intel Core architecture improved SSE processing
to give superscalar execution of bitwise logic operations (3 instructions per cycle vs. 1
in Pentium 4). Upcoming 256-bit AVX technology extends the register set and replaces
destructive two-operand instructions with a nondestructive three-operand form. General
purpose programming on graphic processing units (GPGPU) such as the upcoming 512-bit
Larrabee processor may also be useful for XML applications using parallel bit streams. New
instruction set architectures may also offer dramatic improvements in core algorithms.
Using the relatively simple extensions to support the principle of inductive doubling, a 3X
improvement in several core parallel bit stream algorithms may be achieved [Cameron and Lin 2009]. Other possibilities include direct implementation of parallel
extract and parallel deposit (pex/pdep) instructions [Hilewitz and Lee 2006], and
bit-level interleave operations as in Larrabee, each of which would have important
application to parallel bit stream processing.
Further prospects for XML performance improvement arise from leveraging the
intraregister parallelism of parallel bit stream technology to exploit the interchip
parallelism of multicore computing. Parallel bit stream techniques can support multicore
parallelism in both data partitioning and task partitioning models. For example, the
datasection partitioning approach of Wu, Zhang, Yu and Li may be used to partition blocks
for speculative parallel parsing on separate cores followed by a postprocessing step to
join partial S-trees [Wu et al. 2008].
In our view, the established and expected performance advantages of parallel bit stream
technology over traditional byte-at-a-time processing are so compelling that parallel bit
stream technology should ultimately form the foundation of every high-performance XML
software stack. We envision a common high-performance XML kernel that may be customized to
a variety of processor architectures and that supports a wide range of existing and new XML
APIs. Widespread deployment of this technology should greatly benefit the XML community in
addressing both the deserved and undeserved criticism of XML on performance grounds. A
further benefit of improved performance is a substantial greening of XML technologies.
To complement our research program investigating fundamental algorithms and issues in
high-performance XML processing, our work also involves development of open source software
implementing these algorithms, with a goal of full conformance to relevant specifications.
From the research perspective, this approach is valuable in ensuring that the full
complexity of required XML processing is addressed in reporting and assessing processing
results. However, our goal is also to use this open source software as a basis of
technology transfer. A Simon Fraser University spin-off company, called International
Characters, Inc., has been created to commercialize the results of this work using a
patent-based open source model.
To date, we have not yet been successful in establishing a broader community of
participation with our open source code base. Within open-source communities, there is
often a general antipathy towards software patents; this may limit engagement with our
technology, even though it has been dedicated for free use in open source.
A further complication is the inherent difficulty of SIMD programming in general, and
parallel bit stream programming in particular. Considerable work is required with each new
algorithmic technique being investigated as well as in retargetting our techniques for each
new development in SIMD and multicore processor technologies. To address these concerns, we
have increasingly shifted the emphasis of our research program towards compiler technology
capable of generating parallel bit stream code from higher-level specifications.
A Catalog of Parallel Bit Streams for XML
In this section, we introduce the fundamental concepts of parallel bit stream
technology and present a comprehensive catalog of parallel bit streams for use in XML
processing. In presenting this catalog, the focus is on the specification of the bit
streams as data streams in one-to-one correspondence with the character code units of an
input XML stream. The goal is to define these bit streams in the abstract without
initially considering memory layouts, register widths or other issues related to
particular target architectures. In cataloging these techniques, we also hope to convey
a sense of the breadth of applications of parallel bit stream technology to XML
Basis Bit Streams
Given a byte-oriented text stream represented in UTF-8, for example, we define a
transform representation of this text consisting of a set of eight parallel bit streams
for the individual bits of each byte. Thus, the
Bit0 stream is the stream
of bits consisting of bit 0 of each byte in the input byte stream,
the bit stream consisting of bit 1 of each byte in the input stream and so on. The set
Bit7 are known as the basis
streams of the parallel bit stream representation. The following table
shows an example XML character stream together with its representation as a set of 8
XML Character Stream Transposition.
Depending on the features of a particular processor architecture, there are a number
of algorithms for transposition to parallel bit stream form. Several of these algorithms
employ a three-stage structure. In the first stage, the input byte stream is divided
into a pair of half-length streams consisting of four bits for each byte, for example,
one stream for the high nybble of each byte and another for the low nybble of each byte.
