[ltt-dev] [RFC] Common Trace Format Proposal for Linux (v1)

Mathieu Desnoyers mathieu.desnoyers at efficios.com
Tue Oct 5 15:37:12 EDT 2010

The goal of the present document is to propose a trace format that suits the
needs of the embedded, telecom, high-performance and kernel communities.  It is
based on the Common Trace Format Requirements (v1.4) document. It is designed to
be natively generated by tracing of a Linux kernel and Linux user-space
applications written in C/C++.

A reference implementation of a library to read and write this trace format is
being implemented within the BabelTrace project, a converter between trace
formats. The development tree is available at:

  git tree:   git://git.efficios.com/babeltrace.git
  gitweb:     http://git.efficios.com/?p=babeltrace.git

Comments are welcome! (proposal document below)



1. Preliminary definitions

  - Trace: An ordered sequence of events.
  - Section: Group of events, containing a subset of the trace event types.
  - Packet: A sequence of physically contiguous events within a section.
  - Event: This is the basic entry in a trace. (aka: a trace record).
    - An event identifier (ID) relates to the class (a type) of event within
      a section.
        e.g. section: high_throughput, event: irq_entry.
    - An event (or event record) relates to a specific instance of an event
        e.g. section: high_throughput, event: irq_entry, at time X, on CPU Y

2. High-level representation of a trace

A trace is divided into multiple trace streams, each representing an information
stream specific to:

 - a section,
 - a processor.

A trace "section" consists of a collection of trace streams (typically one trace
stream per cpu) containing a subset of the trace event types.

Because each trace stream is appended to while a trace is being recorded, each
is associated with a separate file for disk output. Therefore, a trace stored to
disk can be represented as a directory containing one file per section.

A metadata section contains information on trace event types. It describes:

- Trace version.
- Types available.
- Per-section event header description.
- Per-section event header selection.
- Per-section event context fields.
- Per-event
  - Event type to section mapping.
  - Event type to name mapping.
  - Event type to ID mapping.
  - Event fields description.

3. Trace Section

A trace section is divided in contiguous packets of variable size. These
subdivisions allow the trace analyzer to perform a fast binary search by time
within the section (typically requiring to index only the packet headers)
without reading the whole section. These subdivisions have a variable size to
eliminate the need to transfer the packet padding when partially filled packets
must be sent when streaming a trace for live viewing/analysis. Dividing sections
into packets is also useful for network streaming over UDP and flight recorder
mode tracing (a whole packet can be swapped out of the buffer atomically for

The section header is repeated at the beginning of each packet to allow
flexibility in terms of:

  - streaming support,
  - allowing arbitrary buffers to be discarded without making the trace
  - allow UDP packet loss handling by either dealing with missing packet or
    asking for re-transmission.
  - transparently support flight recorder mode,
  - transparently support crash dump.

The section header will therefore be referred to as the "packet header"
thorough the rest of this document.

4. Types

4.1 Basic types

A basic type is a non-compound type, as described in this section.

4.1.1 Type inheritance

Type specifications can be inherited to allow deriving concrete types from an
abstract type. For example, see the uint32_t type derived from the "integer"
abstract type below ("Integers" section). Concrete types have a precise binary
representation in the trace. Abstract types have methods to read and write these
types, but must be derived into a concrete type to be usable in an event field.

Concrete types inherit from abstract types. Abstract types can inherit from
other abstract types.

4.1.2 Alignment

We define "byte-packed" types as aligned on the byte size, namely 8-bit.
We define "bit-packed" types as following on the next bit, as defined by the
"bitfields" section.
We define "natural alignment" of a basic type as the lesser value between the
type size and the architecture word size.

All basic types, except bitfields, are either aligned on their "natural"
alignment or byte-packed, depending on the architecture preference.
Architectures providing fast unaligned writes byte-packed basic types to save
space, aligning each type on byte boundaries (8-bit). Architectures with slow
unaligned writes align types on the lesser value between their size and the
architecture word size (the type "natural" alignment on the architecture).

Note that the natural alignment for 64-bit integers and double-precision
floating point values is fixed to 32-bit on a 32-bit architecture, but to 64-bit
for a 64-bit architecture.

Metadata attribute representation:

  align = value;                                /* value in bits */

4.1.3 Byte order

By default, target architecture endianness is used. Byte order can be overridden
for a basic type by specifying a "byte_order" attribute. Typical use-case is to
specify the network byte order (big endian: "be") to save data captured from the
network into the trace without conversion. If not specified, the byte order is

Metadata representation:

  byte_order = native OR network OR be OR le;	/* network and be are aliases */

4.1.4 Size

Type size, in bits, for integers and floats is that returned by "sizeof()" in C
multiplied by CHAR_BIT.
We require the size of "char" and "unsigned char" types (CHAR_BIT) to be fixed
to 8 bits for cross-endianness compatibility.

