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.Dd June 1, 2022
.Dt TSLOG 4
.Os
.Sh NAME
.Nm tslog
.Nd Boot-time event tracing facility
.Sh SYNOPSIS
To compile this boot-time event tracing facility into the kernel,
place the following line in the kernel configuration file:
.Bd -ragged -offset indent
.Cd "option TSLOG"
.Ed
.Sh DESCRIPTION
.Nm
is a boot-time event tracing facility.
It is suitable for tracing recursive events
based on function entries and exits.
Its purpose is to ease pinpointing and reducing the overall
.Fx
boot time by generating detailed timing information.
.Pp
.Nm
is able to trace the boot loader, kernel initialization, and userland processes.
.Pp
In userland, it records the following details for each process ID:
.Bl -dash
.It
The timestamp of the
.Xr fork 2
which creates the given process ID and the parent process ID.
.It
The path passed to
.Xr execve 2 ,
if any.
.It
The first path resolved by
.Xr namei 9 ,
if any.
.It
The timestamp of the
.Xr exit 3
which terminates the process.
.El
.Sh SYSCTL VARIABLES
The following
.Xr sysctl 8
variables are available:
.Bl -tag -width indent
.It Va debug.tslog
Dump the
.Nm
buffer of recorded loader and kernel event timestamps.
.It Va debug.tslog_user
Dump the
.Nm
buffer
of recorded userland event timestamps.
.El
.Sh FLAMEGRAPHS
The
.Nm
buffer dumps
can be used to generate flamegraphs of the
.Fx
boot process for visual analysis.
See
.Lk https://github.com/cperciva/freebsd-boot-profiling
for more information.
.Sh SEE ALSO
.Xr dtrace 1 ,
.Xr boottrace 4 ,
.Xr ktr 4
.Sh HISTORY
.Nm
first appeared in
.Fx 12.0 .
Support for tracing boot loaders and userland process
was added in
.Fx 13.2 .
.Ss TSLOG vs. Boottrace
.Nm
is oriented towards system developers while
.Xr boottrace 4
is meant to be easy to use by system administrators.
Both faciliities provide an overview of timing and resource usage of the boot
process.
.Ss TSLOG vs. DTrace
.Xr dtrace 1
is not always the right tool for profiling early kernel initialization.
The reason is it requires some kernel subroutines
which are not yet available early in the boot process, e.g.:
traps, memory allocation, or thread scheduling.
.Nm
depends on fewer kernel subroutines than
.Xr dtrace 1
and because of that can trace early kernel initialization.
.Ss TSLOG vs. KTR
.Xr ktr 4
has a couple of limitations which prevent it from
being able to run at the start of the boot process.
In contrast,
.Nm
is designed for logging timestamped events for boot
profiling.
.Sh AUTHORS
.An -nosplit
.Nm
was written by
.An Colin Percival Aq Mt cperciva@FreeBSD.org .
.Pp
This manual page was written by
.An Mateusz Piotrowski Aq Mt 0mp@FreeBSD.org .