In the second stage, these streams of four bits per byte are each divided into streams
consisting of two bits per original byte, for example streams for the
Bit6/Bit7 pairs. In the final stage, the streams are further subdivided
in the individual bit streams.
Using SIMD capabilities, this process is quite efficient, with an amortized cost of
1.1 CPU cycles per input byte on Intel Core 2 with SSE, or 0.6 CPU cycles per input byte
on Power PC G4 with Altivec. With future advances in processor technology, this
transposition overhead is expected to reduce, possibly taking advantage of upcoming
parallel extract (pex) instructions on Intel technology. In the ideal, only 24
instructions are needed to transform a block of 128 input bytes using 128-bit SSE
registers using the inductive doubling instruction set architecture, representing an
overhead of less than 0.2 instructions per input byte.
This section describes bit streams which support basic processing operations.
Deletion Mask Streams
DelMask (deletion mask) streams marks character code unit positions for deletion.
Since the deletion operation is dependency free across many stages of XML processing,
it is possible to simply mark and record deletion positions as deletion mask streams for future processing. A single
invocation of a SIMD based parallel deletion algorithm can then perform the deletion of
positions accumulated across a number of stages through a bitwise ORing of deletion
masks. For example, deletion arises in the replacement of predefined entities with a
single character, such as in the replacement of the & entity, with the
& character. Deletion also arises in XML
end-of-line handling, and CDATA section delimeter processing. Several algorithms to
delete bits at positions marked by DelMask are possible [Cameron 2008].
The following table provides an example of generating a DelMask in the context of
bit stream based parsing of well-formed character references and predefined entities.
The result is the generation of a DelMask stream.
DelMask Stream Generation
Error Flag Streams
Error flag streams indicates the character code unit positions of syntactical
errors. XML processing examples which benefit from the marking of error positions
include UTF-8 character sequence validation and XML parsing [Cameron 2008].
The following table provides an example of using bit streams to parse character
references and predefined entities which fail to meet the XML 1.0 well-formedness
constraints. The result is the generation of an error flag stream that marks the
positions of mal-formed decimal and hexical character references respectively.
Error Flag Stream Generation
> &#, &#x;
Lexical Item Streams
Lexical item streams differ from traditional streams of tokens in that they are bit
streams that mark the positions of tokens, whitespace or delimiters. Additional bit
streams, such as the reference streams and callout streams, are subsequently constructed
based on the information held within the set of lexical items streams. Differentiation
between the actual tokens that may occur at a particular point (e.g., the different XML
tokens that begin “<”) may be performed using multicharacter recognizers on the
bytestream representation [Cameron, Herdy and Lin 2008].
A key role of lexical item streams in XML parsing is to facilitate fast scanning
operations. For example, a left angle bracket lexical item stream may be formed to
identify those character code unit positions at which a “<” character occurs.
Hardware register bit scan operations may then be used by the XML parser on the left
angle bracket stream to efficiently identify the position of the next “<”. Based
on the capabilities of current commodity processors, a single register bit scan
operation may effectively scan up to 64 byte positions with a single instruction.
Overall, the construction of the full set of lexical item stream computations
requires approximately 1.0 CPU cycles per byte when implemented for 128 positions at a
time using 128-bit SSE registers on Intel Core2 processors [Cameron, Herdy and Lin 2008].
The following table defines the core lexical item streams defined by the Parabix XML
Lexical item stream descriptions.
| LAngle || Marks the position of any left angle bracket character.
| RAngle || Marks the position of any right angle bracket character.
| LBracket || Marks the position of any left square bracker character.
| RBracket || Marks the position of any right square bracket
| Exclam || Marks the position of any exclamation mark character.
| QMark || Marks the position of any question mark character.
| Hyphen || Marks the position of any hyphen character.
| Equals || Marks the position of any equal sign character.
| SQuote || Marks the position of any single quote character.
| DQuote || Marks the position of any double quote character.
| Slash || Marks the position of any forward slash character
| NameScan || Marks the position of any XML name character.
| WS || Marks the position of any XML 1.0 whitespace character.
| PI_start || Marks the position of the start of any processing instruction
at the '?' character position.
| PI_end || Marks the position of any end of any processing instruction
at the '>' character position.
| CtCD_start || Marks the position of the start of any comment or CDATA
section at the '!' character position.
| EndTag_start || Marks the position of any end tag at the '/' character
| CD_end || Marks the position of the end of any CDATA section at the '>'
| DoubleHyphen || Marks the position of any double hyphen character.
| RefStart || Marks the position of any ampersand character.
| Hash || Marks the position of any hash character.
| x || Marks the position of any 'x' character.
| Digit || Marks the position of any digit.
| Hex || Marks the position of any hexidecimal character.
| Semicolon || Marks the position of any semicolon character.