Metadata representation:

  size = value;    (value is in bits)

4.1.5 Integers

Signed integers are represented in two-complement. Integer alignment, size,
signedness and byte ordering are defined in the metadata. Integers aligned on
byte size (8-bit) and with length multiple of byte size (8-bit) correspond to
the C99 standard integers. In addition, integers with alignment and/or size that
are _not_ a multiple of the byte size are permitted; these correspond to the C99
standard bitfields, with the added specification that the CTF integer bitfields
have a fixed binary representation. A MIT-licensed reference implementation of
the CTF portable bitfields is available at:


Binary representation of integers:

- On little and big endian:
  - Within a byte, high bits correspond to an integer high bits, and low bits
    correspond to low bits.
- On little endian:
  - Integer across multiple bytes are placed from the less significant to the
    most significant.
  - Consecutive integers are placed from lower bits to higher bits (even within
    a byte).
- On big endian:
  - Integer across multiple bytes are placed from the most significant to the
    less significant.
  - Consecutive integers are placed from higher bits to lower bits (even within
    a byte).

This binary representation is derived from the bitfield implementation in GCC
for little and big endian. However, contrary to what GCC does, integers can
cross units boundaries (no padding is required). Padding can be explicitely
added (see 4.1.6 GNU/C bitfields) to follow the GCC layout if needed.

Metadata representation:

  abstract_type integer {
    signed = true OR false;                     /* default false */
    byte_order = native OR network OR be OR le; /* default native */
    size = value;                               /* value in bits, no default */
    align = value;                              /* value in bits */

Example of type inheritance (creation of a concrete type uint32_t):

type uint32_t {
  parent = integer;
  size = 8;
  signed = false;
  align = 32;

Definition of a 5-bit signed bitfield:

type int5_t {
  parent = integer;
  size = 5;
  signed = true;
  align = 1;

4.1.6 GNU/C bitfields

The GNU/C bitfields follow closely the integer representation, with a
particularity on alignment: if a bitfield cannot fit in the current unit, the
unit is padded and the bitfield starts at the following unit. We therefore need
to express the extra "unit size" information.

Metadata representation:

abstract_type gcc_bitfield {
  parent = integer;
  unit_size = value;

As an example, the following structure declared in C compiled by GCC:

struct example {
  short a:12;
  short b:5;

Would correspond to the following structure, aligned on the largest element
(short). The second bitfield would be aligned on the next unit boundary, because
it would not fit in the current unit.

type struct_example {
  parent = struct;
  fields = {
      type {
        parent = gcc_bitfield;
        unit_size = 16;				/* sizeof(short) */
        size = 12;
        signed = true;
        align = 1;
      type {
        parent = gcc_bitfield;
        unit_size = 16;				/* sizeof(short) */
        size = 5;
        signed = true;
        align = 1;

4.1.7 Floating point

The floating point values byte ordering is defined in the metadata.

Floating point values follow the IEEE 754-2008 standard interchange formats.
Description of the floating point values include the exponent and mantissa size
in bits. Some requirements are imposed on the floating point values:

- FLT_RADIX must be 2.
- mant_dig is the number of digits represented in the mantissa. It is specified
  by the ISO C99 standard, section 5.2.4, as FLT_MANT_DIG, DBL_MANT_DIG and
  LDBL_MANT_DIG as defined by <float.h>.
- exp_dig is the number of digits represented in the exponent. Given that
  mant_dig is one bit more than its actual size in bits (leading 1 is not
  needed) and also given that the sign bit always takes one bit, exp_dig can be
  specified as:

  - sizeof(float) * CHAR_BIT - FLT_MANT_DIG
  - sizeof(double) * CHAR_BIT - DBL_MANT_DIG
  - sizeof(long double) * CHAR_BIT - LDBL_MANT_DIG

Metadata representation:

abstract_type floating_point {
   exp_dig = value;
   mant_dig = value;
   byte_order = native OR network OR be OR le;

Example of type inheritance:

type float {
  exp_dig = 8;         /* sizeof(float) * CHAR_BIT - FLT_MANT_DIG */
  mant_dig = 24;       /* FLT_MANT_DIG */
  byte_order = native;

TODO: define NaN, +inf, -inf behavior.

