docs/cpuprofile-fileformat.adoc

= Gperftools CPU Profiler Binary Data File Format
:reproducible:

[.normal]
This file documents the binary data file format produced by the
gperftools CPU Profiler. It is one of "legacy" formats supported by
the pprof tool. For information about using the CPU Profiler, see
link:cpuprofile.html[its user guide].

The profiler source code, which generates files using this format, is at
`src/profiler.cc`.

== CPU Profile Data File Structure

CPU profile data files each consist of four parts, in order:

* Binary header
* Binary profile records
* Binary trailer
* Text list of mapped objects

The binary data is expressed in terms of "slots." These are words large
enough to hold the program's pointer type, i.e., for 32-bit programs
they are 4 bytes in size, and for 64-bit programs they are 8 bytes. They
are stored in the profile data file in the native byte order (i.e.,
little-endian for x86 and x86_64).

== Binary Header

The binary header format is show below. Values written by the profiler,
along with requirements currently enforced by the analysis tools, are
shown in parentheses.

[cols=",",options="header",]
|===
|slot |data
|0 |header count (0; must be 0)
|1 |header slots after this one (3; must be >= 3)
|2 |format version (0; must be 0)
|3 |sampling period, in microseconds
|4 |padding (0)
|===

The headers currently generated for 32-bit and 64-bit little-endian (x86
and x86_64) profiles are shown below, for comparison.

[cols=",,,,,",options="header",]
|===
| |hdr count |hdr words |version |sampling period |pad
|32-bit or 64-bit (slots) |0 |3 |0 |10000 |0

|32-bit (4-byte words in file) |`0x00000` |`0x00003` |`0x00000`
|`0x02710` |`0x00000`

|64-bit LE (4-byte words in file) |`0x00000 0x00000`
|`0x00003 0x00000` |`0x00000 0x00000` |`0x02710 0x00000`
|`0x00000 0x00000`
|===

The contents are shown in terms of slots, and in terms of 4-byte words
in the profile data file. The slot contents for 32-bit and 64-bit
headers are identical. For 32-bit profiles, the 4-byte word view matches
the slot view. For 64-bit profiles, each (8-byte) slot is shown as two
4-byte words, ordered as they would appear in the file.

The profiling tools examine the contents of the file and use the
expected locations and values of the header words field to detect
whether the file is 32-bit or 64-bit.

== Binary Profile Records

The binary profile record format is shown below.

[cols=2*]
|===
|slot
|data

|0
|sample count, must be >= 1

|1
|number of call chain PCs (num_pcs), must be >= 1

|2 .. (num_pcs + 1)
|call chain PCs, most-recently-called function first.
|===

The total length of a given record is 2 + num_pcs.

Note that multiple profile records can be emitted by the profiler having
an identical call chain. In that case, analysis tools should sum the
counts of all records having identical call chains.

*Note:* Some profile analysis tools terminate if they see _any_ profile
record with a call chain with its first entry having the address 0.
(This is similar to the binary trailer.)

=== Example

This example shows the slots contained in a sample profile record.

[cols=",,,,",]
|===
|5 |3 |0xa0000 |0xc0000 |0xe0000
|===

In this example, 5 ticks were received at PC 0xa0000, whose function had
been called by the function containing 0xc0000, which had been called
from the function containing 0xe0000.

== Binary Trailer

The binary trailer consists of three slots of data with fixed values,
shown below.

[cols=",",options="header",]
|===
|slot |value
|0 |0
|1 |1
|2 |0
|===

Note that this is the same data that would contained in a profile record
with sample count = 0, num_pcs = 1, and a one-element call chain
containing the address 0.

== Text List of Mapped Objects

The binary data in the file is followed immediately by a list of mapped
objects. This list consists of lines of text separated by newline
characters.

Each line describes one mapping as produced by SaveProcSelfMaps. For
example:

....
  40000000-40015000 r-xp 00000000 03:01 12845071   /lib/ld-2.3.2.so
....

The first address must start at the beginning of the line. This is
essentially the same format as Linux's `/proc/<pid>/maps` file.
Recent Linux systems have this format documented in
link:https://man7.org/linux/man-pages/man5/proc_pid_maps.5.html[`man 5
proc_pid_maps`]. Less recent systems document it under `man 5 proc`.

Tools ignore minor:major number and inode number. And only executable
mappings really need to be present. See
`src/base/proc_maps_iterator.{h,cc}` for how it is produced.

Unrecognized lines should be ignored by analysis tools.

Note, original pprof tool also supported processing `$build`
"variable" when processing mappings, but we never produced such
mappings. So we don't document this anymore.

'''''

Original authror: Chris Demetriou (cgd) +
Last update by: Aliaksei Kandratsenka +