The following illustrates a number of the lexical item streams.
<tag><tag> text <
UTF-8 Byte Classification, Scope and Validation Streams
An XML parser must accept the UTF-8 encoding of Unicode [XML 1.0].
It is a fatal error if an XML document determined to be in UTF-8 contains byte sequences
that are not legal in that encoding. UTF-8 byte classification, scope, XML character
validation and error flag bit streams are defined to validate UTF-8 byte sequences and
support transcoding to UTF-16.
UTF-8 Byte Classification Streams
UTF-8 byte classification bit streams classify UTF-8 bytes based on their role in
forming single and multibyte sequences. The u8Prefix and u8Suffix bit streams
identify bytes that represent, respectively, prefix or suffix bytes of multibyte
UTF-8 sequences. The u8UniByte bit stream identifies those bytes that may be
considered single-byte sequences. The u8Prefix2, u8Prefix3, and u8Prefix4 refine the
u8Prefix respectively indicating prefixes of two, three or four byte
UTF-8 Scope Streams
Scope streams represent expectations established by UTF-8 prefix bytes. For
example, the u8Scope22 bit stream represents the positions at which the second byte of a
two-byte sequence is expected based on the occurrence of a two-byte prefix in the
immediately preceding position. The u8scope32, u8Scope33, u8Scope42, u8scope43, and
u8Scope44 complete the set of UTF-8 scope streams.
The following example demonstrates the UTF-8 character encoding validation
process using parallel bit stream techniques. The result of this validation process
is an error flag stream identifying the positions at which errors occur.
A Text in Farsi: ى ك م ت ن ف ا ر س ى
UTF-8 Validation Streams
Proper formation of UTF-8 byte sequences requires that the correct number of
suffix bytes always follow a UTF-8 prefix byte, and that certain illegal byte
combinations are ruled out. For example, sequences beginning with the prefix bytes
0xF5 through 0xFF are illegal as they would represent code point values above 10FFFF.
In addition, there are constraints on the first suffix byte following certain special
prefixes, namely that a suffix following the prefix 0xE0 must fall in the range
0xA0–0xBF, a suffix following the prefix 0xED must fall in the range 0x80–0x9F, a
suffix following the prefix 0xF0 must fall in the range 0x90–0xBF and a suffix
following the prefix 0xF4 must fall in the range 0x80–0x8F. The task of ensuring that
each of these constraints hold is known as UTF-8 validation. The bit streams xE0,
xED, xF0, xF4, xA0_xBF, x80_x9F, x90_xBF, and x80_x8F are constructed to flag the
aforementioned UTF-8 validation errors. The result of UTF-8 validation is a UTF-8
error flag bit stream contructed as the ORing of a series of UTF-8 validation tests.
XML Character Validation Streams
The UTF-8 character sequences 0xEF 0xBF 0xBF and
0xEF 0xBF 0xBE correspond to the Unicode code points 0xFFFE
and 0xFFFF respectively. In XML 1.0, 0xFFFE and 0xFFFF represent characters outside
the legal XML character ranges. As such, bit streams which mark 0xEF, 0xBF, and 0xBE
character are constructed to flag illegal UTF-8 character sequences.
UTF-8 to UTF-16 Transcoding
UTF-8 is often preferred for storage and data exchange, it is suitable for
processing, but it is significantly more complex to process than UTF-16 [Unicode]. As such, XML documents are typically encoded in UTF-8 for
serialization and transport, and subsequently transcoded to UTF-16 for processing
with programming languages such as Java and C#. Following the parallel bit stream
methods developed for the u8u16 transcoder, a high-performance standalone UTF-8 to
UTF-16 transcoder [Cameron 2008], transcoding to UTF-16 may be achieved by
computing a series of 16 bit streams. One stream for each of the individual bits of a
UTF-16 code unit.