4.1.8 Enumerations

Enumerations are a mapping between an integer type and a table of strings. The
numerical representation of the enumeration follows the integer type specified
by the metadata. The enumeration mapping table is detailed in the enumeration
description within the metadata.

abstract_type enum  {
  .parent = integer;
  .map = {
    { value , string },
    { value , string },
    { value , string },

4.1.9 Strings

Strings have a variable size and are terminated by a '\0' "NULL" character.
Their encoding is described in the metadata. In absence of encoding attribute
information, the default encoding is UTF-8.

abstract_type string {
  encoding = UTF8 OR ASCII;

4.2 Compound types

4.2.1 Structures

Structures are aligned on the largest alignment required by basic types
contained within the structure. (This follows the ISO/C standard for structures)

Metadata representation:

abstract_type struct {
  fields = {
    { field_type, field_name },
    { field_type, field_name },


type struct_example {
  parent = struct;
  fields = {
      type {                 /* Nameless type */
        parent = integer;
        size = 16;
        signed = true;
        align = 16;
      uint64_t,              /* Named type declared in the metadata */

The fields are placed in a sequence next to each other. They each possess a
field name, which is a unique identifier within the structure.

4.2.2 Arrays

Arrays are fixed-length. Their length is declared in the type declaration within
the metadata. They contain an array of "inner type" elements, which can refer to
any type not containing the type of the array being declared (no circular

Metadata representation:

abstract_type array {
  length = value;
  elem_type = type;


type example_array {
  parent = array;
  length = 10;
  elem_type = uint32_t;

4.2.3 Sequences

Sequences are dynamically-sized arrays. They start with an integer that specify
the length of the sequence, followed by an array of "inner type" elements.

abstract_type sequence {
  length_type = type;	/* Inheriting from integer */
  elem_type = type;

The integer type follows the integer types specifications, and the sequence
elements follow the "array" specifications.

5. Trace Packet Header

- Aligned on page size. Fixed size. Fields aligned on their natural size or
  packed (depending on the architecture preference).
  No padding at the end of the trace packet header. Native architecture byte
- Magic number (CTF magic numbers: 0xC1FC1FC1 and its reverse endianness
  representation: 0xC11FFCC1) It needs to have a non-symmetric bytewise
  representation. Used to distinguish between big and little endian traces (this
  information is determined by knowing the endianness of the architecture
  reading the trace and comparing the magic number against its value and the
  reverse, 0xC11FFCC1). This magic number specifies that we use the CTF metadata
  description language described in this document. Different magic numbers
  should be used for other metadata description languages.
- Session ID, used to ensure the packet match the metadata used.
  (note: we cannot use a metadata checksum because metadata can be appended to
   while tracing is active)
- Packet content size (in bytes).
- Packet size (in bytes, includes padding).
- Packet content checksum (optional). Checksum excludes the packet header.
- Per-section packet sequence count (to deal with UDP packet loss). The number
  of significant sequence counter bits should also be present, so wrap-arounds
  are deal with correctly.
- Timestamp at the beginning and end of the packet. Should include all
  event timestamps contained therein.
- Events discarded count
  - Snapshot of a per-section free-running counter, counting the number of
    events discarded that were supposed to be written in the section prior to
    the first event in the packet.
    * Note: producer-consumer buffer full condition should fill the current
            packet with padding so we know exactly where events have been
- Compression scheme used for the packet content. Applied directly to raw data.
  0: no compression scheme
  1: bzip2
  2: gzip
- Cypher used for the packet content. Applied after compression.
  0: no encryption
  1: AES
- Checksum scheme used for the packet content. Applied after encryption.
  0: no checksum
  1: md5
  2: sha1

type packet_header {
  parent = struct;
  fields = {
    { uint32_t, magic },
    { uint32_t, session_id },
    { uint32_t, content_size },
    { uint32_t, packet_size },
    { uint32_t, checksum },
    { uint32_t, section_packet_count },
    { uint64_t, timestamp_begin }
    { uint64_t, timestamp_end }
    [ uint32_t, events_discarded },
    { uint8_t,  section_packet_count_bits },	/* Significant counter bits */
    { uint8_t,  compression_scheme },
    { uint8_t,  encryption_scheme },
    { uint8_t,  checksum },

6. Event Structure

The overall structure of an event is:

  - Event Header (as specifed by the section metadata)
  - Extended Event Header (as specified by the event header)
  - Event Context (as specified by the section metadata)
  - Event Payload (as specified by the event metadata)

6.1 Event Header

One major factor can vary between sections: the number of event IDs assigned to
a section. Luckily, this information tends to stay relatively constant (modulo
event registration while trace is being recorded), so we can specify different
representations for sections containing few event IDs and sections containing
many event IDs, so we end up representing the event ID and timestamp as densely
as possible in each case.

We therefore provide two types of events headers. Type 1 accommodates sections
with less than 31 event IDs. Type 2 accommodates sections with 31 or more event

The "extended headers" are used in the rare occasions where the information
cannot be represented in the ranges available in the event header.

