[Request for review] Profiling the FreeBSD kernel boot

Colin Percival cperciva at tarsnap.com
Fri Dec 22 09:44:54 UTC 2017

Hi everyone,

For the past few months I've been working on code for profiling the FreeBSD
"kernel boot", i.e., everything between when kernel code starts running and
when we first enter userland as init(8).  This is not trivial since it's
impossible to use tools like dtrace to monitor things prior to when said
tools are running.  The goal of this exercise is to help me track down the
places where we're wasting time during the boot, and then to fix them.

The approach I've taken is to add some macros -- most notably TSENTER() and
TSEXIT() -- which by default compile to nothing, but if the TSLOG kernel
option is enabled they compile to code which logs the cycle count (e.g., on
x86 the value from the RDTSC instruction) along with some other data (in the
case of TSENTER and TSEXIT, the fact that we're entering/exiting a function).
This can then be dumped via a sysctl (debug.tslog) and processed in userland
to convert function entries/exits into stacks and to visualize the time spent
in the boot process.

Two examples:

A flame chart of my laptop booting HEAD:

A flame chart of an EC2 c5.4xlarge instance booting 11.1-RELEASE:

The patches (10 of them, to be applied in order), userland scripts, and very
brief usage instructions are at:

I hope to commit the patches in the next week, since I'm planning on writing
a paper to submit to AsiaBSDCon (which has a deadline of December 31st); so
if anyone has interest/time to look at this in the near future (I mean, it's
not like anyone is going to be busy this weekend, right?) I'd love to have
some feedback before it goes into the tree.

Colin Percival
Security Officer Emeritus, FreeBSD | The power to serve
Founder, Tarsnap | www.tarsnap.com | Online backups for the truly paranoid

More information about the freebsd-current mailing list