The bit streams for UTF-16 are conveniently divided into groups: the eight streams
u16Hi0, u16Hi1, ..., u16Hi7 for the high byte of each UTF-16 code unit and the eight
streams u16Lo1, ..., u16Lo7 for the low byte. Upon conversion of the parallel bit
stream data back to byte streams, eight sequential byte streams U16h0, U16h1, ...,
U16Hi7 are used for the high byte of each UTF-16 code unit, while U16Lo0, U16Lo1,...,
U16Lo7 are used for the corresponding low byte. Interleaving these streams then
produces the full UTF-16 doublebyte stream.
UTF-8 Indexed UTF-16 Streams
UTF-16 bit streams are initially defined in UTF-8 indexed form. That is, with sets
of bits in one-to-one correspondence with UTF-8 bytes. However, only one set of
UTF-16 bits is required for encoding two or three-byte UTF-8 sequences and only two
sets are required for surrogate pairs corresponding to four-byte UTF-8 sequences. The
u8LastByte (u8UniByte , u8Scope22 , u8Scope33 , and u8Scope44 ) and u8Scope42 streams
mark the positions at which the correct UTF-16 bits are computed. The bit sets at
other positions must be deleted to compress the streams to the UTF-16 indexed form.
Control Character Streams
The control character bit streams marks ASCII control characters in the range
0x00-0x1F. Additional control character bit streams mark the tab, carriage return, line
feed, and space character. In addition, a bit stream to mark carriage return line
combinations is also constructed. Presently, control character bit streams support the
operations of XML 1.0 character validation and XML end-of-line handling.
XML Character Validation
Legal characters in XML are the tab, carriage return, and line feed characters,
together with all Unicode characters and excluding the surrogate blocks, as well as hexadecimal OxFFFE and
OxFFFF [XML 1.0]. The x00_x1F bit stream is constructed and used in
combination with the additional control character bit streams to flags the positions
of illegal control characters.
XML 1.0 End-of-line Handling
In XML 1.0 the two-character sequence CR LF (carriage return, line feed) as well as
any CR character not followed by a LF character must be converted to a single LF
character [XML 1.0].
By defining carriage return, line feed, and carriage return line feed bit streams,
dentoted CR, LF and CRLF respectively, end-of-line normalization processing can be
performed in parallel using only a small number of logical and shift operations.
The following example demonstrates the generation of the CRLF deletion mask. In
this example, the position of all CR characters followed by LF characters are marked
for deletion. Isolated carriage returns are then replaced with LF characters.
Completion of this process satisfies the XML 1.0 end-of-line handling requirements.
For clarity, this example encodes input data carriage returns as
C characters, whereas line feed characters are shown as
XML 1.0 End-of-line Handling
first line C second line CL third line L one more C nothing
Call Out Streams
Call out bit streams mark the extents of XML markup structures such as comments,
processing instruction and CDATA sections as well as physical structures such as character and
entity references and general references. Call out streams are also formed for logical markup structures such
start tags, end tags and empty element tags.
Comment, Processing Instruction and CDATA Section Call Out Streams
Comments, processing instructions and CDATA sections call out streams, Ct_Span,
PI_Span and CD_Span respectively, define sections of an XML document which
contain markup that is not interpreted by an XML processor. As such, the union of
Ct_Span, PI_Span and CD_Span streams defines the regions of non-interpreteable markup.
The stream formed by this union is termed the CtCDPI_Mask.
The following tables provides an example of constructing the CtCDPI_Mask.
<?php?> <!-- example --> <![CDATA[ shift: a<<1 ]]>
With the removal of all non-interpreteable markup, several phases of parallel bit
stream based SIMD operations may follow operating on up to 128 byte positions on
current commondity processors and assured of XML markup relevancy. For
example, with the extents identification of comments, processing instructions and
CDATA sections, XML names may be identified and length sorted for efficient symbol
As an aside, comments and CDATA sections must first be validated to ensure
that comments do not contain "--" sequences and that CDATA sections do not contain illegal
"]]>" sequences prior to ignorable markup stream generation.