Types uintX_t represent an X-bit unsigned integer.

6.1.1 Type 1 - Few event IDs

  - Aligned on 32-bit (or 8-bit if byte-packed, depending on the architecture
  - Fixed size: 32 bits.
  - Native architecture byte ordering.

type event_header_1 {
  parent = struct;
  fields = {
    { uint5_t, id },	/*
			 * id: range: 0 - 30.
			 * id 31 is reserved to indicate a following
			 * extended header.
    { uint27_t, timestamp },

The end of a type 1 header is aligned on a 32-bit boundary (or packed).

6.1.2 Extended Type 1 Event Header

  - Follows struct event_header_1, which is aligned on 32-bit, so no need to
  - Fixed size: 96 bits.
  - Native architecture byte ordering.

type event_header_1_ext {
  parent = struct;
  fields = {
    { uint32_t, id },		/* 32-bit event IDs */
    { uint64_t, timestamp },	/* 64-bit timestamps */ 

The end of a type 1 extended header is aligned on the natural alignment of a
64-bit integer (or 8-bit if byte-packed).

6.1.3 Type 2 - Many event IDs

  - Aligned on 32-bit (or 8-bit if byte-packed, depending on the architecture
  - Fixed size: 48 bits.
  - Native architecture byte ordering.

type event_header_2 {
  parent = struct;
  fields = {
    { uint32_t, timestamp },
    { uint16_t, id },	/*
			 * id: range: 0 - 65534.
			 * id 65535 is reserved to indicate a following
			 * extended header.

The end of a type 2 header is aligned on a 16-bit boundary (or 8-bit if

6.1.4 Extended Type 2 Event Header

  - Follows struct event_header_2, which alignment end on a 16-bit boundary, so
    we need to align on 64-bit integer natural alignment (or 8-bit if
  - Fixed size: 96 bits.
  - Native architecture byte ordering.

type event_header_2_ext {
  parent = struct;
  fields = {
    { uint64_t, timestamp },	/* 64-bit timestamps */ 
    { uint32_t, id },		/* 32-bit event IDs */

The end of a type 2 extended header is aligned on the natural alignment of a
32-bit integer (or 8-bit if byte-packed).

6.2 Event Context

The event context contains information relative to the current event. The choice
and meaning of this information is specified by the metadata "section"
information. For this trace format, event context is usually empty, except when
the metadata "section" information specifies otherwise by declaring a non-empty
structure for the event context. An example of event context is to save the
event payload size with each event, or to save the current PID with each event.

6.2.1 Event Context Description

Event context example. These are declared within the section declaration within
the metadata.

type per_section_event_ctx {
  parent = struct;
  fields = {
    { uint, pid },
    { uint16_t, payload_size },

6.3 Event Payload

An event payload contains fields specific to a given event type. The fields
belonging to an event type are described in the event-specific metadata
within a structure type.

6.3.1 Padding

No padding at the end of the event payload. This differs from the ISO/C standard
for structures, but follows the CTF standard for structures. In a trace, even
though it makes sense to align the beginning of a structure, it really makes no
sense to add padding at the end of the structure, because structures are usually
not followed by a structure of the same type.

This trick can be done by adding a zero-length "end" field at the end of the C
structures, and by using the offset of this field rather than using sizeof()
when calculating the size of a structure (see section "A.1 Helper macros").

6.3.2 Alignment

The event payload is aligned on the largest alignment required by types
contained within the payload. (This follows the ISO/C standard for structures)

7. Metadata

The meta-data is located in a tracefile section named "metadata". It is made of
"packets", which each start with a packet header. The event type within the
metadata section have no event header nor event context. Each event only
contains a null-terminated "string" payload, which is a metadata description
entry. The events are packed one next to another. Each packet start with a
packet header, which contains, amongst other fields, the session ID and magic

The metadata can be parsed by reading through the metadata strings, skipping
spaces, newlines and null-characters.

trace {
  major = value;	/* Trace format version */
  minor = value;

section {
  name = section_name;
  event {
    /* Type 1 - Few event IDs; Type 2 - Many event IDs */
    header_type = type1 OR type2;
    context {
      event_size = true OR false;  /* Includes event size field or not */

event {
  name = event_name;
  id = value;			/* Numeric identifier within the section */
  section = section_name;
  fields = type inheriting from "struct" abstract type.

/* More detail on types in section 4. Types */

/* Named types */
type typename {

/* Unnamed types, contained within compound type fields */
type {

A.1 Helper macros

The two following macros keep track of the size of a GNU/C structure without
padding at the end by placing HEADER_END as the last field.

#define HEADER_END		char end_field[0]
#define header_sizeof(type)	offsetof(typeof(type), end_field)

Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.

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