Reference Call Out Streams
The reference call out streams are the GenRefs, DecRefs, and HexRefs streams. This
subset of the call out streams marks the extents of all but the closing semicolon of
general and character references.
(<,>,&,',") and numeric character
references (&#nnnn;, &#xhhhh;) must be replaced by a single character
[XML 1.0]. As previously shown, this subset of call out streams enables the construction of a DelMask for
Tag Call Out Streams
Whereas sequential bit scans over lexical item streams form the basis of XML
parsing, in the current Parabix parser a new method of parallel parsing has been
developed and prototyped using the concept of bitstream addition. Fundamental to this
method is the concept of a cursor stream, a bit stream marking
the positions of multiple parallel parses currently in process.
The results of parallel parsing using the bit stream addition technique produces a
set of tag call out bit streams. These streams mark the extents of each start tag,
end tag and empty element tag. Within tags, additional streams mark start
and end positions for tag names, as well as attribute names and values. An error flag
stream marks the positions of any syntactic errors encountered during parsing.
The set of tag call out streams consists of the ElemNames, AttNames, AttVals, Tags,
EmptyTagEnds and EndTags bit streams. The following example demonstrates the bit
stream output produced which from parallel parsing using bit stream addition.
a1='foo' a2 =
SIMD Beyond Bitstreams: Names and Numbers
Whereas the fundamental innovation of our work is the use of SIMD technology in
implementing parallel bit streams for XML, there are also important ways in which more
traditional byte-oriented SIMD operations can be useful in accelerating other aspects of
Efficient symbol table mechanisms for looking up element and attribute names is
important for almost all XML processing applications. It is also an important technique
merely for assessing well-formedness of an XML document; rather than validating the
character-by-character composition of each occurrence of an XML name as it is
encountered, it is more efficient to validate all but the first occurrence by first
determining whether the name already exists in a table of prevalidated names.
The first symbol table mechanism deployed in the Parabix parser simply used the
hashmaps of the C++ standard template library, without deploying any SIMD technology.
However, with the overhead of character validation, transcoding and parsing dramatically
reduced by parallel bit stream technology, we found that symbol lookups then accounted
for about half of the remaining execution time in a statistics gathering application
[Cameron, Herdy and Lin 2008]. Thus, symbol table processing was identified as a major
target for further performance improvement.
Our first effort to improve symbol table performance was to employ the splash tables
with cuckoo hashing as described by Ross [Ross 2006], using SIMD
technology for parallel bucket processing. Although this technique did turn out to have
the advantage of virtually constant-time performance even for very large vocabularies,
it was not particularly helpful for the relatively small vocabularies typically found in
XML document processing.
However, a second approach has been found to be quite useful, taking advantage of
parallel bit streams for cheap determination of symbol length. In essence, the length of
a name can be determined very cheaply using a single bit scan operation. This then makes
it possible to use length-sorted symbol table processing, as follows. First, the
occurrences of all names are stored in arrays indexed by length. Then the length-sorted
arrays may each be inserted into the symbol table in turn. The advantage of this is that
a separate loop may be written for each length. Length sorting makes for very efficient
name processing. For example hash value computations and name comparisons can be made by
loading multibyte values and performing appropriate shifting and masking operations,
without the need for a byte-at-a-time loop. In initial experiments, this length-sorting
approach was found to reduce symbol lookup cost by a factor of two.
Current research includes the application of SIMD technology to further enhance the
performance of length-sorted lookup. We have identified a promising technique for
parallel processing of multiple name occurrences using a parallel trie lookup technique.
Given an array of occurrences of names of a particular length, the first one, two or
four bytes of each name are gathered and stored in a linear array. SIMD techniques are
then used to compare these prefixes with the possible prefixes for the current position
within the trie. In general, a very small number of possibilities exist for each trie
node, allowing for fast linear search through all possibilities. Typically, the
parallelism is expected to exceed the number of possibilities to search through at each
node. With length-sorting to separate the top-level trie into many small subtries, we
expect only a single step of symbol lookup to be needed in most practical instances.
The gather step of this algorithm is actually a common technique in SIMD processing.
Instruction set support for gather operations is a likely future direction for SIMD
Many XML applications involve numeric data fields as attribute values or element
content. Although most current XML APIs uniformly return information to applications in
the form of character strings, it is reasonable to consider direct API support for
numeric conversions within a high-performance XML engine. With string to numeric
conversion such a common need, why leave it to application programmers?
High-performance string to numeric conversion using SIMD operations also can
considerably outperform the byte-at-a-time loops that most application programmers or
libraries might employ. A first step is reduction of ASCII bytes to corresponding
decimal nybbles using a SIMD packing operation. Then an inductive doubling algorithm
using SIMD operations may be employed. First, 16 sets of adjacent nybble values in the
range 0-9 can be combined in just a few SIMD operations to 16 byte values in the range
0-99. Then 8 sets of byte values may similarly be combined with further SIMD processing
to produce doublebyte values in the range 0-9999. Further combination of doublebyte
values into 32-bit integers and so on can also be performed using SIMD operations.
Using appropriate gather operations to bring numeric strings into appropriate array
structures, an XML engine could offer high-performance numeric conversion services to
XML application programmers. We expect this to be an important direction for our future
work, particularly in support of APIs that focus on direct conversion of XML data into
APIs and Parallel Bit Streams
The ILAX Streaming API
The In-Line API for XML (ILAX) is the base API provided with the Parabix parser. It
is intended for low-level extensions compiled right into the engine, with minimum
possible overhead. It is similar to streaming event-based APIs such as SAX, but
implemented by inline substitution rather than using callbacks. In essence, an extension
programmer provides method bodies for event-processing methods declared internal to the
Parabix parsing engine, compiling the event processing code directly with the core code
of the engine.
Although ILAX can be used directly for application programming, its primary use is
for implementing engine extensions that support higher-level APIs. For example, the
implementation of C or C++ based streaming APIs based on the Expat [Expat] or general SAX models can be quite directly implemented. C/C++ DOM
or other tree-based APIs can also be fairly directly implemented. However, delivering
Parabix performance to Java-based XML applications is challenging due to the
considerable overhead of crossing the Java Native Interface (JNI) boundary. This issue
is addressed with the Array Set Model (ASM) concept discussed in the following section.
With the recent development of parallel parsing using bitstream addition, it is
likely that the underlying ILAX interface of Parabix will change. In essence, ILAX
suffers the drawback of all event-based interfaces: they are fundamentally sequential in
number. As research continues, we expect efficient parallel methods building on parallel
bit stream foundations to move up the stack of XML processing requirements. Artificially
imposing sequential processing is thus expected to constrain further advances in XML
Efficient XML in Java Using Array Set Models
In our GML-to-SVG case study, we identified the lack of high-performance XML
processing solutions for Java to be of particular interest. Java byte code does not
provide access to the SIMD capabilities of the underlying machine architecture. Java
just-in-time compilers might be capable of using some SIMD facilities, but there is no
real prospect of conventional compiler technology translating byte-at-a-time algorithms
into parallel bit stream code. So the primary vehicle for delivering high-performance
XML processing is to call native parallel bit stream code written in C through JNI
However, each JNI call is expensive, so it is desirable to minimize the number of
calls and get as much work done during each call as possible. This mitigates against
direct implementation of streaming APIs in Java through one-to-one mappings to an
underlying streaming API in C. Instead, we have concentrated on gathering information on
the C side into data structures that can then be passed to the Java side. However, using
either C pointer-based structures or C++ objects is problematic because these are
difficult to interpret on the Java side and are not amenable to Java's automatic storage
management system. Similarly, Java objects cannot be conveniently created on the C side.
However, it is possible to transfer arrays of simple data values (bytes or integers)
between C and Java, so that makes a reasonable focus for bulk data communication between
C and Java.
Array Set Models are array-based representations of information
representing an XML document in accord with XML InfoSet [XML Infoset] or
other XML data models relevant to particular APIs. As well as providing a mechanism for
efficient bulk data communication across the JNI boundary, ASMs potentially have a
number of other benefits in high-performance XML processing.
Prefetching. Commodity processors commonly support hardware and/or software
prefetching to ensure that data is available in a processor cache when it is
needed. In general, prefetching is most effective in conjunction with the
continuous sequential memory access patterns associated with array
DMA. Some processing environments provide Direct Memory Access (DMA)
controllers for block data movement in parallel with computation. For example,
the Cell Broadband Engine uses DMA controllers to move the data to and from the
local stores of the synergistic processing units. Arrays of contiguous data
elements are well suited to bulk data movement using DMA.
SIMD. Single Instruction Multiple Data (SIMD) capabilities of modern
processor instruction sets allow simultaneous application of particular
instructions to sets of elements from parallel arrays. For effective use of
SIMD capabilities, an SoA (Structure of Arrays) model is preferrable to an AoS
(Array of Structures) model.
Multicore processors. Array-oriented processing can enable the effective
distribution of work to the individual cores of a multicore system in two
distinct ways. First, provided that sequential dependencies can be minimized or
eliminated, large arrays can be divided into separate segments to be processed
in parallel on each core. Second, pipeline parallelism can be used to implement
efficient multipass processing with each pass consisting of a processing kernel
with array-based input and array-based output.
Streaming buffers for large XML documents. In the event that an XML document
is larger than can be reasonably represented entirely within processor memory,
a buffer-based streaming model can be applied to work through a document using
sliding windows over arrays of elements stored in document order.
Saxon-B TinyTree Example
As a first example of the ASM concept, current work includes a proof-of-concept to
deliver a high-performance replacement for building the TinyTree data structure used
in Saxon-B 6.5.5, an open-source XSLT 2.0 processor written in Java [Saxon]. Although XSLT stylesheets may be cached for performance, the
caching of source XML documents is typically not possible. A new TinyTree object to
represent the XML source document is thus commonly constructed with each new query so
that the overall performance of simple queries on large source XML documents is
highly dependent on TinyTree build time. Indeed, in a study of Saxon-SA, the
commercial version of Saxon, query time was shown to be dominated by TinyTree build
time [Kay 2008]. Similar performance results are demonstrable for the
Saxon-B XSLT processor as well.
The Saxon-B processor studied is a pure Java solution, converting a SAX (Simple
API for XML) event stream into the TinyTree Java object using the efficient Aelfred
XML parser [Ælfred]. The TinyTree structure is itself an
array-based structure mapping well suited to the ASM concept. It consists of six
parallel arrays of integers indexed on node number and containing one entry for each
node in the source document, with the exception of attribute and namespace nodes
[Saxon]. Four of the arrays respectively provide node kind, name
code, depth, and next sibling information for each node, while the two others are
overloaded for different purposes based on node kind value. For example, in the
context of a text node , one of the overloaded arrays holds the text buffer offset
value whereas the other holds the text buffer length value. Attributes and namespaces
are represented using similiar parallel array of values. The stored TinyTree values
are primarily primitive Java types, however, object types such as Java Strings and
Java StringBuffers are also used to hold attribute values and comment values
In addition to the TinyTree object, Saxon-B maintains a NamePool object which
represents a collection of XML name triplets. Each triplet is composed of a Namespace
URI, a Namespace prefix and a local name and encoded as an integer value known as a
namecode. Namecodes permit efficient name search and look-up using integer
comparison. Namecodes may also be subsequently decoded to recover namespace and local
Using the Parabix ILAX interface, a high-performance reimplementation of TinyTree
and NamePool data structures was built to compare with the Saxon-B implementation. In
fact, two functionally equivalent versions of the ASM java class were constructed. An
initial version was constructed based on a set of primitive Java arrays constructed
and allocated in the Java heap space via JNI New<PrimitiveType>Array
method call. In this version, the JVM garbage collector is aware of all memory
allocated in the native code. However, in this approach, large array copy operations
limited overall performance to approximately a 2X gain over the Saxon-B build time.
To further address the performance penalty imposed by copying large array values,
a second version of the ASM Java object was constructed based on natively backed
Direct Memory Byte Buffers [Hitchens 2002]. In this version the JVM garbage
collector is unaware any native memory resources backing the Direct Memory Byte
Buffers. Large JNI-based copy operations are avoided; however, system memory must be
explicitly deallocated via a Java native method call. Using this approach, our
preliminary results show an approximate total 2.5X gain over Saxon-B build time.
An important focus of our recent work is on the development of compiler technology to
automatically generate the low-level SIMD code necessary to implement bit stream processing
given suitable high-level specifications. This has several potential benefits. First, it
can eliminate the tedious and error-prone programming of bit stream operations in terms of
register-at-a-time SIMD operations. Second, compilation technology can automatically employ
a variety of performance improvement techniques that are difficult to apply manually. These
include algorithms for instruction scheduling and register allocation as well as
optimization techniques for common subexpression expression elimination and register
rematerialization among others. Third, compiler technology makes it easier to make changes
to the low-level code for reasons of perfective or adaptive maintenance.
Beyond these reasons, compiler technology also offers the opportunity for retargetting
the generation of code to accommodate different processor architectures and API
requirements. Strategies for efficient parallel bit stream code can vary considerably
depending on processor resources such as the number of registers available, the particular
instruction set architecture supported, the size of L1 and L2 data caches, the number of
available cores and so on. Separate implementation of custom code for each processor
architecture would thus be likely to be prohibitively expensive, prone to errors and
inconsistencies and difficult to maintain. Using compilation technology, however, the idea
would be to implement a variety of processor-specific back-ends all using a common front
end based on parallel bit streams.
Character Class Compiler
The first compiler component that we have implemented is a character class compiler,
capable of generation all the bit stream logic necessary to produce a set of lexical
item streams each corresponding to some particular set of characters to be recognized.
By taking advantage of common patterns between characters within classes, and special
optimization logic for recognizing character-class ranges, our existing compiler is able
to generate well-optimized code for complex sets of character classes involving numbers
of special characters as well as characters within specific sets of ranges.
Regular Expression Compilation
Based on the character class compiler, we are currently investigating the
construction of a regular expression compiler that can implement bit-stream based
parallel regular-expression matching similar to that describe previously for parallel
parsing by bistream addition. This compiler works with the assumption that bitstream
regular-expression definitions are deterministic; no backtracking is permitted with the
parallel bit stream representation. In XML applications, this compiler is primarily
intended to enforce regular-expression constraints on string datatype specifications
found in XML schema.
Unbounded Bit Stream Compilation
The Catalog of XML Bit Streams presented earlier consist of a set of abstract,
unbounded bit streams, each in one-to-one correspondence with input bytes of a text
file. Determining how these bit streams are implemented using fixed-width SIMD
registers, and possibly processed in fixed-length buffers that represent some multiple
of the register width is a source of considerable programming complexity. The general
goal of our compilation strategy in this case is to allow operations to be programmed in
terms of unbounded bit streams and then automatically reduced to efficient low-level
code with the application of a systematic code generation strategy for handling block
and buffer boundary crossing. This is work currently in progress.
Parallel bit stream technology offers the opportunity to dramatically speed up the core
XML processing components used to implement virtually any XML API. Character validation and
transcoding, whitespace processing, and parsing up to including the full validation of tag
syntax can be handled fully in parallel using bit stream methods. Bit streams to mark the
positions of all element names, attribute names and attribute values can also be produced,
followed by fast bit scan operations to generate position and length values. Beyond bit
streams, byte-oriented SIMD processing of names and numerals can also accelerate
performance beyond sequential byte-at-a-time methods.
Advances in processor architecture are likely to further amplify the performance of
parallel bit stream technology over traditional byte-at-a-time processing over the next
decade. Improvements to SIMD register width, register complement and operation format can
all result in further gains. New SIMD instruction set features such as inductive doubling
support, parallel extract and deposit instructions, bit interleaving and scatter/gather
capabilities should also result in significant speed-ups. Leveraging the intraregister
parallelism of parallel bit stream technology within SIMD registers to take of intrachip
parallelism on multicore processors should accelerate processing further.
Technology transfer using a patent-based open-source business model is a further goal of
our work with a view to widespread deployment of parallel bit stream technology in XML
processing stacks implementing a variety of APIs. The feasibility of substantial
performance improvement in replacement of technology implementing existing APIs has been
demonstrated even in complex software architectures involving delivery of performance
benefits across the JNI boundary. We are seeking to accelerate these deployment efforts
both through the development of compiler technology to reliably apply these methods to a
variety of architectures as well as to identify interested collaborators using open-source
or commercial models.
This work is supported in part by research grants and scholarships from the Natural
Sciences and Engineering Research Council of Canada, the Mathematics of Information
Technology and Complex Systems Network and the British Columbia Innovation Council.
We thank our colleague Dan Lin (Linda) for her work in high-performance symbol table
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